GB2048491A

GB2048491A - Method for evaluating corrosion of coated metallic material

Info

Publication number: GB2048491A
Application number: GB8008963A
Authority: GB
Original assignee: Nippon Paint Co Ltd
Current assignee: Nippon Paint Co Ltd
Priority date: 1980-03-17
Filing date: 1980-03-17
Publication date: 1980-12-10
Also published as: GB2048491B

Abstract

Corrosion of coated metallic material is evaluated by placing the coated side of a metallic specimen plate, in contact with a given corrosive medium; detecting the spontaneous electrode potential of said specimen plate through a first reference electrode and potentiostatically electrolysing said film side at said spontaneous electrode potential; causing said film side to be selectively polarized by a cathodic-anodic pulsation polarization method and by a linear potential sweep polarization method with the help of a first counter-electrode thereby to detect the polarization current and thereafter to detect the existence of any film defect; and selectively subtracting from said polarization current a current portion correspondingly caused by the electric resistance specific to said film, thereby detecting an infinitesimally small current-potential variation so as further to detect said polarization current caused by the application of said cathodic-anodic pulsation polarization to said specimen plate when film defects are not present, and, when film defects are present, detecting said polarization current alternatively caused by the application of said cathodic-anodic polarization to said specimen plate to determine selectively either one or both of a cathodic polarization curve and an anodic polarization curve.

Description

SPECIFICATION Corrosion evaluation testing method of coated metallic material and apparatus employed therefor BACKGROUND OF THE INVENTION This invention relates to corrosion evaluation, and more particularly, to a corrosion evaluation testing method for coated metallic material, and to the apparatus employed therefor.

In general, metallic material is mainly coated on its surface for the purpose of corrosion protection, and especially to prevent corrosion. The corrosion preventing performance of a coated film is highly dependent upon the resistance-polarization which is itself dependent upon the electrical resistance of the coated film. Accordingly, various high resistance films are, in practice, currently coated on to steel plate, for example. However, water still cannot be completely shut out even upon application of such coating films of the above-descirbed type onto a metallic material. Accordingly, corrosion progresses under the film. Also, when defects exist in the coated film, the corrosion which is caused in the defective portion of the film progresses more strongly in proportion to increases in the electrical resistance of the film.Thus, so-called chipping corrosion which occurs as a result of film damage and/or cracks in the film presents a serious problem in the art of metal corrosion, and especially the corrosion of film coated metal.

When the problems concerning the reaction to corrosion of a coated metallic material are considered in general, in addition to the resistance-polarization effect which is described above, it is also necessary to take into account the physicochemical effects caused by a corrosive liquid under environmental conditions where the coated film is involved, for example, the selfpreventing or promoting effects which are inherently caused by substances which have melted and been discharged from the film, including pigments.

At present, so far as the corrosion evaluation method of a coated steel plate is concerned, tests such as a spontaneous exposure test or a weathering exposure test, a dipping or immersion test and an artifical accelerated test have been widely employed. However, according to these methods, injury or damage is first applied to the specimens which are to be tested and thereafter the results are simply visually observed. As a result, the phenomenological mechanism relating to the coated steel plate and the method for testing corrosion thereof have still not been sufficiently clarified. Therefore, significant information for developing these technical aspects relating to film coating which could be useful in corrosion prevention cannot be expected to be available with the help of the conventional methods described above.

In addition, in response to an anodic reaction which is involved in the corrosion of the coated steel plate, a cathodic reaction is generally caused in the neighbourhood of the region in which the anodic reaction is taking place. Thus, the corrosion phenomenon of hydrogen fragility corrosion or of hydrogen cracking, caused by the diffusion of atomic hydrogen into the steel, which results from the cathodic reaction, should not be overlooked from the viewpoint of preventing the structure from being cracked by the corroision phenomena. Furthermore, it is important to know the corrosion preventing capability of the film in various corrosive liquids (for example, in crude oil) apart from the electrolytes in various soils or in corrosive gases, for example.

In order quantitatively to analyse the corrosion phenomenon relating to the coated steel plate, the phenomenological data of the physicochemical quantities which are concerned in the corrosion have first to be obtained. In particular, an electrochemical method by which the process of corrosion reaction can be easily traced is suitable for this type of corrosion evaluation method. However, since the film coated onto the steel plate is high in resistance, reliable measuring results necessary for estimating the corrosion reaction mechanism cannot be obtained by the same electrochemical method as may be used in the case of the measurement relating to the bare steel.

We have now developed a method of obtaining the polarization curve, a method of detecting the extremely small amount of current-potential variation or of detecting an infinitesimally small current-potential variation, and a method of detecting the film resistance specific to the coated film, and the results of the potentiostatic electrolysis of the coated steel plate at the spontaneous electrode potential which is based upon the latter determination. Moreover, a method of detecting the electrolysis current of the discharge of the atomic hydrogen which is diffused into the non-film side from the film side, with the non-film coated side of the coated plate being potentiostatically electrolysed at the spontaneous electrode potential, has also been developed.

Specifically, we have established a corrosion evaluation testing method for coated metallic material and apparatus for this purpose. These comprise a method of detecting electrochemical data significant to the corrosion evaluation of the coated metallic material through the application to the coated metallic material of a series of electrochemical testing method steps developed according to the present invention, and an apparatus for automatically performing the detecting method.

Up to the present, with respect to the corrosion evaluation testing method when the film has defects, a method of first providing cross-cuts on the film, thereby visually observing the blistering and rust width near the cuts, has been conventionally employed. However, according to the conventional method just mentioned, it is difficult to judge or to confirm, from the outside, the corrosion forms, i.e., whether the corrosion tends to spread under the film and along the film in relation to the film defect portion or whether the corrosion tends to progress in depth through the film. Moreover, it is not practical to apply such a corrosion evaluation test when the coated metallic articles or workpieces are buried in water or in the soil.On the other hand, an electrochemical method of measuring the electrode potential, for example, of the coated metallic article or workpiece has also been conventionally performed. However, such a method is still insufficient for use in the corrosion evaluation test, wherein the electrochemical method inherently involves complicated analyses.

As regards the defects which accompany the conventional method, we have found out, after various experiments on the polarization behaviour of a coated metallic face by the use of the newly developed method of the present invention, that a peak or a stage portion will appear in the polarization curve when the metallic material coated with the film which contains defects is polarized in a cathodic mode. As a result of our analysis, we have also found that there can exist a certain relationship between the existence of the peak or the stage portion and the corrosion form in the film defect portion Thus, according to the present invention we provide an electrochemical method and apparatus for detecting the corrosion form or structure in the film defect portion of the coated metallic plate material, either or both of which can be used in the corrosion evaluation.

As described earlier, as long as the corrosion preventing performance of the film is properly quantitatively treated, it is necessary electrochemically to measure the corrosion preventing performance of the film in corrosive gases including steam, in soil and in media such as crude oils but with the exception of electrolytes.

SUMMARY OF THE INVENTION Accordingly, a particular feature of the present invention is the provision of a corrosion evaluation testing method for coated metallic material which comprises detecting the electrochemical data significant to the corrosion evaluation of the coated metallic material through the application to the coated metallic material of a series of electrochemical testing steps developed according to the present invention, and an apparatus for automatically performing the detecting method.

Another important feature of the present invention is to provide a corrosion evaluation testing method for coated metallic material, and apparatus therefor of the above-described type, which achieve the electrochemical detection of the corrosion form in the film defect portion of a coted metallic material which is used in the corrosion evaluation.

A further feature of the present invention is to provide a corrosion evaluation testing method for coated metallic material and apparatus therefor of the above-described type, which make it possible electrochemically to measure the corrosion preventing performance of the film even in such media as corrosive gases, and various steams, soils and crude oils.

A still further feature of the present invention is to provide a corrosion evaluation testing method of coated metallic material and apparatus employed therefor of the above-described type, which overcome various disadvantages of the prior art specifically described above.

According to the present invention, there is provided a corrosion evaluation testing method for coated metallic material, which comprises the steps of: applying, upon at least one face of a given metallic plate, paint whose film performance is to be examined, and arranging to bring the paint into contact with a given corrosion medium; potentiostatically electrolysing the film side of the specimen plate at the resulting spontaneous electrode potential; polarizing the film side face of the specimen plate through pulse-polarization or equivalent means to detect whether or not the film face has defects;; detecting any infinitesimally small current-potential variation, with values correspondingly affected by the polarization due to a film resistance of the coated film having been compensated, when the film has no defects, and, on the condition being reversed, detecting an anodic and/or cathodic polarization curve; and bringing a non-film face on the reverse face of the specimen plate in relation to the film face of the plate into contact with an alkaline solution whether or not the specimen plate is a specimen for testing resistance such as sulfide stress cracking resistance or embrittlement resistance, whereby the electrolysis current resulting from ionization in the nonfilm face is detectable when an evaluation of the behaviour of the film in the said corrosion medium cannot be made or is not needed, the said medium being a solids, a gas or a liquid with the exception of an electrolyte.

For the sake of concisely summarizing the present invention, in order to distinguish between the electrolysis current recovering on the film-face side and the electrolysis current occurring on the non-film-face side, the former is designated as a polarization current whilst the latter is designated as an electrolysis current.

In the above method, when the pulse polarization method is adopted at the above-described second step, the pulse current flowing between the film face of the coated metallic plate and a counter electrode is used to measure the electrical resistance of the film, or, in short, to detect the water absorption or permeation rate of the film by the use of the above resistance. The use of pulse polarization or the linear potential sweep polarization makes it possible to detect whether or not the film has defects.

When the film has no defects, the polarization curve is rate-determined or governed by the electrical resistance specific to the film and becomes almost straightline. The value of the corrosion current can be obtained through the provision of the resultant extremely small amount of current-potential variation, wherein a value corresponding to the polarization due to the electric resistance specific to the film resistance has been subtracted from the above variation.

When the film has defects, the value of the corrosion current can be obtained from the cathodic and/or anodic polarization curve.

In particular, a peak or stage-portion may appear in the cathodic polarization curve. We have found that the variation in the value of the peak potential or the stage potential with respect to the immersion time corresponds to the variation in the rust width of the film defect portion, and the aging variation in the current value of the peak or the stage portion can be correlated with the aging variation in the corrosion current. Accordingly, detection of the peak potential and peak current of the peak or stage portion, or of their aging variations makes it possible to obtain electrochemical data such as the aging variation in the rust width and corrosion current, i.e., the corrosion amount with respect to the immersion time of the coated metallic material.

Also, according to the present invention, we have confirmed that the area relating to the peak or stage portion in the cathodic polarization curve corresponds to the rust width. Accordingly, the area or its aging variation is traced so as further to clarify the corrosion behaviour of the coated metallic material. The variational features of the cathodic polarization curves with respect to the immersion time can be roughly classified into two types. Therefore, the areas relating to the peak or the stage portion can be obtained by respective methods correspondingly suitable for the respective variational modes.

As regard apparatus for performing the method described above, the apparatus according to the present invention comprises: a pair of measuring cell vessels for seizing the specimen plate from both sides; a first potentiostat means for potentiostatically electrolysing the film side of the specimen plate at the spontaneous electrode potential; a device for detecting the existence of the film defects through pulse polarization or equivalent phenomena; a second potentiostat means for potentiostatically electrolysing the non-film side of the specimen plate at relative zero potential level with reference to the potential of a reference electrode; a control means for controlling the overall operation of the apparatus; and means for collecting, recording and calculating the input signals each coming from the detecting device to display on a displaying means the outputs necessary for the corrosion evaluation.

In particular, when the device for detecting the existence of the film defects further comprises a pulse potential applying means and a pulse polarization current detecting means, the signals from both means can output the electric resistance signal of the film, in short, the water permeation signal of the film, with the help of the means for collecting, recording and calculating (a processor means) when the above signals input into the latter means.

When the value of the corrosion current is obtained from the polarization curve or the extremely small amount of current-potential variation as described above, the output signal of the polarization current detecting means, or the output signals of a detecting means for an extremely small amount of current and of a linear potential applying means are input to the processor means to calculate the corrosion current.

In addition, when the peak potential or the current is required to locate where the peak or the stage portion occurs in the cathodic polarization curve, a peak detecting means is first connected both to the polarization current detecting means and to the control means. By this arrangement, upon receipt of a peak-existing signal, the peak detecting means actuates a specially provided peak potential and/or current detecting means, so as to input the detected peak potential and/or the current signal into the processor means. On the other hand, when the peak area is required to obtain, the peak area of the cathodic polarization curve which is being formed and has been formed within the processor means is required to be calculated in response to the peak-existing signal.

In addition to the peak detecting mens capable of differentiating the electrolysis current variation, for example, a peak area detecting means capable of integrating the electrolysis current variation and a time-counting means capable of counting the time after the measuring start may be further provided in the apparatus according to the present invention. Thus, the peak signal to be effected from the peak detecting means actuates the time counting means, through the control means, to store and to display the time duration of the time of the given peak signal.Furthermore, the signal described above also actuates the peak area detecting means, so that the electrolysis current variation between the given peak signals is first integrated and, then, the integrated value is stored and displayed so as to provide the desired measuring value of the electrolysis current values from the electrolysis current variation.

As regards the electrochemical method and apparatus for detecting the corrosion form in the film defect portion of the coated metallic material, the existence of the peak or the stage portion in the cathodic polarization curve of the coated metallic material having the defective film is first confirmed and, then, its related spontaneous electrode potential is compared with the spontaneous electrode potential of a substrate metal, thereby confirming whether it is cathodic or anodic in comparison with the substrate metal. Since the judgement of the corrosion form as described above is qualitative, the result described above and the information related to the corrosion current, for example, the corrosion volume, may be combined according to the present invention to provide a stereoscopic feature of the corrosion portion.Furthermore, the relationship between the existence of the peak or the stage portion and the corrosion form in the film defect portion region may become opposite, subject to the existence of a metal-plated layer or a corrosion prevention film around the cathode between the film and the substrate metallic surface, and the absence of the metal plating or the like. However, the existence of the metal-plated layer and the existence of the corrosion prevention film around the cathode may in turn be determined by the above-described comparison and/or the detection of the existence of the peak or the stage portion in the anodic polarization curve.

In addition, in order further to determine the information related to the rust width of the film defect portion (in the case of crevice corrosion) and the pitting depth, the potential and the current area of the peak are required to be detected when the peak exists in the cathodic polarization curve. The quantity of electricity relating to a second peak and its subsequent portion of the aging variation of the hydrogen ionized current, or the quantity of electricity relating to the third peak out of the aging variation of the hydrogen ionized current around the non-film face are arranged to obtain without any dependence upon the existence of the peaks, for the purpose described above.

As regards an apparatus embodying the method, the peak detecting means to be employed for the cathodic polarization curve, a first spontaneous electrode potential detecting means for detecting the spontaneous electrode potential of the coated metallic material and spontaneous electrode potential signal inputting means of the substrate metal are connected to the calculating means or the processor means, thereby to display the corrosion form with the display means.

As regards further advantages, the judgement of the existence of the metal-plated layer can be performed by the detection of the peak or the stage portion in the anodic polarization curve with the peak detecting means. Also, if the signal on the corrosion current is input to the processor means, the corrosion form can be stereoscopically displayed on the display means. Furthermore, if, as for the information on the rust width or the pitting depth, the peak potential, current and/or area signal to be effected by the use of the cathodic polarization curve and/or the quantity of electricity of the second peak and its subsequent portion relating to the electrolysis current variation or the quantity of electricity of the third peak are input to the processor means, the display of the corrosion-form becomes much more detailed.

As regards the measurement of the corrosion preventing performance of the film in a corrosive gas, steam, soil or crude oil except for an electrolyte, this object can be achieved by contacting an alkaline solution with the non-film face on the side opposite to the film side, to under the non-film face potentiostatically electrolysed, whereby the corrosion reaction on the film side can be estimated by obtaining the hydrogen-ionized electrolysis current information (the electrolysis current caused by the ionization of the hydrogen).

As will be clear from the above description, the present invention has already established a method of detecting electrochemical information significant to the corrosion evaluation of a coated metallic material, and apparatus-for automatically performing the latter detection method.

Thus, the present invention makes it possible quantitatively to treat the corrosion evaluation of a coated material.

BRIEF DESCRIPTION OF THE DRAWINGS Various features and advantages of the present invention will become apparent from the following description of various preferred embodiments thereof, with reference to the accompanying drawings, in which: Figure 1 is a block diagram of a corrosion evaluation testing apparatus according to the present invention, which is used to carry out a method of the present invention; Figure 2 is a schematic, cross-sectional view of paired measuring cells according to the present invention which are suitable for use as measuring cells in the apparatus of Fig. 1; Figure 3 is a schematic diagram illustrating the principle of the present invention according to which a film side of a coated metallic material is first potentiostatically electrolysed at the spontaneous electrode potential and is then polarized through the linear potential sweep method;; Figure 4 is a block diagram of an apparatus which is arranged to automatically perform an operation according to the apparatus principle shown in Fig. 3; Figure 5is a circuit diagram of the apparatus of Fig. 4; Figure 6 is a circuit diagram of a detection apparatus for detecting corrosion current and corrosion potential according to the present invention; Figure 7 is a graph illustrating a method of obtaining the corrosion current and corrosion potential according to the present invention, the method being significantly different from the method adopted for the embodiment of Fig. 6; Figure 8 is a principle-diagram of an apparatus for potentiostatically electrolyzing a non-film side of the coated metallic material at a spontaneous electrode potential;; Figure 9 is a circuit diagram of an apparatus which can automatically perform the principle shown in Fig. 8; Figures 10(a) and 10(b) are a practical operating flow chart of the apparatus of Fig. 1 while especially specifiying the relationship among data (1) to data (21); Figures 11(a) and 11(b) are a flow chart for stereoscopically drawing a corrosion sectional model diagram; Figure 12 is a chart illustrating respective pulse wave forms of a train of pulses which are used in the pulse polarization method according to the present invention; Figure 13 is a chart illustrating respective pulse wave forms of a train of current pulses which are obtained through the use of the pulse potential train of Fig. 12; Figure 14 is a graph showing the variation in the potential applied with respect to time, which is used in the linear potential sweep method;; Figure 15is a graph showing the variation in the polarization current correspondingly caused in accordance with the variation in the potential of Fig. 14; Figure 16 is a graph showing the variation in an extremely small amount of current versus potential (the principle for obtaining the real relationship between current and potential by eliminating the ohmic drop); Figure 1 7 is a graph showing the variation in a cathodic polarization curve obtained in the method according to the present invention; Figure 18 and Figure 19 are graphs each illustrating a method for obtaining the peak area of a cathodic polarization curve; Figure 20 is a partial view schematically illustrating a defect portion sectional model in a case where the corrosion form is "pitting";; Figure 21 is a partial view schematically illustrating a defect portion sectional model in a case where the corrosion form is "crevice"; Figure 22 is a diagrammatic view illustrating an embodiment of the method according to the present invention; Figure 23 is a graph showing cathodic polarization curves with an immersion time being chosen as the parameter according to EMBODIMENT 1 of the present invention; Figure 24 is a cross-sectional view schematically showing the corrosion form in the film defect portion (wherein dotted lines show a pitting and solid lines show the lateral expanse of a rust width or a crevice corrosion);; Figure 25 is a graph correlating among the aging variation in peak potential, the aging variation in rust width (X mark), and film peeled-off width (0 mark) with respect to the cathodic potential according to the EMBODIMENT 1 of Fig. 23; Figure 26 is a graph showing one (immersion time: 500 hours) of the cathodic polarization curves of specimen steel plates A and B according to EMBODIMENT 2 of the present invention; Figure 27 is a graph showing the aging variation in the peak potential with respect to the immersion time according to EMBODIMENT 2 of Fig. 26; Figure 28 is a graph showing the aging variation in the rust width with respect to the immersion time according to EMBODIMENT 2 of Fkg. 26; Figure 29 is a graph showing one (immersion time: 100 hours) of the cathodic polarization curves of specimen steel pipes C and D according to an EMBODIMENT 3 of the present invention; Figure 30 is a graph showing the aging variation in the corrosion current in EMBODIMENT 3 of Fig. 29; Figure 31 is a graph showing the aging variation in the peak current in EMBODIMENT 3 of Fig. 29; Figure 32 is a graph showing the aging variation in the rust width in EMBODIMENT 3 of Fig.

29; Figure 33 is a graph showing one (immersion time: 20 hours) of the cathodic polarization curves of specimen plates E, F and F' according to EMBODIMENT 4 of the present invention; Figure 34 is a graph showing the aging variation in the rust width in EMBODIMENT 4 of Fig.

33; Figure 35 is a graph showing the aging variation in the peak potential in EMBODIMENT 4 of Fig. 33; Figures 36(a) and 36(b) are graphs each showing one (immersion time: 250 hours) of the cathodic and anodic polarization curves of specimen plates G and H according to EMBODIMENT 5 of the present invention; Figure 37 is a graph showing the aging variation of the rust width in EMBODIMENT 5 of Figs. 36(a) and 36(b); Figure 38 is a graph showing the aging variation of peak potential in EMBODIMENT 5 of Figs. 36(a) and 36(b); Figure 39 is a graph showing the cathodic polarization curve of specimen plate H according to EMBODIMENT 6 of the present invention, while the immersion time is chosen as the parameter;; Figure 40 is a graph showing the cathodic polarization curve of a specimen plate I according to EMBODIMENT 6 of the present invention, while the immersion time is chosen as the parameter; Figure 41 is a graph showing an anodic polarization curve of specimen plate I in EMBODIMENT 6 of Fig. 39; Figure 42 is a view for schematically illustrating a normal clearance corrosion form; Figure 43 is a view illustrating the corrosion form according to the present invention; Figures 44 and 45 are graphs each showing the aging variation in an electrolysis current measured with respect to the immersion time according to the present invention; Figures 46, 47 and 48 are graphs each showing the comparison between filiform corrosion length and hydrogen concentration melted in a steel substrate due to corrosion which occurs on the film face side; the corrosion width of the crevice corrosion and the above-described hydrogen concentration; and the depth of the pitting and the above-described hydrogen concentration; Figure 49 is a graph showing the aging variation in the electrolysis current measure with respect to the immersion time according to the present invention; and Figure 50 is a block diagram of an apparatus for detecting the existence of the peak in the electrolysis current variation and the area of the peak.

Before the description of the present invention proceeds, it is to be noted that like parts are designated by like reference numerials throughout several view of the accompanying drawings, unless otherwise denoted.

DETAILED DESCRIPTION OF THE INVENTION Fig. 1 is a block diagram of an apparatus according to the present invention, which confirms the corrosion state of a film coated metal. A film coated metallic plate W is firmly held between two measuring cells 1 and 1'. One face of the metallic plate W is coated with paint, while the other face is left uncoated. A corrosive medium such as an NaCI solution of 3 wt% is placed in the left-hand measuring cell 1, so as to be in contact with the film face side of the coated metallic plate W. On the other hand, for example, an alkaline solution which is low in hydrogen ion concentration is placed in the right-hand side measuring cell 1', so as to be in contact with the non-film side of the metallic plate.

To obtain a polarization curve through a linear potential sweep method after the film side has been potentiostaticaliy electrolysed at the spontaneous electrode potential, a reference electrode R and a counter-electrode C are immersed in the measuring cell 1. The electrodes thus arranged together with the coated metallic plate W are connected with a potentiostat 2, respectively, so as to form a film-side measuring system (hereinafter referred to as an anodic reaction measuring system). On the other hand, to measure the electrolysis current which is obtained by electrolysis of the non-film side at a hydrogen ionization electrode potential after the non-film side has been potentiostatically electrolysed at the spontaneous electrode potential, a reference electrode R' and a counter-electrode C' are immersed in the measuring cell 1'. The electrodes thus arranged and the coated metallic plate W are respectively connected with a potentiostat 2', thereby to form a non-film side measuring system (hereinafter referred to as a cathodic reaction measuring system).

In the anodic reaction measuring system, a spontaneous electrode potential detecting circuit 3 is connected with the potentiostat 2. A signal which is of opposite polarity but of the same absolute value as a potential signal output from the circuit 3 is transmitted through an auto-set circuit of the spontaneous electrode potential 4 to the potentiostat 2, so that the coated metallic plate W is potentiostatically electrolysed at the spontaneous electrode potential by the electrostat 2.

Also, a pulse potential generating circuit 5 is connected with the potentiostat 2 to obtain data on the electric resistance of the film for use in detection of the film defects through application of the pulse potential to the coated metallic plate W after the potentiostatic electrolysis of the plate W at the spontaneous electrode potential. In addition, a linear potential applying circuit 6 is connected with the potentiostat 2 to polarize the coated metallic plate W at a given potential sweep rate.

Pulse currents from the potentiostat 2 are input to a pulse current storage circuit 8 through an electrolysis current detecting circuit 7, the pulse currents being adapted to flow between the counter-electrode and the coated metallic plate W when the coated metallic plate W is pulsepolarized by a train of pulse potentials from the pulse potential generating circuit 5. When alternative, cathodic and anodic pulse potentials each having the same absolute value are applied to the coated metallic plate W, the signals of the pulse currents are also input to a pulse current value comparing circuit 9 to determine whether or not the film face of the coated metallic plate W is damaged.Hence, film defect detecting signals are output from the comparing circuit 9, wherein no defects are determined to exist either when the absolute values of pulse currents correspond to the anodic and cathodic pulse currents themselves or when the values of the electric resistance of the film calculated therefrom stay in the range of a relative difference of 20% or less. However, the defects are determined to exist when the calculated values of the electric resistance of the film are above 20%. According to a linear potential sweep method which is described hereinafter in detail, no defects are determined to exist when the relationship (E vs, i) is approximately linear, while defects are determined to exist when the (E vs. i) relationship has significantly deviated from a linear relationship (in i = aE, a is + 10% or more).However, according to the pulse polarization method of the present invention, the electric resistance of the film and the permeation rate of water relative to the film can both be detected.

Accordingly, a detecting circuit 10 in respect of the electric resistance of the film and a detecting circuit 11 in respect of the permeation rate of water are both provided.

Also, polarization currents from the potentiostat 2, owing to the potential sweep concerning the coated metallic plate W, are input to a peak detecting circuit 1 2 through the electrolysis current detecting circuit 7 on the one hand, and they are input to an (E vs. i) storage circuit 1 3 and/or an (E vs. log i) storage circuit 1 4 on the other hand. The peak detection circuit 1 2 may be composed of a differentiation circuit.Upon detection of a peak or stage portion in the polarization curve through the peak detection circuit 12, the peak potential and/or the current at this time is detected by a detection circuit 1 5. Furthermore, the signal in response to the peak from the peak detection circuit 1 2 is sent to a peak area detection circuit 1 6 to actuate the circuit 16, thereby to detect the peak area from the (E vs. log i) polarization curve, which the (E vs. log i) storage circuit 1 4 stores. Corrosion currents and corrosion potentials are both detected from the (E vs. log i) polarization curve by a detection circuit 1 7.

A polarization resistance (or the electric resistance of a paint film) is detected from the (E vs. i) polarization curve stored in the (E vs. i) storage circuit 1 3 by a detection circuit 26 in response to an output signal from the detection circuit 10 for the electric resistance of the film.

Lastly, a potential signal from the electrode potential detection circuit 3 is stored once in a spontaneous electrode potential storage circuit 18, and is compared with the spontaneous electrode potential of a substrate metal such as iron by a comparing circuit 19, whereby the -comparing signal is output.

In the cathodic reaction measuring system, an electrolysis current detection circuit 7' is connected with the potentiostat 2'. A hydrogen electrolysis current signal is input to a storage circuit 21 through the circuit 7'. On the other hand, the signal is input to a second peak detection circuit 22 to detect the existence of the second peak in accordance with the aging variation of the electrolysis current, i.e., the quantity of electricity of the second peak.

Furthermore, the electrolysis current signal is input to a third peak detection circuit 23 to detect the rising time of the third peak, i.e. the quantity of electricity of the third peak.

Each of the detection signals is input once to a processor circuit 20 and is sent to a display circuit 25 through an operation circuit 24 so as collectively to display the corrosion condition, including the corrosion form, of the coated metallic material W, which is the specimen to be examined.

Each of the circuit means described above are now described in greater detail. So far as the cell for the polarization measurement is concerned, it is preferable to use measuring cells 1 and 1' each being the construction shown in Fig. 2. The cells 1 and 1' are composed of a left-hand cell for the polarization measurement is concerned, it is preferable to use measuring cells 1 and 1' each being the construction shown in Fig. 2. The cells 1 and 1' are composed of a left-hand cell chamber 101 and a right-hand cell chamber 102 of the same configuration as the cell A of the same area as that of the measuring face. The rear side (the right-hand side in Fig. 2) of the specimen W is also protected with an elastic insulating plate 104, which has a punched opening B of the same material and size as the measuring face, or a punched opening larger in area than the measuring area.The respective exterior sides of the elastic insulating plates 103 and 104 are pressed from both sides with a flange portion 101 a provided for the left-hand cell chamber 101 and a flange portion 1 02a provided for the right-hand cell chamber 102, the flange portions 101 a and 1 02a being respectively grasped by a grasping means (not shown), so that both of the chambers 101 and 102 are fixedly assembled as shown in Fig. 2.

The left-hand side cell chamber 101 has a first trough-like tube 105 and a second trough-like tube 106 on its upper portion side face. The first tube 105 allows the reference electrode R to be set in the chamber. The reference electrode R has to indicate the stable electrode potential, and may be, e.g., a calomel electrode or a silver-silver chloride electrode. The second tube 106 allows the counter-electrode C of platinum, silver carbon or the like to be inserted into the chamber 101. It will be understood that the counter-electrode C together with the reference electrode R can be installed in the first tube 105 by omitting the second tube 106 as shown in the right-hand cell chamber 102. No influences are applied to the measured value even if the reference electrode R' and the counter-electrode C' are made to approach each other.In addition, a helically formed heating tube 107 is provided in the chamber 101. A given amount of warm water is continuously fed into the heating tube 107 from an entrance 107a located on one end face, and is discharged out of an exit 107b, so that an electrolyte contained in the chamber 101 may be kept in a predetermined temperature condition.

Since the cell chamber 102 on the right-hand side is approximately the same in construction as the cell chamber 101 on the left-hand side, like parts are designated by the same reference numerals.

NaCI solution of a predetermined concentration is placed inside the left-hand side cell chamber 101, while NaOH solution of a predetermined concentration is placed inside the righthand side cell chamber 102. In the left-hand side measuring cell 1, the electrolysis which confirms the polarization behaviour of the film side or coated side of the coated metallic plate W is performed. In the right-hand side measuring cell 102, the hydrogen electrolysis of the nonfilm face side is performed.

The advantages of using the present measuring cells 101 and 102 include the following: (1) the measuring area or expoed area can be precisely and easily set by the determination of the area of the punched opening A, and the measured values can be correctly compared with respect to each other; (2) the solutions which fill the measuring cells 101 and 102 can be made different in temperature to perform accelerated tests under a thermal gradient condition; (3) since the electrolytic cell 101 and the electrolytic cell 102 are separated by elastic insulating materials 103 and 104, the cathodic reaction and the anodic reaction with respect to the coated metallic plate W can be traced with a compact unit.

(4) since an alkali solution such as an NaOH solution or the like fills the measuring cell 102, the behaviour of the electrolysis on the non-coated face does not have any influences upon the corrosion behaviour on the coated face side; and (5) the solutions in the measuring cells 101 and 102 are not electrically short-circuited therebetween.

To polarize the coated face side on the coated metallic plate W through the linear potential sweep method after the potentiostatic electrolysis of the coated face side has been performed at the spontaneous electrode potential, an apparatus based on such a principle as that shown in Fig. 3 is used.

The reference electrode R is connected in series first to a battery V of variable potential and then to an amplifier AMP. The output side of the amplifier AMP is connected to the counterelectrode C through an ammeter A. On the other hand, the output side of the ammeter A is connected to a means B for reading the changes in current, for example, a recording meter, included in a device such as a computer or a logarithmic transducer, for example. Also, to obtain the potential difference between the reference electrode R and the coated metallic plate W, a voltmeter E is connected between the reference electrode R and an electrical earthing member. The output side of the voltmeter E is connected to a recording meter B' although it may be connected to the means B to illustrate the variation in the logarithmic value (V vs. log i) of the current with respect to the potential variation.

Upon polarization of the coated metallic plate W, the potential difference (Eo) between the reference electrode R and the coated metallic plate W is first measured to adjust the output of the battery V to the - Eo potential of opposite polarity at the same value as that of the Eo potential, thereby to set, to approximately zero, the potential difference between the inputs (a-b) of the amplifier AMP. Since the output of the AMP becomes approximately zero through this adjustment, no current flows to the counter-electrode C. On the other hand, currents hardly flow even to the coated metallic plate W. Thus, it can be said to be in a condition where the potentiostatic electrolysis is performed at the spontaneous electrode potential.

After the spontaneous electrode potential has been set as described above, the potential variable battery V is adjusted. The coated metallic plate W is polarized in accordance with the changes in potential with a given potential sweep rate [(potential variation)Av/(time variation)t] to provide the polarization curve of the coated metallic plate W, in particular the cathodic polarization curve. Owing to the potentiostatic electrolysis of the coated metallic plate W at the spontaneous electrode potential as described above, the measuring error current relating to the polarizing current which is otherwise involved as a result of the difference in spontaneous electrode potentials between the coated metallic plate W and the counter-electrode C can be prevented from flowing.When the film defects exist, the polarization currents relating to the defect regions are larger than that which relates to the other film portion. Therefore, the polarization current in the other film portion can be eliminated from the measured value.

Figs. 4 and 5 show an apparatus based upon the measuring principle shown in Fig. 3 for automatically performing the polarizing operation of the coated metallic plate W with the slight film defects, after the plate W has been potentiostatically electrolysed at the spontaneous electrode potential.

A means 202 for measuring electrode potentials comprises a high impedance operational amplifier 203 (see Fig. 5) and a voltmeter 216 (see Fig. 5). The high impedance operational amplifier 203 has an input resistance (10'452and above) which is sufficiently higher than a film resistance (the resistance to be effected between the coated metallic plate W and the reference electrode R). The voltmeter 216 (see Fig. 5) is connected to the reference electrode R through the high impedance operational amplifier 203 at its one end, and is connected to the electrical earthing member at its other end.Due to the arrangement described above, the electrolysis current caused by the application of the electrical potential is caused not to be input to the means 202 in any way, thereby to correctly measure the variation of the spontaneous electrode potential of the coated metallic plate W.

An auto-set circuit of electrode potential 204 for setting, to an approximate zero condition, the currents caused by the differences in the electrode potentials between the electrode C and the plate W, so as to cause the plate W to be electrolysed at the spontaneous electrode potentials in the corrosive solution, comprises a servomechanism 208, a first differential amplifier 205 and a means 206 for imparting a compensation signal. More specifically, the servomechanism 208 is connected in parallel to the first differential amplifier 205 (see Fig. 4), while being arranged to be connected in series to the amplifier 203 (see Fig. 5), and the output voltage of the means 206 (see Fig. 5) is set with the help of the servomechanism 208. Further details are given below with reference Fig. 5.The first differential operational amplifier 205 is connected, on its input side, to the high impedance operational amplifier 203 and a potentiometer P1 of the means 206, and, on its output side, to the counter-electrode C so as to operate to cancel the output (potential signal) from the amplifier 203 and the output (compensation signal) from the means 206 to approximately zero the error currents effected by the differences in the electrode potentials between the coated metallic plate W and the counter-electrode C.

The means 206 (see Figs. 4 and 5) for imparting a compensation signal comprises the potentiometer P1, a DC power supply source El and distributed resistors R9, R10, these being connected in parallel with respect to each other and grounded through a normally open contact L6 from a branch point of the distributed resistors R9 and R10. The means 206 is connected on its output side through the potentiometer P1 to the differential operational amplifier 205, and is grounded through a normally closed contact L7. In the case where the contact L7 is in its ON mode (closed mode), the output voltage (compensation signai) is adapted not to be input to the differential operational amplifier 205.In the case where the contact L7 is in its OFF mode (open mode), with the normally open contact L6 being simultaneously in its ON mode (closed mode), the compensation signal is adapted to be input to the differential operational amplifier 205.

Furthermore, the servomechanism 208 comprises a comparator 209, a servomotor 210 and a means 211 for imparting a comparison signal, constituting a negative feedback circuit. Thus, the comparator 209 is a servo amplifier, which is connected on its input side to the high impedance operational amplifier 203 together with the means 211 for imparting a comparison signal, while it is connected on its output side to the servomotor 210 through a normally open contact LO (see Fig. 5). The comparator 209 compares the output from the high impedance operational amplifier 203 (potential signal) is compared with the output (comparsion signal) from the means 211, thereby to input a signal which is a difference signal to the servomotor 210 and to drive the servomotor 210.

The means 211 (see Figs. 4 and 5) for imparting a comparison signal compriss a potentiometer P2, a DC power supply source E2 and distributed resistors R7, R8, which are connected in parallel. A branch point of the distributed resistors R7 and R8 is earthed. The means 211 for imparting a comparison signal is connected through the potentiometer P2 to the comparator 209 so as to input comparison signals. In addition, the servomotor 210 is adapted simultaneously to displace the potentiometer P2 of the means 211 for imparting a comparison signal, and the potentiometer P1 of the means 206 for imparting a comparison signal. The servomotor 210 is driven by means of the output of the comparator 209 to set a compensation signal capable of cancelling the potential signal described above with the potential signal itself.

The normally open contact LO is switched over from ON to OFF to cut off the input from the comparator 209 to the servomotor 210, thereby to retain the established compensation signal.

The above-described measuring preparation is performed under the above-described construction. Then, a power supply source 207 for applying a low DC voltage of cathodic or anodic potential gradient between the coated metallic plate W and the reference electrode R comprises a potentiometer P3, a DC power supply source E3 and distributed resistors R1 1, R12, which are connected in parallel. The potentiometer P3 on the output side is connected to the branch point of the distributed resistors R9, R10 included in the means 206 for imparting a comparison signal through a normally open contact L8. A branch point of the distributed resistors R1 1, R12 is earthed.Accordingly, the low DC voltage of the positive or negative potential gradient is output from the power supply source 207 by the displacement of the potentiometer P3 to achieve a superposition upon the compensation signal, thereby to input to the first differential operational amplifier 205; the output potential between the coated metallic plate W and the reference electrode R is applied at the counter-electrode C.

Figs. 4 and 5 further show an apparatus 212 (see Fig. 4), which measures a very small amount of polarization currents flowing through the film defect portion, includes a second differential operational amplifier 217 and a means 218 (see Fig. 5) for imparting a correction signal. More specifically, the second differential operational amplifier 217 is connected in parallel to the variable resistor R7. The input side thereof is connected through an amplifier 214 to the coated metallic plate and the output side thereof is grounded through an ammeter 220.

Fig. 5 shows a multistage parallel resistor 21 3 wherein R1 to R4, respectively connected in series with normally open contacts L1 to L4, with R5 being connected in series with a normally closed contact L5, are collectively connected in parallel. Power drops are available through the change-over switching operation of the contacts.

The means 218 for imparting a correction signal comprises a potentiometer P4, a DC power supply source E4 and distributed resistors R13, R14, these being in parallel, and is connected through the potentiometer P4 on the output side to the second differential operational unit 217.

A branch point of the distributed resistors R13 and R14 is earthed.

The potentiometer P3 of the power supply source 207 and the potentiometer P4 of the means 218 for imparting a correction signal are adapted to be simultaneously displaced at a given speed by a synchronous motor 219. As described hereinbelow, when the synchronous motor 21 9 is driven so that the low DC voltage of positive or negative potential gradient is applied between the counter-electrode C and the reference electrode R, the corresponding variations of the electrolysis currents specific to the signal of the external polarization of the filmed metallic plate W receive the proper voltage drop by the multistage resistor 21 3 to input to the amplifier 214.Hence, it is amplified and input, as the measuring signal, to the second differential operating unit 21 7. On the other hand, in the means 218 for imparting a correction signal, the output voltage of the DC power supply source E4 varies with the same gradient as that of the DC low voltage applied as described above due to the displacement of the potentiometer P4 synchronized with the displacement of the potentiometer P3. The potential output from E4 is input to the second differential operational unit 217 as a correction signal, whereby a difference signal is output to the ammeter 220 to compare the currents flowing over the film portion, thereby to measure the current signal flowing only into the defect portion of the film.Also, when the DC power supply source E4 is arranged to keep the condition such that the current-potential gradient specific to the electric resistance of the film resistance, prior to the film being damaged, may be shown as the correction signal by the displacement of the potentiometer P4, the external polarization current difference before and after the film being damaged can be detected. Also, in the above-described apparatus, the current value showing the electric resistance specific to the film resistance is adapted to be compensated.However, when the currents flowing over the film defect portion are sufficiently larger than the currents flowing into the film portion (for example, approximately 100 times can be neglected); the measurement can be directly made through the second differential operational amplifier 217 by an ammeter 215, which is connected at its one end to the amplifier 214 and is grounded at its other end, through opening a contact L10 without obtaining the external polarization currents of the film defect portion.

The operational characteristics of the polarization current measuring apparatus, of the film defect portion, under the above-described construction will be in the sequence now described.

The contacts L5 and L7 (see Fig. 5) are closed and the contacts except for L5 and L7 are opened. The spontaneous electrode potential of the coated metallic plate W is measured by the spontaneous electrode potential measuring means 2.

More particularly, the electrode potential (spontaneous electrode potential) with respect to the reference electrode R of the coated metallic plate W is output to the voltmeter 216 through the high impedance operational amplifier 203, so that the variation of the spontaneous electrode p6atiil is recorded.

Successively, the contacts L6 and LO are closed, thereby to operate the auto-set circuit 204 of the electrode potential (Fig. 4), so that the current caused by the difference of the electrode potential between the coated metallic plate W and the counter-electrode C is then set to approximate zero. Thus, the output (potential signal) of the high impedance operational amplifier 203 is input to the first operational unit 205 and to the comparator 209. The comparison signal which supplies a voltage - Vs cancelling the potential signal is, first, input to the comparator 209. The compensating signal - Vs is kept inputted even to the first operational unit 205. No output is provided from the comparator 209. The output of the first operational unit 205 does not exist with the servomotor 210 being in an OFF mode.

Let us assume that the coated metallic plate W has been corroded and that the electrode potential has risen more than Vs by dV. Such being the case, the potential signal (Vs+Al') is input to the first operational unit 205 and the comparator 209. The difference portion AV with respect to the comparison signal outputs from the comparator 209 as well as from the first operational unit 205. However, the signal dVfrom the comparator 209 is input to the servomotor 210. The servomotor 210 is driven to displace the potentiometers P1 and P2 to output the comparison signal of - (Vs + AV) from the means 211 for imparting a comparison signal, and the comparison signal is input to the comparator 209 so as to zero the output.Upon suspension of the driving operation of the servomotor 210, the compensation signal of the - (Vs + dV) is output from the means for imparting a compensation signal and is then input to the operational unit 205 to cancel the input of the potential signal, whereby the output of the operational unit 205 becomes zero.

Accordingly, the current (error current), which tends to flow between the coated metallic plate W and the counter electrode C is set approximately to zero. This condition is considered to be that in which the coated metallic plate W is electrolysed at the spontaneous electrode potential of the coated metallic plate according to the present invention.

After the measuring preparation has been finished as described above, the contacts LO and L6 are both opened, with the contact L8 being closed. Thus, the potentiometer P3 of the power supply source 207 is displaced by the synchronous motor 219 and then the potential applied is varied, with the cathodic or anodic gradient of the potential being accompanied. Hence, the current flowing between the coated metallic plate W and the counter electrode C is input to the second operational unit 217, and the correction signal is input to the second operational unit 217 from the means 218 provided with the potentiometer P4, which is simultaneously displaced by the synchronous motor 219. The difference in current di between the measuring signal i and the compensation signal is successively output and then amplified so as to be measured by the ammeter 220.

When the contact LO is open, the connection between the comparator 209 and the servomotor 210 is cut off. Hence, the current caused by the DC voltage power source is not input to the servomotor 210, whereby the voltage of the compensation signal is retained and prevents deviations from the electrolysing condition at the spontaneous electrode potential.

When the contact L6 is then opened and the contact L8 is closed, a predetermined potential is applied between the counter electrode C and the reference electrode R.

Thereafter, in accordance with the drive of the synchronous motor 21 9 and the displacement of the potentiometer P3 in the direction of A in Fig. 5, the application of potential increases with the positive gradient of the potential, whereby the current signal of - i of an anodic external polarization curve of the coated metallic plate W is adapted to be input to the second operational unit 217. On the other hand, when the potentiometer P3 is displaced in the direction of B in Fig. 5, the application potential decreases with the negative gradient of the potential, so that the current signal of + i of the cathodic external polarization curve of the coated metallic plate W is input to the second operational unit 217.

The synchronous motor 21 9 is driven so as to cause the potentiometer P4 of the means 21 8 for imparting a correction signal to be displaced in the A' direction in synchronization with the displacement of the potentiometer P3 in the A direction. On the contrary, the potentiometer P4 is displaced in the B' direction in synchronization with the displacement of the potentiometer P3 in the B direction. The DC power supply source E4 is set so that the currents (correction signal) of the current-potential with a predetermined gradient, which are obtained through the application of the potential variation from the application voltage suppiying means upon the premeasured film resistance, may be supplied by the displacement of the potentiometer P4.

Accordingly, a correction signal is input to the secondary operational unit 217 from the means 218 for imparting a correction signal. The difference current Ai, in which a correction signal (the correction signal is input to the second operational unit 21 7 as the voltage signal, and the film resistance value is converted to a current value within the unit 217) corresponding to the film resistance from the current signal + i of the anodic or cathodic external polarization curve is subtracted, is output from the second operational unit 21 7 and is measured by the ammeter 220.When the current signal Ai is recorded and correlated with the electrode potential of the coated metallic plate W, the intended polarization curve relating to the defect portion of the very small amount of film defects of the coated metallic plate W is provided.

According to the present invention, the above-described apparatus is also provided with the function of measuring the electric resistance of the film of the coated metallic plate W. After the operation of the potentiostatic electrolysis (measuring preparation) at the spontaneous electrode potential, the electric resistance of the film is obtained by the pulse polarization method with a sufficiently correct measured value being guaranteed even if the voltage is made low enough not to disturb the measuring system.

Thus, as seen from Figs. 4 and 5, after the measuring preparation as described above, a contact L9 is closed with the contacts LO and L6 being simultaneously opened so as to supply the pulse signal from a pulse power supply source 207a to measure the electric resistance of the film by the electrolysis current measuring means 212 (see Fig. 4).

The contact LO is first opened and the connection between the comparator 209 and the servomotor 210 is broken, thereby preventing the pulse signal from being input to the servomotor 210 and its resultant error operation (namely, when the pulse signal is input to the servomotor 210, such a compensation signal as to cancel the pulse signal is adapted to be output, with the result that the electrolysis current as the signal of the electric resistance of the film is not available), and thereafter output the pulse signal VP from the pulse power supply source 207a, wherein the pulse signal VP is superimposed upon the compensation signal by the means 206 for imparting a compensation signal and is then input to the operational unit 205.

However, since the compensation signal is cancelled by the potential signal described above, only the pulse signal is output from the operational unit 205 and is input to the counterelectrode C.

Such being the case, the voltage of VP is applied between the reference electrode R and the counter-electrode C. The electrolysis current signal flowing between the counter-electrode C and the coated metallic plate W, which is a current corresponding to the voltage of this VP properly dropped by the multistage resistor 213, is amplified by the amplifier 21 4 and is measured by the ammeter 215 so as to be recorded. Since the potential of this pulse signal is set sufficiently small when compared with the spontaneous electrode potential of the coated metallic plate, no influences are applied to the spontaneous electrode potential. Therefore, the spontaneous electrode potential of the coated metallic plate can be measured under the state in which the pulse signals are also being continuously applied accordingly in an intermittent application mode.Furthermore, the pulse signals may be combined with those each having an opposite polarity or the application wave of the potential may take stagewise forms. Thus, for example, the DC resistance difference which is caused by the difference in the direction of the current flowing in the coated metallic plate can be easily detected.

Also, in the above-described contact switching operation, the contacts L5 and L7 are first opened from an instant to to an instant t, to measure the electrode potential of the coated metallic plate W. Then, from the instant t1 to an instant t2, the contacts LO, L5 and L6 are closed to set the error currents, which flow due to the difference in the electrode potential between the coated metallic plate W and the counter-electrode C, to an approximate value of zero. In this way, the measuring preparation of the electric resistance of the film for electrolysing the coated metallic plate at the spontaneous electrode is successively performed. The contacts LO and L6 are then opened from the instant t2 and an instant t5, with the contact L9 being simultaneously closed.The pulse potential of + Up is applied from the instant t2 to an instant t3, and the pulse potential of - Up is applied from an instant t4 to the instant t5 to measure the electrolysis currents 4 and ip flowing between the coated metallic plate W and counterelectrode C. When this operation is compiled for a programme and then is repeatedly performed through the programme, in addition to the spontaneous electrode potential, the electric resistance of the film can be automatically measured for a long period while the electrolysis operation is being performed at the spontaneous electrode potential.

The detecting circuit 1 7 of corrosion currents and corrosion potentials may be constructed as shown in Fig. 6.

Fig. 6 shows a measuring cell 1. The cell 1 contains a specimen plate W, which has been coated by a polymer or resin film containing an inorganic or organic substance on its surface, the polymer or resin film having been partially damaged with a sharp blade; a reference electrode R, which has a stable electrode potential, e.g. a calomel electrode or a silver-silver chloride electrode; and a counter-electrode C composed of an uncoated metallic plate e.g. of platinum, for example all immersed in a corrosion solution such as a 3 to 5 per cent solution of a salt.

A means 302, which electrolyses the specimen plate W at the spontaneous electrode potential while setting to zero the error currents, which lead to measuring errors, substantially comprises an operation amplifier 303. As shown in Fig. 6, a DC power supply source El grounded at its one end through a switch SW1 is connected to the input side of the operational amplifier 303 through a potentiometer P1. The reference electrode R is also connected to the input side. The counter-electrode C is connected through a proper variable resistor Ro to the output side.By the arrangement described above, the voltage of a polarity opposite to a potential (the electrode potential difference of the specimen W with reference to the reference electrode R) indicated by a voltmeter 304 grounded at its one end is input to the operational amplifier 303 through adjustment of the output voltage of the power supply source El by the potentiometer P1 thereby to zero the output, so that the very small amount of polarization currents during application of the low-level potential can be correctly measured.

A means 306 is provided for applying to the coated specimen plate W an excessive potential, which changes with a linear trend, thereby to polarize the specimen. The means 306 comprises a DC power supply source E3, a potentiometer P3, a switch SW3 and an integration circuit 307. One end of the DC power supply source E3, which is earthed at its other end, is connected to the integration circuit 307 through the potentiometer P3 and the switch SW3. By the above arrangement, an output voltage which is gradually increased by the integration circuit 307 is adapted to be added to the output voltage of the DC power supply source El through a connection to a contact (c) by the change-over operation of the switch SW1.

A means 308 is provided for applying a slight excess potential ranging from 0 to 10 mV to the coated metallic plate W, wherein a DC power supply source E4 is connected to a contact (b) through a potentiometer P4 and, thus, the slightly excessive potential or voltage described above is applied to the coated metallic plate W through a change-over operation of the switch SW1. Hence, the predetermination of the degree of the proper current amplification in accordance with the maximum polarization current during the measurement of the corrosion current is arranged to be easily accomplished.Thus, when the polarization current of approximately 100 times is adapted to flow upon the measurement of the polarization current during application of 10 mV, it is observed that almost all the specimens W are polarized to a range sufficient to obtain a Tafel slope, whereby the polarization current which becomes a reference for obtaining the maximum polarization current value is provided.

A comparator circuit 309 is connected to the output side of a integration circuit 307 so that the application voltage may be input to the comparator circuit. Another input of 10 mV is input thereto as the comparison potential. When the difference between these inputs has become zero, the signal carrying an information of this comparing result is input to a switch SW4 and a holding circuit 311 which is connected, through the switch SW4 and a multistage current amplifier 310, to the specimen plate W. Therefore, when the specimen plate W is polarized at the potential of 10 mV, the switch SW4 is opened, while the output voltage of the current amplifier 310 at this time is adapted to be retained in the holding circuit 311.

As is shown in Fig. 6, the specimen W is earthed. Five comparator circuits 312 to 316 are connected in parallel with respect to each other between the counter-electrode C and the variable resistor Ro. A comparison-reference power supply source E5 is provided as another input in each of the comparator circuits 312 to 316, and is connected to each of the split points of a potentiometer P5. The comparison-reference power supply source E5 corresponds in capacity to the voltage variation in IR drop of the variable resistor Ro caused by the polarization current during the final application of a potential application means 308.More particularly, when the IR drop is E(V) and the output voltage of the reference power supply source E5 is then set in E(V), respective potentials having each the logarithmically equally divided distance, i.e., 1OE(V)/100 for the comparator circuit 312, 17. (V)/100 for the comparator circuit 313, 31 .6E (V)/ 100 for the comparator circuit 314, 56. 3E (V)/ 1 00 for the comparator circuit 31 5.

and 100E (V)/100 for the comparator circuit 316, are input with the help of the potentiometer P5. Thus, when the IR drop of the resistor Ro caused by the polarization current coincides with another input voltage, is signal is adapted to be imparted. At this time, the input potential to each of the comparator circuits 312 to 316 is fixed. The potential drop by the preferable maximum, polarization current established by the specimen W is to set the resistance value of the variable resistance Ro so that the potential drop may coincide with the output voltage of the reference power supply source E5.

Each output-side of the comparator circuits 312 to 316 is connected to each input-side of holding circuits 318 to 322, while respective input-sides of the holding circuits are each connected to the counter electrode R. The coincident signal of each of the comparator circuits is sequentially input. The electrode potential difference between the reference electrode R and the specimen plate W at that time is adapted to be sequentially stored and retained in each of the holding circuits 318 to 322.

Meanwhile, each output-side of the holding circuits 311 and 31 8 to 322 is connected to a digital panel meter 324 through a gate circuit 323 to indicate the voltage in each of the measuring points.

A case in which the corrosion currents of each of various specimen plates are measured by the use of the corrosion current measuring apparatus of the above-described character is now described.

REFERENCE EXAMPLE 1 As the specimen plate, a coated plate was used, which was a piece of steel plate with dimensions of 7 X 14 x 0.08 cm coated with an electrodeposition paint for use in undercoating the bodies of general purpose vehicles. The steel plate which was coated was a piece of polished steel plate (Japanese Industrial Standards, G.3141). The electrodeposition paint was substantially composed of a malein oil with a solids content of 10 per cent and had a pH-value of 8. The steel plate was dipped in the paint at 30"C with DC 200 V being applied for three minutes. After the coating operation, it was washed with water. Subsequently, it was dried at a temperature of 170"C for thirty minutes to harden the paint, whereby the coated steel plate was made. The film thickness was 25 y.

Successively, the steel plate was injured with a sharp knife or blade to such an extent that the injury reached from the film surface of the coated steel plate to the steel plate itself. The injury was a straight line 10 mm long and 3.2 X 10-2 cm2 in area (where the steel was in contact with the liquid) as determined by microscopic measurement. The coated surface was sealed with a water-proof tape with the exception of a circle of 2 cm in radius which, however, included the injury therein, so that the entry of water into the film was prevented (to omit influences on the measurement which might be caused by other injuries, the ends of the steel plate, and the reverse face of the steel plate). The plate thus treated was dipped in the measuring liquid.The measuring liquid employed was an NaCI solution of 3 per cent and was at a temperature of 50"C. Platinum as the counter-electrode and a saturation calomel electrode (S.C.E.) as the reference electrode were also dipped in the measuring liquid at the same time. The injured coated steel plate was linearly subjected to voltage at a potential scanning or sweep rate of 50 mV per minute and was polarized. The polarization was a so-called anodic polarization, the current flowing from the specimen to the counter-electrode in the liquid. The variation in the current during this polarization was detected and measured.

First, the application of 10 mV was performed upon the specimen plate W by the applying means 308. Measurement of the reference polarization current showed an approximate value of 2.0 X 10-5A. As described previously, such being the case, to set the maximum polarization current of 2 x 1 0 - 5A flowing through the variable resistor Ro as the final measuring point (100%), and to set the polarization current values where the logarithmic values become equally different as each of the measuring points, i.e., 0.2 X 10-5A (10%), 0.356 X 10-5A (17.8%), 0.632 X 10-5A (31.6%) and 1.126 X 10-5A (56.3%), the variable resistor Ro was set to 105so so that these might correspondingly coincide with the comparison voltages or potentials divided in advance into the logarithmically equal different spaces, i.e., the maximum voltage or potential drop might become 2 V.

The overpotential corresponding to a polarization current flowing through the resistor Ro at this time of 2 X 10-5A was approximately 200 mV. The specimen plate W was polarized to such an extent that the logarithmic values of the overpotential and the polarized current became linear. The integration circuit 307 was so arranged that the overpotential might be increased with the linear scanning rate of 50 mV per minute.

After the specimen plate W had been immersed in the corrosion liquid for forty-eight hours, the specimen plate W was anodically polarized from the spontaneous electrode potential by the overpotential applying means 306, while the specimen plate W was being electrolysed at the spontaneous electrode potential. The polarization current when the overpotential of 10 mV was applied was recorded as 0.835 V with respect to a full scale of voltage of 1.0 volt on the holding circuit 311. Accordingly, the polarization current difference corresponding to the overpotential difference of 10 mV was 1.67 X 10-7A.

In accordance with a further polarization operation of the specimen plate W, the overpotential with respect to the polarization current at each of the measuring points was recorded sequentially on each of the holding circuits 318 to 322 as shown in Table 1.

TABLE 1 Established Electrode Measuring polarization potential Overpotential point current value Holding of specimen difference (%) (A) circuit plate (mV) (mV) 10 0.2 x10-5 318 - 505 17.8 0.365 X 10-5 319 -473 32 31.6 0.632X10-5 320 -440 33 56.3 1.126 X 10-5 321 -403 37 100 2 10-5 322 -367 36 As regards the optimum overpotential difference, it was estimated that the values of the overpotential and the logarithmic polarization current were in a linear relationship in the measuring points of 56.3% or more due to the fact that the overpotential difference 37 mVof the measuring point 56.3% and that of the measuring point of 100% were almost equivalent.

Therefore, so far as the optimum overpotential difference for this case is concerned, 37 mV was adopted. Since a logarithmic value of 1 of the difference in the polarization current value, i.e., 1.0, was divided into the four equal divisions or equally divided differences as described above, the Tafel slope became a value of 148 mV (= 37 X 4).

On the other hand, the Tafle slope which was obtained by an extrapolation of the curve of log i vs. Ewas also 148 mV wherein the curve described above correlates the logarithmic values of the polarization currents against the respective values of the overpotential, and is recorded as a correlation of log i vs. Eas previously described.

Accordingly, it can be said that the measuring result is sufficiently reliable.

Similarly, the Tafel coefficient according to the cathodic reaction can be obtained by the following relationship:

wherein dE is the overpotential difference, di is the polarization current difference corresponding to dE, ba is an anodic Tafel coefficient and bc is a cathodic Tafel coefficient.

From the relationship (1), the corrosion current (i,,,) can be obtained. In this example, the following simplified equation can be used for the estimation:

According to equation (2), the corrosion current icy,, for the present case takes the following value:

According to the above embodiment, the Tafel slope was obtained with the equally split current value as a reference. On the other hand, the Tafel slope can be obtained with the equally split voltage as a reference.

In this case, the potential is equally divided while being base on Ecorr. to set El, E2, E3,...Ex points so as to detect the corresponding logarithmic values of the polarization currents in each of the points, i.e., log i" log i2, log i3,. . . log i,. Successively, the differences #, (= log ii - 0), 2 (= log i2iog log i1), and so on are obtained, whereby the same values occurring among the respective gradients (a relative difference of 10% or less is allowed) become a Tafel slope.For example, referring now to Fig. 7, respective gradient values of 83 to 86 have each approximately equal value in an anodic polarization curve as shown in the left-hand side in Fig. 7. Thus, a line ()) becomes the Tafel line of the anode polarization curve extending through the points 2 to 6 for this case. Similarly, in respect of a cathodic polarization curve on the right-hand side in Fig. 7, a line 0 becomes a Tafel line. Since an ordinate of an intersection between the lines ()) and , i.e., log icy", is correspondingly equivalent to the corrosion current i,orr' it is read out from Fig.

7. As a matter of fact, once both Tafel slopes have been obtained, the potential is further divided into potential spaces each being equal as described previously in a direction designated by (i) in Fig. 7. The current value of each of the potential points thus prepared is obtained by the use of the Tafel slope on the anodic side. The Tafel line is extended onto the cathodic side.

potential is further divided into potential spaces which are all equal as described previously in a direction designated by (i3 in Fig. 7. The current value of each of the potential points thus prepared on the cathodic side is obtained by the use of the Tafel slope on the cathodic side.

Accordingly, the Tafel (tangential) line is extended onto the anodic side. These extrapolated current values are compared with respect to each other by the comparator circuit according to the present invention, to detect a coincident value as the corrosion current.

Referring now to Fig. 8, there is shown a circuit diagram principle according to the present invention, wherein the anodic reaction measuring system (film side measuring system), (see Fig.

3) and a cathodic reaction measuring system (non-film side measuring system) are combined.

The potentiostat 2' in the cathodic reaction measuring system and the electrolysis current detecting circuit 7' are constructed as shown on the right-hand side of Fig. 8. The reference electrode R' inserted into the right-hand side measuring cell 1' is connected through the DC power supply source V' to the potentiostat 2'. The output side of the potentiostat 2' is connected, through the electrolysis current detecting circuit 7', to the counter-electrode C'. A relative electrode potential on the non-film side face of the coated metallic plate W is set by the DC power supply source V' to become approximately zero with respect to the reference electrode, so that a condition may obtain on the non-film side face in which atomic hydrogen is ionized. The initil potential of the non-film side face of the coated metallic plate W is maintained by the potentiostat 2'.Thus, the electrolysis currents caused when the atomic hydrogen is ionized on the non-fiim side face flow between the coated metallic plate W and the counterelectrode C', and are detected by the detecting circuit 7'.

The characteristic features of the present apparatus are described as follows.

(1) Depending upon the fact that a constant-potential electrolysis operation is performed with the relative electrode potential on the non-film face of the coated metallic plate W being retained to approximate zero with respect to the reference electrode, the atomic hydrogen is ionized on the non-film face and the atomic hydrogen concentration on an interface of the non-film face becomes approximately zero. Thus, a concentration gradient of atomic hydrogen is established in the steel from the film face to the nonfilm face. The driving force for the diffusion of the atomic hydrogen is caused by this concentration gradient and, thus, the penetration of the hydrogen can be caused without forcibly polarizing the coated steel plate.

(2) In addition, since the non-film face is in contact with an alkaline solution, the H + ion concentration is small and the atomic hydrogen is likely to be ionized.

(3) Furthermore, since the non-film face is in contact with the alkaline solution, the corrosion currents on the non-film face can be controlled. Accordingly, the hydrogen electrolysis current can be correctly measured.

According to the present invention, in order automatically to set the above-described relative electrode potential on the non-film side face of the coated metallic plate to approximately zero with respect to the reference electrode, portions of the apparatuses described in Figs. 4 and 5 can be used. The necessary circuit provisions are shown in Fig. 9, wherein the same numerals are used for the same components as in Fig. 5.

Referring now to Fig. 9, in this circuit, when the contacts L5 and L7 are closed under the condition that the contacts LO and L6 are opened, the electric power required to cause the electric potential level of the non-film side to be relative zero with respect to the electrode potential of the reference electrode R' is output through the operational amplifier 205. This means that the reverse side relative to the coated side of the coated metallic plate W, i.e. the non-coated or non-film side, is potentiostatically electrolysed under the condition that it is potentiostatically set to a zero potential level with respect to the electrode potential of the reference electrode R'.Thus, if the electrolysis current to flow between the non-coated side and the counter-electrode C' is detected with the help of the means 7' (which substantially comprises the voltmeter), the resultant detected value itself is the discharged current value of the atomic hydrogen, which is discharged through the substrate metal of the coated specimen plate W. Respective means designated as 206, 209, 210 and 211 and their related means are all prepared to set the potential level of the non-coated side of the coated specimen plate W at the spontaneous electrode potential.

Referring to Fig. 9, in order to switch over the measuring range subject to the respective magnitudes of the electrolysis currents, a rotary solenoid switch L is provided which is capable of being connected to each of the resistors arranged in parallel of, for example, a multistage resistor 213. By this arrangement, when the voltage drop of To of the multistage resistor 213 becomes larger than the output voltage of V1 of a comparison power supply source 221 (VO > V1), a comparator 222 accordingly imparts a difference signal. The difference signal thus produced is amplified by an amplifier 223 so as to actuate a relay, whereby the proper switching operation (L5 L4. . or L,) is correspondingly effected in accordance with the magnitude of the difference signal.

The operation of the corrosion evaluation measuring apparatus according to the present invention, which is shown in Fig. 1, is now described with reference to Figs. 10 and 11.

Fig. 10 shows an operation flow chart wherein the same numerals are given to the same components as those of Fig. 1. All the controlling operations may be performed by a microcomputer. The specimen is coated by a given film on its one side, while it is free from any coating of film on its other side, and is set between the measuring cells 1 and 1'.

When the specimen is selected from the group of the sulfide stress cracking resistance materials and the embrittlement resistance materials, either of a corrosive gas (SO2 gas or steam) a corrosive liquid (crude oil) or a solid (soil), with an electrolyte being, however excluded, is placed in the measuring cell 1. A. given alkaline solution (for example, NaOH solution) is placed in the measuring cell 1'. Accordingly, in this case, the measurement on the non-film side is performed, but the measurement on the film side is not performed or cannot be performed.

When the judgement of the phenomenological behaviour related to the specimen is required in the electrolyte, a given electrolyte (for example, NaCI solution) is placed in the measuring cell 1 to perform the measurement in respect of the film side. When the measurement concerning the non-film side is required or a temperature promoting test on the film side is performed, a given alkaline solution is placed in the measuring cell 1'.

Accordingly, given test conditions are input to a step controlling means (not shown here). The step controlling means controls measuring steps as follows. When the specimen is neither the one for sulfide stress cracking resistance, nor the one for embrittlement resistance (in the case of NO) and the judgement of the behaviour in the electrolyte is required (in the case of YES), the measurement of the film side is arranged to be performed.In the case of the film side measurement (hereinafter referred to as the anodic measurement for the sake of brevity), the electrode potential of the specimen plate W is measured by the spontaneous electrode potential detecting circuit 3 and is once stored by the spontaneous electrode potential storing circuit 1 8 to obtain the data (9), while the specimen plate is potentiostatically electrolysed at the spontaneous electrode potential with electrode potential being the reference. The proper state for the potentiostatic electrolysis described above is retained by the potentiostat 2.

In the state of this potentiostatic electrolysis, cathodic and anodic pulses are applied between the specimen plate W and the counter-electrode C with the pulse potential generating circuit to pulse-polarize the specimen plate W. The cathodic and anodic pulse currents flowing between the specimen plate W and the counter-electrode C subject to the operation of the pulse polarization are stored in the storage circuit 8 and are fed to the comparing circuit 9 to judge whether or not the cathodic and anodic pulse-currents are the same. When they are almost or substantially the same (in the case of YES), no film-defects exist (data (10)) in the specimen plate W.Thus, the cathodic and anodic pulse current values which are stored in the storage circuit 8 are divided by cathodic and anodic pulse voltage values, at a division circuit 27 to detect the electric resistance (data (12)) of the film and the water permeating rate (data (13)) in the film. On the other hand, when they are judged not to be the same substantially (in the case of NO), the film defects (data (11)) are displayed.

The electric resistance in the film and the water permeating rate in the film are detected as described below.

When the pulse polarization is performed by + Ep and - Ep as shown in Fig. 12, such pulse polarization current changes as shown in Fig. 13 are obtained. Since a time rof 0.3679 times the maximum value i which is initially generated in the course of the operation of the pulse polarization, becomes a time constant value, the film resistance Rf is detected, according to the relationship of R, = Ep/i, by the use of the current value it or 5time. When the film resistance R, is obtained, the electrostatic capacity C of the film, which is expressed as C = r/R" can be obtained.Also, since the attenuation curve of the pulse current in connection with the operation of the pulse polarization is expressed as i = EXPf- (1/RfCf)tJ, C" and C0 are both obtained from this equation. The water permeating rate is obtained by the following expression, i.e., Ap = 100 log (C/C0)/log 80 (%).

After the operation of the pulse polarization is over, the specimen plate W is polarized by the potential scanning polarization method at a given rate with the linear potential sweep applying circuit 6. In cases where no film-defects are confirmed in the specimen plate W, the DC voltage is applied between the coated metallic plate W and the counter-electrode C by the linear potential sweep applying circuit 6. The application voltage is varied with positive and negative grade. The electrolysis current flowing between the coated metallic plate W and the counterelectrode C is input to a differential circuit 28 as a measured signal.However, the DC voltage which varies with the mode relatively equivalent to the variational mode of the application voltage to be output from the circuit 6 is input to the differential circuit 28 as the correction signal of the result of dividing by the electric resistance in the film. The difference signal between the measured signal and the correction signal is taken out by the differential circuit 28 so as to detect a very small or an infinitesimally small current-potential variation (data (14)).

Thus, when film defects are substantially not present in the specimen W, the variation of the potential current (Fig. 1 5) which almost linearly varies is correspondingly obtained as the result of the linear potential sweep polarization (Fig. 14). However, when the very small currentpotential variation (see Fig. 16) is taken out, the corrosion current i,orr. can be obtained from the following expression: i,orr. = Ep2/(AEAiip2)' 1/fax,,. ff(X!lE)} When the film defects exist in the specimen plate W, the positive and negative log. i vs.E polarization curve is stored in the storage circuit 1 4 so that it is not only to be displayed, but also to be utilized for detecting the corrosion current and corrosion amount (data (16)) with the help of the circuit means 1 7. In addition, the peak detection circuit 12 detects whether or not the peaks exist on the cathodic polarization curve.

The peak detection circuit 1 2 may be composed of a differentiation circuit. The d(log i)/d(E) variation which defines the cathodic polarization curve turns its plus mode (23 to its minus mode 0 when the peak in such polarization curve as shown in Fig. 1 7 is formed and, thereafter, turns to the plus mode (3 again. Thus, respective differential values become zero at both point A and point B. Accordingly, when a zero output is output from the differentiation circuit twice, the existence of the peak can be confirmed. When the potential and the current at an instance of the first zero output are detected by the detecting circuit 15, detected values give a peak potential and a peak current, respectively.The peak potential and current values of each polarization curve (the polarization curve of the specimen plate W is measured for every given time period after contact with the corrosion liquid) are stored at a storage circuit 29 to measure their variations with time (data (21)). The peak area in the cathodic polarization curve is obtained with the circuit means 1 6.

So far as the variation of the peak associated with the cathodic polarization with time is concerned, there are two types. According to one case, the cathodic polarization curve including its peak portion (i.e., the polarization current increases) comes out upwardly in succession, i.e., Gto to (2) as can be seen in Fig. 18, wherein the given time period is a parameter. On the other hand, according to the other case, only the peak portion of the cathodic polarization curve substantially lies upwardly, i.e., from Qto O as can be seen in Fig. 19.In the former case, the Tafel slope of the polarization curve ()) is obtained by the use of the same technique as described earlier to decide a Tafel tangential line (D. The area surrounded by the polarization curve higher than the tangential line (X)' is obtained with the help of an integration circuit. The area is defined and obtained by the following expression, i.e.,

wherein an integral starting point is a potential point where the measured value has deviated from the tangential line, and an integral terminating point is a potential EB where the differentiation circuit of the peak detection circuit 1 2 outputs the second zero output.However, f(EO) is a function representing the polarization curve, while f (E(v). is a function representing the tangential line. In the latter case, the ara is defined and obtained by the following expression, i.e.,

wherein intersecting points of the polarization curve (X) and the polarization curve (2) or (ss) are the integral starting point E,and the integral terminating point E2, respectively.

Furthermore, when the peak exists (in the case of YES), the electrode potential of the specimen plate W which has not yet treated by the polarization operation is compared with that of the substrate metal, thereby to confirm whether the electrode potential of the above-described specimen plate is more cathodic or anodic with respect to the electrode potential of the substrate metal. With respect to the electrode potential of the substrate metal, as the substrate metal to be adopted for the specimen plate W is normally iron, the spontaneous electrode potential of iron is chosen. Since the potential of the iron varies in accordance with the corrosion circumstances which are affected by e.g. the pH and the temperature conditions, the electrode potential of bare iron in the circumstances wherein the experiments are conducted is chosen as the reference.In the case of the cathodic mode (in the case of YES), the film defect portion is likely to be a pitting corrosion. Accordingly, the pitting corrosion (data (17)) is displayed. The corrosion amount or dimensions and its variation with time are taken into consideration so as stereoscopically to display the pitting corrosion state of the defect portion. In the case of the anodic mode (in the case of NO), the corrosion of the film defect portion is likely to spread over in the relatively lateral direction (crevice corrosion). Accordingly, the crevice corrosion (data (19)) is displayed, wherein the corrosion amount and the peak potential, the peak current and/or the peak area or its variational feature with time are taken into consideration, thereby to stereoscopically display the crevice corrosion state of the damaged section.

On the other hand, when the peak does not exist (in the case of NO), the electrode potential of the specimen plate W which has not yet been treated by the polarization operation is compared with the electrode potential of the substrate metal by the comparator circuit 1 9. In the case of the cathodic mode (in the case of YES), the film defect portion is likely to be corroded in the relatively lateral direction. In the case of the anodic mode (in the case of NO), the film defect portion is likely to be the pitting corrosion. As is clear from the above description, it is to be noted here that the corrosion condition becomes reversed subject to the condition of whether the cathodic polarization curve involves the peak or not.

In addition, when the information as to whether or not a metallic plating layer or a cathodic corrosion preventing film exists between the film and the substrate metallic surface is intended to be obtained, the existence of the peak in the anodic polarization curve or the existence of the difference in the spontaneous electrode potential between the substrate metal and the coated metal are confirmed.

When the peak or the stage portion exists in the anodic polarization curve, the plating layer exists, wherein the polarization step requires to be polarized by 500 mV in the anodic mode with respect to the level Ecorr. and then, it is confirmed whether or not the peal or the stage portion exists in this range.

The defect relating to the specimen plate must exist even in the metal plating layer. Therefore, in the case wherein there is no defect in the metal plating layer, the metal plating layer itself is judged to be the metallic substrate in the initial dipping stage of the specimen plate.

Accordingly, in the case where the defect is deep enough to reach the substrate metal portion, the plating layer is first confirmed to exist.

More specifically, a procedure shown in Fig. 11 is used for the detection purpose described above. The specimen steel plate W is polarized by approximately 100 mV in the cathodic mode with respect to the potential Ecorr. by the use of the linear potential sweep polarization method to obtain the cathodic polarization curve, whereas the plate is polarized by approximately 500 mV in the anodic mode with respect to the potential Ecorr. to obtain the anodic polarization curve.

Successively, the existence of the peak or the stage portion is detected from the cathodic polarization curve to obtain either an existence signal 9 or an absence signal (ss). On the other hand, the existence of the peak or the stage portion is also detected from the anodic polarization curve to obtain either of an existence signal (2) and absence signals (!) and (D. In addition, as is shown in Fig. 11, the electrode potential Ecor, of the specimen steel plate W is compared with the electrode potential EFe of the bare steel plate under the same corrosion circumstance to obtain either the cathodic signal 1 Ecorr. < EFe or the anodic signal of Ecorr > EFe.

In the case where the peak-existing signal (t is obtained in the cathodic polarization curve, with the meal plating layer-absent signal Q7 being simultaneously obtained for the reason described above, the corrosion form of crevice corrosion is then confirmed, subject to the condition that the anodic signal ( of the Ecorr. > EFe is obtained. In the case where the plating-layer existing signal (t is obtained, the corrosion form is confirmed to be pitting corrosion when the cathodic signal 0 of Ecor, < EFe is obtained.

Also, even when a cathodic corrosion- E < E preventing-film existing signal (ss), instead of that of the metallic plating layer, is obtained (a metal-plating-layer-absent-signal (#) and the cathodic signal 0 corresponding to Ecorr. < EFe are both obtained), the corrosion form is confirmed to be the pitting corrosion form. Furthermore, the Ecorr. of the specimen steel plate W becomes more cathodic than the EFe signal despite the absence of the metal plating layer, since the corrosion preventing film will show the same functional characteristics as when the metal plating layer exists.

In the case where the peak-absent signal ( is obtained in the cathodic polarization curve, with the metal-plating-layer-absent signal (t) being simultaneously obtained, when the anodic signal (3 of Ecor, > EFe is obtained, the corrosion form of pitting corrosion is confirmed. Furthermore, when the metal-plating-existing signal Q2 and the cathodic signal (3 of the Ecorr. < EFe (the cathodic corrosion-preventing-film-existing signal (ss) is obtained) are both obtained, the corrosion is confirmed to be crevice corrosion.

According to the present invention, a model figure Qi which stereoscopically displays the corrosion section of the film defect portion is drawn as follows.

As for signals (data) to be taken into consideration for the above-described purpose, there are enumerated corrosion volume Gi, corrosion configuration () of pitting corrosion or crevice corrosion, peak area 9, peak potential and peak current (9.

The corrosion volume 9 is a corrosion quantity (Wg = ccrr. hr/k; where k is the electrochemical equivalent amount and hr is the immersion time) divided by the specific gravity p. Accordingly, the following expression: i.e., VcO,,(cm3) = ;COrr hr/k.p is obtained. According to the present invention, the corrosion current icerrcan be obtained as previously described (see Fig. 7), and thus the corrosion volume V can be obtained as long as the immersion time h, is measured, and the electrochemical equivalent amount k and the specific gravity of the substrate metal are both given as constant in advance.

Referring now to Figs. 20 and 21, in the case of pitting, since a rust width I spread laterally in the interface of the film and the metal surface from the film defect portion can be considered to be substantially absent, the surface area D of the film defect portion is required to be only taken into consideration to model the size of the corrosion volume V With respect to the surface area D, if the corrosion current density K, (iCOr/cm2) of the bare substrate metal, which is in the same environmental condition as those for the specimen plate W, is measured in advance, the actual measurement of the corrosion current com (which is referred to as K2) immediately after the dipping of the specimen plate W in the corrosion medium permits it to be estimated by the following relationship, i.e., D = K2/K1(cm2).

In the case of crevice corrosion, as will be described hereinbelow, the more the peak potential is cathodic, with the difference between the peak current value with respect to the peak potential and the current value represented by the Tafel tangential line (substantial height of the peak) being correspondingly larger, the iarger the rust width becomes (see Fig. 21). Accord ingly, when the relationship between respective variations of peak potential, current and/or peak area with time and rust width are appropriately taken into consideration, the accuracy of the model figure is improved. The height denoted by H is generally in the range of from several micrometers to several tens of micrometers, whereby it is not difficult to draw the model figure even if the exact values of the height described above are not obtained.

The peeled-off area A, of the film under the condition of non-artificial prevention of spontaneous corrosion can be estimated from the peak potential PE and the peak area PA of the cathodic polarization curve. Namely, among A1, PA and PE, there can exist the following relationship, i.e., A, = K' (PA X Pre)' wherein K' is a constant related to the peak potential together with the peak area under the peeled-off area A,.

The relationship between the existence of the peak in the cathodic polarization curve and the corrosion form, the peak potential, the peak current, the peak area, the variation of the peak area with time and the rust width will be clarified with reference to the following embodiments.

EMBODIMENT 1 As the specimen plate, a steel plate (JIS. G, 3131: mild steel plate having the dimensions of 15 X ) < 7 X 0.08 cm) coated, to 200 p in film thickness, with epoxide resin paint of amine hardening type (Copon EA-9, trade name, marketed by Nippon Paint Co., Ltd.) was used. Each coated steel plate W had a defect portion H on its one side portion (the film defect of approximately 1 X 10-3cm2 was confirmed in advance through microscopic observation). As illustrated in Fig. 22, a silver-silver chloride reference electrode R and a platinum counter electrode C were set adjacent the defect portion H.

When the specimen steel plate W was polarized in the cathodic mode (50 mV per minute) for each immersion time (50, 100, 200, 500, 1000 hours), peaks P" P2, P P4, P P,as shown P Fig. 23 appeared in each of the cathodic polarization curves. It was observed that the potential of each of the peaks P, to P5 was likely to shift in the cathodic direction with the increase of the lapse of the immersion time.

On the other hand, the rust width a of the defect portion H region and the film peeled-off width (a+ 2b), as shown in Fig. 24, for each dipping time was measured according to the conventional method.

As shown in Fig. 25, it was found out that an approximately linear relationship exists between the variation of the rust width (x marks in the drawing) with time and the variation of the peak potential with time, while it was further found out that an approximately linear relationship also exists between the variation of the peeled-off width (O marks in the drawing) with time and the variation of the peak potential with time.

Thus, by the measurement of the variation of the peak potential with time when the peaks appear in the cathodic polarization curve, it can be understood that the rust width and the peeled-off width of the film defect portion can be estimated.

EMBODIMENT 2 An epoxide resin paint A (which is a spontaneous drying paint composed of two miscible liquid types of paste and hardening agent, more specifically, composed of 40 per cent PVC; 80 per cent NV; additive ((drip preventing agent + su'rface-active agent), (rust preventing pigment + filter (body)), epoxy resin) or a polyester resin paint B (which is the same as paint A, except that the polyester resin is employed instead of epoxy resin) is coated, to a thickness of 500 z, on respective steel plates each having the same dimensions as those used in embodiment 1.

Each coated steel plate was dipped in 3% aqueous NaCI solution (30on), and polarized under cathodic polarization conditions of 50 mV per minute for each immersion time (approximately 500 hours) to detect the peak potential which appeared in each of the polarization curves and to measure the rust width of the defect portion (the existence of the defective portion of approximately 1 X 10-3cm2 was each time confirmed in advance through observation by the microscope) in each immersing time.

Fig. 26 shows the cathodic polarization curves of the specimen steel plates A and B during 500 hours immersion operation. As will be apparent from Fig. 26, the peaks were seen with respect to the specimen steel plate A. However, no peak appeared in respect of the specimen plate B. It was confirmed that the peaks concerning the specimen plate B did not appear before the lapse of 1,500 hours' immersion time.

According to the observation of the rust width of each of the specimen steel plates A and B during 500 hours' immersion operation, the rust width in the specimen steel plate A was likely to develop into the pitting.

From the result as described above, in the case of the specimen steel plate A, it can be estimated that the rust width is likely to spread in the lateral direction from instant one after the peak and stage portion appear in the cathodic polarization curve.

Referring now to Fig. 27, the aging variation of the peak potential in each of the polarization curves of the specimen steel plates A and B shows the same inclination as that of the aging variation of the rust width (see Fig. 28), whereby the interrelation therebetween can be understood.

EMBODIMENT 3 Steel pipes C and D (Each of these pipes C and D is of 100 mm inner diameter and of 5 mm thickness. The steel pipe C was coated with a zinc-phosphate film prior to the coating of the paint, while the steel pipe C was not so treated), each was coated, to a thickness of several milimeters with polyethylene paint. The pipes were installed in the soil, at levels below approximately 1 m in depth with reference to the road surface and at a relative moisture content of 10 per cent.

The location where the damage was considered to be effected during the installing operation was polarized by the cathodic polarization method with the help of the same method as described in the embodiment 1 above. Referring now to Fig. 29, there are shown the cathodic polarization curves, which are respectively obtained by 100 hours' immersion of the specimen steel plates C and D, respectively. When the corrosion current is obtained from the cathodic polarization curve for each immersion time by the use of an ordinal extrapolation method, the variation of the corrosion current with time can be correlated with respect to the immersion time as shown in Fig. 30. As regards the specimen steel pipes C and D, the rust is likely to be spread in the relatively lateral direction or in a narrow splitting mode from the film defect portion.

Under such the condition as described above, it can be estimated from Fig. 30 that the rust caused in the specimen steel pipe C spreads about ten -times as fast as the rust caused in the specimen steel pipe will spread. This phenomenological feature corresponds to the difference in the current value of the peak portions in each of the poralization curves shown in Fig. 29.

Accordingly, it can be understood that a corrosion estimation correspondingly equivalent to that accomplished by the use of the corrosion current correlation described above can be in turn accomplished by reading the peak currents of the polarization curves. In fact, as can be seen from Figs. 30, 31 and 32, the trend or feature of the aging variation in the peak current of the poralization curve is approximately the same as the trend of the aging variation (see Fig. 32) of the rust width and the trend of the aging variation (see Fig. 30) of the corrosion current.

In general, in the case in which the peak potential appears in the initial stage of the immersion operation, with its peak potential value being entirely cathodic, namely, when the rust spreads extremely in the initial immersion stage, the peak potential shows the difficulty of shifting towards high cathodic values in proportion to the immersion time, being different from the embodiment 2 in which the peak potential becomes gradually more cathodic. Accordingly, in such a case as described above, since the peak current corresponds substantially to the corrosion current as described previously, the peak current can serve as a substitute for judging the variation of the rust width. Thus, the form or configuration and the expanse of the rust can be also estimated from the peak electric current value and its variation with time.

EMBODIMENT 4 Epoxy resin paint (paint on the market for coiouring the zinc-plated sheet metal) was coated (paint-film thickness: approximately SOp, dried for 170"C X 10 minutes) on a commercially available fused zinc-plated steel plate E (15 X 7 X 0.03 cm), a plate F of the same kind finished by the use of the normal chromic acid treatment and a steel plate F' which was treated by means of an ordinarily electroplated zinc finish, whereby specimens E, F and F' were prepared.

With respect to the specimen plates E, F and F', their cathodic polarization curves were respectively obtained, with the immersion time being chosen as the parameter for each of the dipping operations carried out in the same manner as in the embodiment 2. Fig. 33 shows each of the cathodic polarization curves after 20 hours' immersion operation. As can be seen from Fig. 33, the specimen plates E, F and F' show the spontaneous electrode potential of approximately - 1 volt, respectively. With respect to the specimen plates F and F', the peak or stage appears in each polarization curve. But no peak appears in the polarization curve of the specimen plate E and the rising feature of the polarization curve is larger than those of the specimen plates F and F'.On the other hand, the visual corrosion evaluation is further performed on the same specimen plates described above under the same condition. The results obtained from the respective variations of the rust width with respect to time are shown in Fig.

34. With respect to the specimen plates F and F', the increase in the rust width is not remarkable, whereas with respect to the specimen plate E, the increase in the rust width is remarkable. Namely, in the specimen plates F and F', the development of the pitting corrosion in the film defect portion was confirmed.

Accordingly, from the above results, the estimation can be made as follows. In the fused zincplated steel plate, the pitting corrosion behaviour is shown when the peak and the stage portion are effected in the cathodic polarization curve. When the cathodic polarization curve rises without peaks and stage portion, the rust width tends to show increase. Thus, so far as the specimen plate E is concerned, only the plated layer is corroded and the steel plate of the substrate is protected from the corrosion. Accordingly, the quantity of electricity of icy,, of the E corresponds to the variation in the rust width. On the other hand, in the specimen plates F and F', the corrosion inhibiting reaction in the substrate steel plate subject to the plating is not effective.It is needless to say that the aging variation of the depth of the pitting corrosion of the specimen plates F and F' can be substantially estimated from the aging variation (see Fig. 35) of the peak potential of the polarization curve or icy,, It is to be noted here that the results just described above become opposite to the corrosion behaviour estimated from the cathodic polarization curve of the specimen plate, which is not zinc-plated on the steel plate of the embodiment 2.

EMBODIMENTS With respect to specimen plates G and H, the cathodic polarization curves were respectively obtained, with the immersion time being chosen as the parameter for each of the immersion operations carried out in the same manner as in embodiment 2. Commercially available paint G" (35% PVC, 60% NV, epoxy-acrylic resin, non rust-preventing pigment) and H" (commercially available paint G" further including rust-preventing pigment) were coated, to a thickness of appropriate 15 p. on the fused zinc plated steel plate F which was used in embodiment 4, thereby to prepare the specimen plates H and G. Figs. 36(a) and (b) show the cathodic polarization curve of each of the specimen plates G and H after 250 hours' immersion operation.As can be seen from these cathodic polarization curves, a substantially downward peak or stage portion appears, whereby it can be estimated that the rust width is likely to be spread in the lateral direction or in the splitting mode. However, the rising is still recognized in the polarization curve of the specimen plate G (see Fig. 36(a)), whereas the polarization curve of the specimen plate H becomes approximately flat.

In view of the aging variation (see Fig. 37) of the rust width, the rust width of the specimen plate G is constantly larger than the rust width of the specimen plate H, with this feature corresponding to the aging variation (see Fig. 38) of the peak potential. However, according to the comparison with the polarization curve (E of Fig. 33) to be effected by the specimen plate E of the embodiment 4, the current value shows approximately the same variation, while the aging variation of the rust width is approximately one-half. Accordingly, so far as the specimen plate of the present embodiment is concerned, the behaviour of the pitting corrosion can be estimated to overcome the tendency of the increase in the rust width as the peak and the stage portion in the cathodic polarization curve appear.

Therefore, the overall corrosion evaluation can be performed by the confirmation of the existence of the peak in the cathodic polarization curve, and by the use of the potential and current values of the peak. For example, the corrosion behaviour of the specimen plate G can be judged to show the corrosion behaviour of the specimen plates E and F of the embodiment 4 from the existence of the peak, the peak potential and peak current of the polarization curve (G of Fig. 36(a)) of the specimen plate G.

Fig. 36(b) shows the cathodic polarization curves of the specimen plates G and H after 250 hours' immersion. In the initial stage of the polarization operation, the stage portion is seen in the anodic polarization curve, since the dissolving reaction of the zinc-plated film is a ratedetermining step or a rate-controlling step in the initial stage of the polarization operation, but the dissolving reaction is restrained from occurring as the polarization potential becomes more anodic and the polarization current does not follow the increase of the potential. Thereafter, the polarization current tends to increase following the increase of the potential, since the dissolving reaction of the substrate metal of the iron may exist. This phenomenon is unique when the metal plating lever exists between the film and the substrate metal.Accordingly, the detection of the peak or stage portion in the anodic polarization curve makes it possible to detect the existence of the metal plating layer.

EMBODIMENT 6 With respect to the specimen plate H of the embodiment 5 and a specimen plate I (the same paint was coated on steel plate I which is higher with respect to the surface spangle than the zinc-plated steel plate of the specimen plate H), the cathodic polarization curves were measured at the immersion initial stage (0.5 hour) and for each immersion time in the same manner as described in embodiment 2. The results are shown in Fig. 39 (specimen H) and Fig. 40 (specimen 1). As regards Fig. 39, the interrelation can be observed between the aging variation of the area of the peak portion in each polarization curve (the area surrounded by the polarization curve to be obtained in the initial stage of the immersion operation (0.5 hour) and each polarization curve) and the aging variation (see Fig. 37) of the rust width can be observed.

Accordingly, the rust width can be estimated from the area of the peak portion. The interrelation may be shown between the aging variation of the area in respect of the peak portion of Fig. 40 and the aging variation of the rust width. However, the increase rate relating to the area of the peak portion shown in Fig. 39 is larger than the increase rate relating to the area of the peak portion of Fig. 40, showing a trend or feature opposite to the variation of the rust width. Thus, Fig. 39 shows a feature similar to the H of embodiment 4. Fig. 40 shows a feature similar to G.

Accordingly, from the area and/or the aging variation of the peak portion, the corrosion behaviour, especially the rust width and the variation thereof can be estimated.

Fig. 41 shows the anodic polarization curve of the specimen plate I after 0.5 hour's immersion operation. Since the specimen plate I is coated by a metal-plated layer, the peak or the stage portion appears in the anodic polarization curve as can be seen in Fig. 36(b).

The above embodiments show one example of the corrosion evaluation testing methods according to the present invention.

Generally, in the study of clearance corrosion or its practical evaluation, the corrosion behaviour to be caused by the clearance between metal and metal or metal and non-metal has been considered to be serious problem as shown in Fig. 42, wherein a preferred experimental object is a specimen plate whose contact area between the face of a metal 0 and the liquid (;3 is larger. However, so far as the case of the coated metallic material (hereinafter the metallic material is specifically assumed to be steel plate) is concerned, when a very small defect exists in the film (see Fig. 43), the width W or the length of the defect is small.Actually, the liquid Q2 is only in contact with the metallic portion directly below the defective width W even if the metal Q is in contact with the liquid (2). Due to the decrease in the electric resistance, which is caused by the liquid absorbed in the film (ss), the liquid penetrates into the interface of the film 0 and the metal through the water permeation, wherein, when the metal 0 is polarized, the current flows into the film (, but generally is remarkably smaller than the current flowing through the defect portion W.Accordingly, the area into which the current flows to the metal (the cathodic polarization) is smaller in the case represented by Fig. 43 even if respective cases of Fig. 42 and Fig. 43 are both polarized in the cathodic mode under the same condition.

However, when the rust (black portion) is spread from the defect portion and reaches to a certain depth as shown in Fig. 43, the metallic face Qcorresponding to the defect width W is much smaller than the metal face where the rust spreads, even if the H of Fig. 43, including the clearance width is the same in value as the H of Fig. 42.However, in the case of Fig. 42, when the metal face Qa except for the clearance portion, while being in contact with the liquid, is compared with the metal face Qb of the clearance portion, the metal face G is much larger than the metal face (9. Thus, the phenomenological behavior inside the clearance ( of Fig. 42 is substantially subjected to the reaction relating to the metal face Qa and is far from being distinguishedly affected.On the other hand, in the case of Fig. 43, the metal face Gis smaller in area than the metal face Gand thus the reaction inside the clearance (together with the reaction to be effected in the region of the metal face become to be distinguishable. Such being the case, as a result, with respect to oxygen amount, ion concentration or the variational amount of the liquid components due to polarization, there are effected the regional differences between the portion Gand the portion (fi3 or between the liquid 2) and the portions Q (g Thus, the behaviour inside the portion (appears as, for example, peak or stage portion on the cathodic polarization curve.However, when the rust width Gand the peeled-off width are small, subject to such deep corrosion as shown in the dotted line Gin Fig. 43, the portion of the clearance or crevice (0 becomes small. Hence, the behaviour inside approximately the dotted lines is apparently measured overall, its behaviour therefore becoming similar to that represented by the normal cathodic polarization curve having no peak at all.

As is clear from the above description, the peak-causing mechanism is due to the phenomenon of the clearance corrosion or crevice corrosion under the film. From this, the corrosion configuration of the defect portion can be estimated from the existence of the peak, whereby the extremely advantageous corrosion evaluation testing method can be provided according to the present invention.

Respective values of the peak potential and current which both appear in the cathodic polarization curve are likely to increase (the potential is increased in the cathodic mode) in accordance with a rise in temperature. When the preliminary corrosion evaluation is performed, it may be preferable generally to perform the measurements at temperatures ranging from 15 to 40"C, so that the experiments may be performed under conditions relatively corresponding to the natural environmental conditions. Needless to say, it is significant to observe such variation of the peak potential in association with the temperature variation to examine the temperature dependency of the corrosion behaviour.

Furthermore, since the polarization speed gives influences on the polarization behaviour of the specimen plate, it is preferable to select the proper conditions. In general, the polarization speed of 10 to 500 mV per minute is selected. In this range, not only the peak potential varies towards the anodic mode, but also the peak current is likely to increase with an increase in polarization speed.

Also, regarding the method for effecting the polarization curves, the relationship between the current and potential is conventionally arranged to be recorded with the recorder, so that the metal-corrosion preventing reaction per se may be examined. However, the present invention further involves a case wherein the current is arranged to be stored at every certain potential, and the stored values are combined with each other to give the polarization curves correlating the relationships of (i vs. E) and (log i vs. E). In this case, to obtain the peak current or the potential, for example, the current variation from which the polarization curves are established is differentiated with the help of the differentiation circuit thereby to obtain the current value or the potential value when it becomes zero.

The description in the foregoing relates to the measurements on the reaction on the film side.

The measurements on the reaction on the non-film side are now described.

As regards Fig. 10, when the specimen is the specimen prepared for tests relating to hydrogen cracking resistance or to sulfidation attack resistance (the case of YES), or when in the case of NO with the behaviour judgement of the specimen in electrolyte being further unnecessary (the case of NO), the non-film side (reverse side of the specimen plate) is adapted to be in contact with the alkaline liquid, so that it may be potentiostatically electrolysed.First, define the corrosion condition is defined and the potential of the non-film side is set to be zero with respect to the reference and the condition described above is retained as the initial potential with the help of the potentiostat 2', whereby the atomic hydrogen is ionized at the interface between the non-film face and the alkaline liquid to cause the ionized current to flow between the specimen plate W and the counter-electrode C'. This is measured by an electrolysis current measuring apparatus 7' and the variation is stored at the storing circuit 21 and then further displayed (data (1)).

EMBODIMENT 7 As the specimen plate W, a polished steel plate of 0.8 mm in thickness was used, which was treated with the zinc-phosphate formation film and was coated on one face, to 35 p in film thickness, with paint substantially composed of melamine-alkyd resin mixed with 1 % of a rust preventing agent (drying condition: 140"C, drying duration 30 minutes).A 3% NaCI solution was put into the left-hand measuring cell 1 while the solution temperature was maintained at 50"C. On the other hand, an one Normal NaCI solution was placed in the right-hand measuring call 1' while the solution was maintained at 15"C. The specimen plate W was set at an electrode potential of zero with respect to the reference electrode, thus being potentiostatically electrolysed for 25 hours. The variation in the electrolysis current flowing between the counterelectrode C' and the specimen W was shown as a dotted line A in Fig. 44.

The dependency of current relating to steel substrate with immersion period as shown in Fig.

44 can be considered as follows.

Since three peaks appear in the dotted line curve A of Fig. 44, it is assumed that a first peak is P1, a second peak is P2, a this peak is P3, the time required to a rising point of the third peak P3 being tc, and each of P1, P2, P3 and t being considered to show the following characteristic features.

Since the first peak P1 is caused by the variation in the electrolysis current in the initial contact stage of the specimen substrate W with the liquid, the first peak P1 is effected by the corrosion-corrosion preventing reaction between the steel substrate on the non-film coated face and the NaOH solution, judging from the series of reaction steps relating to the specimen plate W. More specifically, the first peak P1 is effected after lapse of time of approximately 0.1 hour with the value of the peak current being an approximately 10 pA, even when both liquids are maintained at the equal temperature of 50"C and the temperature grade is not applied on both sides of the specimen plate W.Therefore, if the hydrogen was dissolved due to corrosion of the steel plate under the film and then penetrated to the non-coated face, the generating time is earlier and the current described above is too excessive.

Actually, since the current value depends upon the surface condition on the non-coated face, the variation relating to the electrolysis at the spontaneous electrode potential is measured, instead of the potentiostatic electrolysis. The result obtained shows that the local minimum variation with respect to the electrode potential appears near the first peak P1, wherein the corrosion-corrosion preventing reaction on the non-coated face must be effected. The electrolysis current through the reaction of the non-coated face with NaOH solution is shown by a curve B in Fig. 44.

Accordingly, from the first peak P1 and the following portion of the curve A, the current, which is caused by the discharge of the atomic hydrogen in the steel substrate out of the non coated face and its successive reaction of ionization, i.e., H = H+ + e-, and the current represented by the above-described curve B are both superimposed so as to form the curve A.

With respect to the second peak P2 and the third peak P3 which can be seen in curve A, the occurrence of these peaks is attributable to the reason described hereinbefore. It can be considered that, on the coated face side, NaCI solution reaches the metal face under the film to cause the corrosion reaction, i.e., H + + e- = H, whereby atomic hydrogen caused by the cathodic reaction penetrates into the metal face to drive out the atomic hydrogen dissolved in the steel substrate from the non-coated face. Moreover, the atomic hydrogen dissolved in the steel through the corrosion reaction starts to flow from the non-coated face. These phenomenological features cause the peaks to be effected in curve A.

In view of the series of the reaction steps, the second peak P2 is provided when the electrolysis current (Fig. 44, curve C) caused in connection with the hydrogen dissolved originally in the steel (when the liquid reaches under the film after the immersion operation) flows into the rear side and the ions are then superimposed on the remaining current of Fig. 44 (b).

The third peak P3 is caused when the variation in electrolysis current as denoted by the curve C is over, the atomic hydrogen caused by the corrosion reaction subsequently caused permeating through the steel, and the electrolysis current consequently caused through the reaction of ionization on the non-coated face (Fig. 44, curve D) being superimposed chiefly on the remaining portion of the current denoted by the curve B.

The above description is confirmed by the following experimental facts. The second peak P2 hardly appears, if the saturated NaCI solution is introduced so as to make the diffusion rate of solution towards the film to be lowered. Furthermore, the rising point to of the third peak P3 corresponds to the time point at which the rust such as visually red rust or the like is brought about.

As is evident from the description concerning Fig. 44, since the curve A represents the overall variation of the electrolysis current superimposedly composed of the respective curves of electrolysis currents B, C, D of Fig. 44, the corrosion resistance of the coated metallic plate, i.e., the corrosion under the film, the hydrogen cracking inside the metal and stress corosion, can be properly estimated and evaluated by simply analysing the over-all curve A.

EMBODIMENT 8 As the specimen, the polished steel plate was used, which was first coated by the zincphosphate-film treatment and was then further coated on its one face with the paint of three time coats and three time bakes painting type. As regards a second coating paint, two types, i.e., easily and hardly corroding paints were both used. The aging variation of the electrolysis current was measured under the same testing conditions as described in the embodiment 7 with the former specimen and the latter specimen being denoted as A and B, respectively. The result is shown in Fig. 45.

According to the graph shown in Fig. 45, the variation in the electrolysis current with respect to the immersion time for both specimens A and B is the same up to the rising point tc of the third peak P3 concerning the specimen B with the easy-to-corrode paint being coated thereon, i.e., up to approximately 1 5 hours after the start of the measurement. However, thereafter, it requires 1 7 to 1 8 hours up to the rising point t, of the third peak P3 for the specimen A. Also, the rising mode towards the third peak P3 to be rendered by the specimen A is not so steep when compared with the rising mode towards the third peak P3 to be rendered by the specimen B.

As regards the fact that the second peak P2 was the same for both specimens A and B, there might exist no remarkable difference in the diffusion rate of water into the respective films between the specimen A and the specimen B. Furthermore, it is estimated that the driving force for driving out the dissolved hydrogen caused by the-effects combined by the corrosion liquid permeation rate on the coated face side and its accompanying corrosion reaction as well as the steel structure through which the melted hydrogen permeates are the same for both cases. The above-described estimations are confirmed as follows: (i) The second peak appeared even when the diffusion of solution into the film was suppressed in a moderate way.On the other hand, when, conversely, in the case of the employment of the saturated NaCI solution, the dehydration step worked and it was difficult to confirm the occurrence of the second peak. Due to the phenomenological features described above, it may be concluded that the second peak can be caused when water molecules, even in only small amounts, have reached the metal under the film. (ii) When the steel plate on the paint-film forming side is electropolished, the surface distortion layer caused by the surface working is removed and the quantity of current to be caused in an operation posterior to the first peak P1 is extremely large. (iii) The relationship between the logarithmic value (log 0 of the time t at which the second peak P2 is effected and the temperature of the immersion liquid on the coated face side becomes linear. The slope of the above relationship becomes the same as the slope which can be obtained when the logarithmic values (log D) of the diffusion coefficient D of the immersion liquid in the film and the measuring temperatures employed are correlated.

Accordingly, the diffusional characteristics of the immersion liquid can be judged by the time t relating to the second peak P2. After having been brought into contact with the film side, the corrosion liquid is diffused into the paint film, and the atomic hydrogen produced through corrosion on the metal surface is diffused in the metal to reach the non-paint film coated metal.

Upon the quantitative calculation of the characteristic features described above, the time t relating to the second peak is considered to be the time required for the atomic hydrogen to reach the non-coated surface. Thus, the relation tp2 = If2/Df + lH2/DH is established, wherein If is the film thickness, 1H is the metallic plate thickness, Df is a diffusion coefficient of the liquid in the film and DH is a diffusion coefficient of the hydrogen in the metal. Such being the case, if the value of DH is given, the value of the diffusion coefficient Df can be determined by the following relationship, i.e., Df = (tp2-/H2/DH)//2' Furthermore, the existence of the second peak P2 is subject to the paint film type to be employed.More specifically, in the case of the dry type, the existing amount of water near the steel face under the film is relatively larger when compared with that for the use of the baking type. The corrosion reaction always proceeds. However, as far as the use of the dry type is concerned, the reactivity of the dry type with the water which newly reaches to the steel face through the film is not so high. In contrast to the situation described above, the reactivity of the baking type with the water is quite high. Accordingly, the occurrence of the second peak P2 is confirmed in the case where the baking type is employed, while it is not confirmed in the case where the dry type is employed. Also, the second peak was not confirmed in the case where the paints of the aging deterioration type are employed as the film.

The rising point tc of the third peak P3 is rendered earlier in the use of the specimen B than in the use of the specimen A. This fact shows that the corrosion can be considered to start relatively earlier for the specimen B, since the time point denoted by to almost coincides with the intersection between the curve C shown in Fig. 44 (the variation of the ionization current which is caused by the discharging or flowing out phenomenon of the hydrogen dissolved in the steel) and the curve D (the variation of the ionization current which is caused by the discharging or flowing out phenomenon of the hydrogen newly melted in the steel through the corrosion reaction). It is confirmed that this fact coincides with the characteristics of the paint used and the production of the red rust.

In particular, in high tensile steel which often causes the corrosion reaction due to the hydrogen, such as hydrogen cracking or the like, the relationship, i.e., T= C tc-k (hrs.) has already been confirmed to be established between the breaking time or the time required for the breaking time T and the rising time 4 of the third peak P3, wherein C is a constant related to the characteristics of the steel material itself (for example, as the tensile strength is larger, it becomes smaller) and the grade or magnitude of the tensile stress applied to the specimen plate W (as the stress being applied is larger, it is smaller), k is a constant related to the thickness of the steel material employed (as the material becomes thicker, it becomes larger).

The steeper rising feature of the third peak P3 with respect to the specimen B shows the faster generating rate of the melted hydrogen. This denotes a relatively faster corrosion reaction.

Also, since the area (which specifies the quantity of electricity Q3) of the third peak P3 shows the resolution amount of the atomic hydrogen in the steel, which is caused through the corrosion reaction on the film side, the area described above relates to the corrosion amount of the coated face. More specifically, with respect to the relation between the area (the resolution amount of the atomic hydrogen in the steel) and the corrosion amount as described above, the relation is further confirmed from the results of Fig. 46 showing the relationship between the length F1 of visually observed filiform rust and the amount of the hydrogen melted in the coated steel plate which is calaulated from the quantity of electricity of the third peak P3, and from the results of Fig. 47 showing the relationship between the visually observed corrosion width lcorr.

according to the salt spray test and the amount of the hydrogen melted in the coated steel plate, and from the results of Fig. 48 showing the relationship between the pitting corrosion depth HP obtained by microscopic observation and the amount of the hydrogen dissolved in the coated steel plate.

Accordingly, a relation between the quantity of electricity Q3 specific to the third peak and the crevice-rust width 1corr in the case of the lateral corrosion is given as follows: /cor. =a,Q,-K, wherein a, is a constant specified by the corrosion occurring environmental conditions; and K, is a constant determined by the corrosion causing environmental conditions together with the pHvalue of a water-soluble material contained in the coated film.

Also, a relation between the quantity of electricity Q3 of the third peak and the filiform rust length Fl on the coated film side can be given as follows: Fl=a2Q wherein a, is the same as described hereinabove, and a2 is a constant specified by the corrosion causing environmental conditions together with the coated film.

Furthermore, the quantity of electricity Q3 of the third peak P3 is proportional to the spontaneous electrode potential (corrosion potential) Ecorr. of the specimen plate W and the relation between them can be given as follows: Ecorr. = a,O, + Eo (volt) wherein a3 and Eo are constants each determined by the corrosion occurring environmental conditions. For example, as the pH-value of the water-soluble material inside the film is smaller, the value of the constant a3 becomes larger and the value of the constant Eo becomes more cathodic. On the other hand, as the pH-value becomes higher, the value of a3 becomes closer to zero.

O, is related to the pH-value and can be correlated as follows: O3 =k(SpH)-8 wherein k is a constant related to the soluble material in the film, the corrosion causing environmental conditions, the film diffusion coefficient specific to the coated film, etc., and (SpH) is the pH-value prevailing over the metal surface under the coated film.

Moreover, when, for example, a zinc-platesd layer or zinc-rich undercoating film exists, Eo shows extremely cathodic (-) in potential. The value of Q3 is generally larger too. When the zinc-plated layer, etc. exists between the film and the metal surface, the corrosion preventing performance is larger. Therefore, if it is assumed that the corrosion preventing performance is designated as Afi the corrosion preventing performance can be given as follows: Af = lEcarri + 03 wherein Ecorr. is normally more cathodic ( - ) in potential when compared with that of the spontaneous electrode potential of the steel under the same corrosion causing environmental conditions.Thus, the corrosion causing performance or capability can be estimated from the corrosion potential Ecorr. and the quantity of electricity Q3 of the third peak.

It is to be noted here that some steel plates do not show such third peak P3 as shown in Figs.

44 and 45. For example, as regards the specimen steel plate, the polished steel plate was first coated or treated by the zinc-phosphate-film treatment and, then, was further coated on its one side face with a styrene-butadiene block terpolymer paint, tannic acid of 1 % by weight being mixed therein as a rust preventing agent. The coated face side was in contact with 3% NaCI solution of 50"C, while the non-coated face side was in contact with an one normal solution of NaOH of 15"C. The specimen plate was immersed for 20 hours so as to measure the electrolysis current (see Fig. 49).In this case, when the visual observation is further taken into account, one can understand the reason why the third peak P3 is not seen stems from the phenomenological feature that red rust was not formed, but alkaline blisters without rust were spotted among the black rusts.

The relationship between the blister on the film side and the quantity of electricity Q3 of the third peak can be given as follows: B= 1/(b.03) 100 (%) wherein b is a constant determined by the corrosion causing environmental conditions and B (%) is the proportion of the blister area related to the measuring area of the specimen plate W.

As will be apparent from the description with reference to Fig. 44, it is to be noted here again that the curve A shown in Fig. 44 is the overall curve superimposedly composed of the curves B, C, D. Therefore, for example, the second peak P2 is difficult to achieve when the rising step of the curve D is relatively earlier than those of the rest curves. Thus, various variational features relating to the curve A are brought about by the combined effects of the characteristics of the coated steel plate and the corrosion liquid.

Accordingly, the observation of the second and third peaks is to be performed for the corrosion detecting purpose.

As regards the second peak detecting circuit, the same differentiation circuit may be used as is used in the detection of the peak or the stage portion of the above-described cathodic or anodic polarization curve. Referring back to Fig. 10, when the second peak exists (in the case of YES), the film of the specimen can be judged as the baking-type film (data (2)). When no second peak exists (in the case of NO), the film of the specimen plate can be judged as the dry type or the aging deterioration film (data (3)).

The third peak detecting circuit 23 may be also composed of the same means as can be found in the second peak detecting circuit 22.

When the third peak exists (in the case of YES), the peak area is measured, so as to detect the hydrogen amount (data (4)) which concerns the corrosion reaction to be effected on the film side. Then, the rising slope of the peak is measured so as to detect the diffusion rate of hydrogen (data (5)). In addition, the rising time point tc of the third peak is detected so as to predict the breaking time (data (6)) of the specimen plate W. On the other hand, when no third peak exists, no rising portion of the third peak exists. Thus, the superior corrosion resistance (data (8)) is displayed.

Referring now to Fig. 50, there is shown a block diagram of an apparatus which can detect the existence of the second peak P2 from the electrolysis current variation, the area (the quantity of electricity) of the second peak P2, the time of the second peak (the second peak generating time), the existence of the third peak P3, the rising time relating to the third peak P3, and the time (the generating time) and area (the quantity of electricity) of the third peak P3.

When the typical electrolysis current variation to be indicated by the dotted curve A of Fig. 44 is input into the apparatus as shown in Fig. 50 through the electrolysis current detecting means 7' shown in Fig. 50 (corresponding to the means of Fig. 1, having the same numeral number), the aging time up to the local maximum, minimum point P1, X, P2, tc and P3 of curve A, namely, time up to the first peak point or apex, time up to the second peak rising point and time up to the second peak point, time up to the third peak rising point and time up to the third peak point are respectively detected. Furthermore, the area of the second peak P2 and the area of the third peak P3 (area after the third peak rising point) are detected. Each of the signals is sent to the processor unit 20 shown in Fig. 1.

More specifically, upon the measurement relating to the non-film side, the electrolysis current signal is input into the differentiation circuit 401 and to a differential circuit 402.

A signal which was input to the differentiation circuit 401 is differentiated here. When the signal at the local maximum point P1 of the curve A is input into the differentiation circuit 401, the output naturally becomes zero. A zero voltage detection circuit 403 is further connected to the output side of the differentiation circuit 401 to detect the existence of the inflection point.

The inflection existence signal is amplified -once by the amplifier 404 and is input into a contact switching circuit 405. Thus, the contact switching circuit 405 is changed over from the contact Al of the measurement start of a multistage contact L1 to the contact A2.

An oscillator 406 is connected to respective contacts Al to A5. The oscillator 406 transmits a pulse signal to a time-counting circuit 407 by a start signal (measuring start), thereby to permit the time-counting circuit 407 to count the number of the pulses and to store or to display the aging time. The counter-up number of the pulses is transmitted to a time-counting circuit 408 to add and to count the counted-up number up to the following local minimum point X. Since the contacts Al to A5 are arranged to be changed over among the respective local minimum and maximum points P1, X, P2 and tc, the aging time up to each of the inflection points described above is stored or displayed in each of the time-counting circuits 407 to 411, respectively.

As described above, the some specimen plates, the second peak P2 may not appear. In this case, it is to be noted here that the respective outputs of the time-counting circuits 408 and 409 concerning the second peak P2 are adopted for, or become, the time of to and the time of P3.

On the other hand, the electrolysis current signal which was input to the differential circuit 402 includes the current portion (shown by the curve B in Fig. 44) which is caused by the corrosion preventing and corroding operations between NaOH and the reverse face of the steel plate, and does not directly concern the present corrosion evaluation purpose. Therefore, to subtract the current portion described above, the variation in the curve B is output from a storage circuit 41 2 which stores the variation of the curve B in advance by a starting signal (measurement start), thereby to input the variation of the curve B to the differential circuit 402.

Accordingly, the output signal from the differential circuit 402 represents the variation provided through a subtraction operation of the curve B from the variation of the curve A, i.e., the combined variation to be effected by the curve C and the curve D. The compensated output is then input to an integration circuit 41 3. The output side of the integration circuit 41 3 is connected to a second peak (P2) area display circuit 41 4 and a third peak (P3) area display circuit 41 5 through a multistage contact L2. The multistage contact L2 per se is controlled by the contact switching circuit 405 described above.Since the second peak (P2) area displaying circuit 414 is connected to respective contacts A2 to A4, the integration value of the curve C from the inflection point X to the inflection point tc is displayed. Since the third peak (P3) area displaying circuit 41 5 is connected to respective contacts A4 to A5 the integral vaue of the curve D after the inflection point tc is displayed. These values of the areas each being specified by the quantity of electricity are sent to the processor unit 20 to perform the above-described purpose.

As described in the foregoing, according to the present invention, the quantitative or qualitative measurement values which are effective to estimate the corrosion of the specimen plate W can be obtained from the measurements concerning either each or both of the film side (surface side) and the non-film side (reverse side) of the specimen plate W. The user or the operator can select desired measured values optionally from the above-described measured values. With respect to the coating metallic material in practical use, the corrosion can be evaluated, with the corrosion causing environmental conditions, the film characteristics (features of the film including the film thickness and details concerning pre-treatment operation), tensile stress and metallic characteristics (metal type and thickness, for example) being simultaneously taken into account. In the apparatus of the present invention, each of the functions of, for example, collecting, recording and calculating i.e., the processor unit, can be arranged to be performed by a plurality of independents mean. Furthermore, with respect to a step controlling means, it is not necessarily composed of means for centrally controlling each of the steps.

Although the present invention has been fully described by way of example with reference to the accompanying drawings, it is to be noted here that various changes and modifications are apparent to those skilled in the art. Therefore, unless otherwise such changes and modifications depart from the scope of the present invention, they should be construed as included therein.

Claims

1. A corrosion evaluation testing method for coated metallic material which method comprises the steps of: (i) placing the film side of a metallic specimen plate, at least one face of which has been coated with a paint film, in contact with a given corrosive medium; (ii) detecting the spontaneous electrode potential of said specimen plate through a first reference electrode and potentiostatically electrolysing said film side at said spontaneous electrode potential; (iii) causing said film side to be selectively polarized by a cathodic-anodic pulsation polarization method and by a linear potential sweep polarization method with the help of a first counter-electrode thereby to detect the polarization current and thereafter to detect the existence of any film defect; and (iv) selectively subtracting from said polarization current a current portion correspondingly caused by the electric resistance specific to said film, thereby detecting an infinitesimally small current-potential variation so as further to detect said polarization current caused by the application of said cathodic-anodic pulsation polarization to said specimen plate when film defects are not present, and, when film defects are present, detecting said polarization current alternatively caused by the application of said cathodic-anodic polarization to said specimen plate to determine selectively either one or both of a cathodic polarization curve and an anodic polarization curve.

2. A method as claimed in Claim 1, wherein, when said specimen plate is made or chosen to be selectively ready for the evaluation of sulphide stress cracking resistance and/or of embrittlement resistance, or when the reaction behaviour of said film side in said corrosive medium is selectively either not necessary or/and even not possible except for said specimen plate which is ready for the evaluation of said resistance, said method comprises: (v) arranging for a non-film face on the reverse face of said film side to be in contact with an alkaline solution; (vi) detecting the spontaneous electrode potential of said specimen plate through a second reference electrode set in said alkaline solution; and potentiostically electrolysing said non-film face of said specimen plate with a second counterelectrode set at zero potential relative to said potential value of said second reference electrode, thereby to detect the electrolysis current with the help of said second counter-electrode.

3. A method as claimed in Claim 2, wherein the temperature of said corrosive medium is set to be higher than the temperature of said alkaline solution to perform accelerated tests.

4. A method as claimed in Claim 2, wherein said method further comprises the steps of using the detected pulse polarization current to measure the electric resistance specific to said film. and measuring the water permeation rate of said film with the help of said film resistance.

5. A method as claimed in Claim 2, wherein, selectively, when the respective cathodic pulse current value and anodic pulse current value, which constitute said pulse current, are relatively substantially the same in the case of the cathodic-anodic pulsation polarization, and, when the slope of the variation of the polarization current against the polarization potential is substantially linear in the case of the linear potential sweep polarization, confirmation of the lack of defects in said film is obtainable by said method.

6. A method as claimed in Claim 2, wherein, when said film has no defects, said method further comprises the step of detecting the corrosion current with the help of said infinitesimaliy small current-potential variation.

7. A method as claimed in Claim 2, wherein, when said film has defects, said method further comprises the step of detecting corrosion current with the help of either one or of both of said cathodic polarization curve and said anodic polarization curve.

8. A method as claimed in Claim 2, wherein, when the peak or stage portion exists in said negative polarization curve, said method further comprises the step of measuring selectively either one or both of the potential and the current value of said peak or said stage portion.

9. A method as claimed in Claim 8, wherein said method further comprises the step of measuring the aging variation in said potential of said peak or said stage portion for each of said cathodic polarization curves which can be obtained for each given time of contact with said corrosive medium for said specimen plate of said coated metallic material.

10. A method as claimed in Claim 8, wherein said method further comprises the step of measuring the aging variation in said current of said peak or said stage portion for each of said cathodic polarization curves which can be obtained for each given time of contact with said corrosive medium for said specimen plate of said coated metallic material.

11. A method as claimed in Claim 2, wherein said method further comprises the step of detecting an area relating to said peak or an area relating to said stage portion, each of which is surrounded by two said polarization curves which can be obtained in an initial contacting stage between said corrosive medium and said specimen plate, and can be obtained after a given time of contact, when said peak or said stage portion exists in said polarization curve.

1 2. A method as claimed in Claim 2, wherein said method further comprises the step of detecting an area relating to a peak or an area relating to stage portion each of which is surrounded by said polarization curve and the Tafel slope specific to said polarization curve.

1 3. A method as claimed in Claim 12, wherein said method furthr comprises the step of obtaining said area of said peak or said area of said stage portion for each immersion time, and measuring the variation of said area with said immersion time.

1 4. A method as claimed in Claim 2, wherein said method further comprises the step of detecting the existence of a second peak in the aging variation of the electrolysis current.

1 5. A method as claimed in Claim 2, wherein when the aging variation of said electrolysis current has at least one peak, said method further comprises the step of detecting the quantity of electricity of each said peak.

1 6. A method as claimed in Claim 2, wherein said method further comprises the step of detecting selectively either one or both of the rising time and the rising slope of a third peak in the aging variation of the electrolysis current.

1 7. A method for coated metallic material which method comprises the steps of: (i) applying DC voltage, at a constant potential sweep speed, between a coated specimen plate having defects and a counter-electrode with a corrosive medium being interposed, thereby effecting cathodic polarization, after a coated specimen plate having defects has been potentiostatically electrolysed at its spontaneous electrode potential, and thereafter detecting the variation in the potential relating to said coated specimen plate with the help of a reference electrode together with the variation in the current flowing between said coated specimen plate and said counter-electrode; (ii) detecting the existence of a peak or stage portion in the cathodic polarization curve formed from said variation in said potential and said variation in said current; and (iii) detecting whether the spontaneous electrode potential of said coated specimen plate is anodic or cathodic with respect to the spontaneous electrode potential of substrate metal obtained under substantially the same electrochemical conditions, whereby the corrosion form is judged.

1 8. A method as claimed in Claim 17, wherein said method further comprises the step of selectively detecting the existence of a peak and/or the existence of a stage portion in the anodic polarization curve relating to said coated specimen plate.

1 9. A method as claimed in Claim 18, wherein said method further comprises the step of detecting the corrosion current from selectively either one or both of a cathodic polarization 'curve and an anodic polarization curve.

20. A method as claimed in Claim 19, wherein said method further comprises the steps of: (iv) detecting peak potential value, peak current value and either one of the area relating to said peak and the area relating to said stage portion for each immersion time; and (v) measuring the variations of said peak potential value, said peak current value and said area with said immersion time.

21. A method as claimed in Claim 17, wherein said method further comprises the steps of: (iv) potentiostatically electrolysing the non-film face side of said specimen plate at a potential necessary for hydrogen ionization of said non-film face side (the reverse side relative to said film face side), with said film face side being under a corroding state, thereby to detect said electrolysis current; and (v) selectively detecting the quantity of electricity relating to a second peak and the following in the aging variation of said electrolysis current, and the quantity of electricity relating to a third peak in said aging variation of said electrolysis current, which all correspond to the hydrogen amount dissolved in said substrate material due to corrosion on said film face side.

22. A corrosion evaluation testing apparatus which comprises: (i) a pair of measuring cell vessels, which are so contructed that a coated metallic plate, at least one face of which is coated with a film, is seized on both sides by respective side edgeportions of said paired measuring cell vessels, a corrosion medium being placed in one of said paired measuring cell vessels on the film-coated side of said coated metallic plate, thereby to establish a first reference electrode and a first counter-electrode in said corrosion medium, the other one of said paired cell vessels, which is positioned on the reverse side relative to said film coated side, being filled with an alkaline solution subject to the condition that the non-film- coated portion is secured on the side face which is opposite to said film coated side of said plate, a second reference electrode and a second counter-electrode being established in said alkaline solution; (ii) a first spontaneous electrode potential detecting means which is connected to said first reference electrode and said coated metallic plate, thereby to detect the electrode potential of the coated metallic plate in said corrosion medium; (iii) a first potentiostat means which is connected to said first counter-electrode and said coated metallic plate thereby to potentiostatically electrolyse said coated metallic plate at its spontaneous electrode potential in response to a potential signal output from said first spontaneous electrode potential detecting means;; (iv) a pulse potential applying means which is connected to said first counter-electrode and said metallic plate, thereby to apply a cathodic-anodic pulsation potential between said first counter-electrode and said coated metallic plate; (v) a linear potential sweep applying means which is connected to said first counter-electrode and said coated metallic plate, thereby to apply a sweep potential between said first counterelectrode and said coated metallic plate, while said sweep potential is arranged to be selectively varied in the anodic mode and in the cathodic mode; (vi) a pulse polarization current detecting means capable of detecting anodic and cathodic pulsation polarization current flowing between said first counter-electrode and said coated metallic plate through the application of said cathodic-anodic pulsation potential;; (vii) a comparator means capable of comparing said anodic and cathodic pulsation polarization currents each being detected by said pulse polarization current detecting means; (viii) a polarization current detecting means capable of detecting polarization current flowing between said first counter-electrode and said coated metallic plate through the application of said sweep potential; (ix) a compensating means which is connected to said linear potential sweep applying means and said pulse polarization current detecting means thereby to subtract a polarization current portion specific to polarization effects caused by an electric resistance specific to said coated film from said anodic and cathodic pulsation polarization currents, or from said polarization current to be caused by the application of said sweep potential;; (x) an infinitesimally small current detecting means capable of detecting infinitesmally small current compensated by said compensating means; (xi) a second spontaneous electrode potential detecting means which is connected to said second reference electrode and said coated metallic plate, thereby to detect the electrode potential of said coated metallic plate; (xii) a second potentiostat means which is connected to said second counter-electrode and said coated metallic plate, thereby to set said coated metallic plate at relative zero potential with respect to said second reference electrode in response to a potential signal output from said second spontaneous electrode potential detecting means;; (xiii) an electrolysis current detecting means which is connected to said second counterelectrode and said coated metallic plate, thereby to detect the electrolysis current flowing between said second counter-electrode and said coated metallic plate; (xiv) a control means connected to each of said means, thereby to control them for the measurement relating to said film coated side in the sequence set forth, said control means actuating said first spontaneous electrode potential detecting means and said first potentiostat means so that said coated metallic plate is potentiastatically electrolysed at its spontaneous electrode potential, said control means actuating said pulse potential applying means, said pulse polarization current detecting means and said comparator means, and said control means actuating said linear potential sweep applying means and said polarization current detecting means, the arrangement being such that, when the signal output from said comparator means is a signal carrying non-defects information thereon, said compensating means and said infinitesmally small current detecting means are simultaneously actuated, whereby said measurement relating to said film-coated side is conducted; before or after said measurement relating to said film-coated side, said control means first actuating said second spontaneous electrode potential detecting means and said second potentiostat means, thereby to set said coated metallic plate to relative zero potential with respect to said second reference electrode, and thereafter to actuate said electrolysis current detecting means; and (xv) a collecting, recording and calculating means which is connected to at least said pulse potential applying means, said pulse polarization current detecting means, said linear potential sweep applying means, said polarization current detecting means, said infinitesimally small current detecting means and said electrolysis current detecting means, thereby to operate selectively said anodic or cathodic polarization curve and said infinitesimally small currentpotential variation curve and, when required, the aging variation of said electrolysis current, and to at least store the operating results.

23. An apparatus as claimed in Claim 22, wherein said collecting, recording and calculating means calculate the electric resistance specific to said coated film in response to the signals which are each output from said pulse potential applying means and said pulse polarization current detecting means, said collecting, recording and calculating means being capable of calculating the water permeation rate from the electric resistance value of said coated film.

24. An apparatus as claimed in Claim 22, wherein said collecting and recording and calculating means are adapted to calculate said corrosion current selectively from said cathodic and anodic polarization curves and from said infinitesimally small current-potential variation curve.

25. An apparatus as claimed in Claim 22, wherein said apparatus further comprises a first peak detecting means for differentiating a polarization current variation curve and a peak potential and/or current detecting means, the respective input sides of the latter means being each connected to the polarization current detecting means with respective output sides being each connected to said collecting, recording and calculating means, the output signal of said peak detecting means being selectively calculated and detected selectively by said collecting, recording and calculating means and a zero output detecting means to obtain a peak existence signal, the output of said peak potential and/or current detecting means being input to said collecting, recording and calculating means with the help of said control means in response to said peak existence signal, and, furthermore, in respect of said peak existence signal. the arrangement being such that the peak area relating to the cathodic polarization curve which is selectively being formed and has been formed within said collecting, recording and calculating means is capable of being calculated and thereafter stored in said collecting, recording and calculating means itself.

26. An apparatus as claimed in Claim 22, wherein said apparatus further comprises an electrolysis current peak detecting means for differentiating electrolysis current variation, an electrolysis current peak area detecting means for integrating said electrolysis current variation and a time counting means for counting the time after the measuring start, said electrolysis current detecting means being connected to respective input sides of said electrolysis current peak detecting means and said electrolysis current peak area detection means with the output side of said electrolysis current peak detecting means being connected selectively to said collecting, recording and calculating means and to said zero output detecting means selectively to calculate and to select said output signal of said electrolysis current peak detecting means, thereby to actuate said time counting means to store and to display the aging time of the generation of said peak signal in response to a peak signal with the help of said control means as well as said electrolysis peak area detecting means so as to integrate said electroiysis current variation between said measuring start time and said time of generation of said peak signal, and then to store and to display the integral result.

27. A corrosion evaluation testing apparatus which comprises: (i) a peak detection means for differentiating the cathodic polarization curve specific to a coated metallic material having film defects therein; (ii) a first spontaneous electrole potential detection means for detecting the spontaneous electrode potential of said coated metallic material in a corrosive medium; (iii) a means for inputing the spontaneous electrode potential of the substrate metal of said coated metallic material which is to be obtained in the same corrosive condition as can be found in said corrosive medium;; (iv) a calculating means whose input side is connected with said peak detecting means, said first spontaneous electrode potential detecting means and said inputting means, said calculating means being arranged to determine whether or not a peak exists in said cathodic polarization curve in response to the signal output from said peak detecting means as well as whether said spontaneous electrode potential of said coated metallic material is relatively more anodic or more cathodic with respect to said spontaneous electrode potential of said substrate metal, thereby to detect either one of pitting corrosion and crevice corrosion through the combination of said peak-existing signal or peak-absent signal and a resultant signal resultantly obtained from said comparison concerning the spontaneous electrode potential; and (v) at least one display means.

28. An apparatus as claimed in Claim 27, wherein said peak detecting means is adapted to differentiate the anodic polarization curve, said calculating means being further capable of adding said signal group selectively to a peak-existing signal in said anodic polarization curve and to a peak-absent signal in said anodic polarization curve, thereby to detect either one of said pitting corrosion and said crevicecorrosion through the resultant combination of all said signals.

29. An apparatus as claimed in Claim 28, wherein a cathodic and/or anodic polarization current inputting means used to obtain corrosion current and a means inputting said corrosion current are further connecting to said input side of said calculating means, information relating to corrosion volume being obtained according to the input signal relating to said corrosion current being combined with the forming signal selectively concerned with said pitting corrosion and said crevice corrosion stereoscopically to illustrate the corrosion form with the help of said display means.

30. An apparatus as claimed in Claim 29, wherein at least one of the means for inputting signals carrying a peak potential, peak current and/or peak areas each relating to said cathodic polarization curve and the means for imputting the quantity of electricity specific to the second peak and its following in the electrolysis current variation is selectively connected to said input side of said calculating means, so that a signal output from said one of the two means can make it possible for said calculating means selectively to operate the rust width when in the case of said crevice corrosion, and to operate the depth of said pitting when in the case of said pitting corrosion, whereby said display means is capable of illustrating said stereoscopic corrosion form in a highly detailed state.

31. A method as claimed in Claim 1, substantially as herein described with reference to the accompanying drawings and/or any of the specific examples.

32. An apparatus as claimed in Claim 22, substantially as herein described with reference to the accompanying drawings and/or any of the specific examples.