JP4593382B2 - Method for measuring corrosion rate of metal and method for preventing metal corrosion by this method - Google Patents

Description

  The present invention relates to a method for measuring a corrosion rate by calculating a corrosion rate parameter of a metal when evaluating the corrosion state of the metal, and a method for preventing the corrosion of a metal by using an anticorrosion or a corrosion inhibitor using the same. is there.

  Conventionally, as an anticorrosion method for metals used in corrosive environments, an electrocorrosion protection method has been frequently used. However, there is no clear theoretical basis for the optimum set potential in cathodic protection.

  In other words, it is necessary to maintain the metal surface potential in the insensitive region known in the thermodynamic potential-pH diagram in electrocorrosion protection, and for this purpose, it is desirable to make the corrosion protection potential as low as possible. It is said that.

  However, setting the anticorrosion potential lower than necessary not only reduces the economic efficiency, but also increases the amount of hydrogen generated on the surface of the anticorrosion target metal. Secondary problems such as causing peeling occur.

  In general, the anodic dissolution reaction rate of a metal is represented by [Formula 4] as described in Non-Patent Document 1, for example.

Where β _a is the Tafel slope of the anode reaction, E _corr is the corrosion potential in the natural corrosion state (ie, no DC polarization is applied), i _corr is the corrosion rate in the natural corrosion state (ie, no DC polarization is applied), E is an arbitrary set potential, and ia is _a corrosion rate at an arbitrary set potential.

Accordingly, if determined [Equation 4] is a specific quantitative indicators, by determining the corrosion rate i _a in the setting potential under cathodic protection conditions to target, the specific values of anticorrosion potential E to be set It becomes possible to decide.

In order to actually measure β _a , since | E | and | i _a | are very large values, that is, when | E | >> 0, [Expression 4] is approximated by [Expression 5]. Using [Equation 5], a method is known in which β _a is determined from _a linear gradient of log | i _a | −E by applying a large current and anodic dissolution macropolarization to an object.

However, in order to accurately measure β _a by this method, it is necessary to have a large polarization that dissolves the metal, that is, accelerates the corrosion state, and the current density needs to be 10 to 100 times that of the natural corrosion state. The above method has not been put to practical use because there is a risk of accelerating corrosion of the object in order to determine β _a .

  Under such circumstances, in Patent Document 1, as a more theoretical method of setting the anticorrosion potential, the polarization resistance is measured while changing the set anticorrosion potential, and the polarization resistance shows the maximum value. An anticorrosion potential setting method in which the potential is an optimum anticorrosion potential is disclosed.

  However, in this method, it is necessary to measure the maximum value of the polarization resistance in an actual cathodic protection system, and a large-scale power supply device more than necessary to perform actual cathodic protection is required for the measurement. From the practical point of view, it includes several issues.

As one of means for evaluating the corrosion rate of a metal, a method of determining from the corrosion rate is known. As a method for measuring the corrosion rate, an electrochemical method for calculating the corrosion rate i _corr by measuring the polarization conductance Y _corr defined by the polarization resistance R _p of the sample metal specimen or its reciprocal is known. ing.

Incidentally, it is known that there is a relation of [Equation 6] between the rate parameter Y _corr of the corrosion reaction, the Faraday current i flowing through the sample metal piece, and the minute polarization value E.

On the other hand, theoretically, the relationship as in [Equation 7] is established between the polarization conductance Y _corr and the corrosion rate i _corr .

In this equation, β _a and β _c are Tafel gradients of the anode reaction and the cathode reaction, respectively.

On the other hand, there is a relationship of [Equation 8] between the corrosion rate i _corr and the mass reduction rate V.

  In [Expression 8], M is the atomic weight of the sample metal, Z is the ionic valence of the eluted metal, F is the Faraday constant, and ρ is the density of the metal.

Here, it is said that the Tafel gradients β _a and β _c are determined by a combination of a metal and a corrosive environment.

Therefore, in the macro, β _a and β _c are constants determined by the metal and the corrosive environment, but the actual corrosive environment changes the concentration of ions contained as the corrosion progresses, Strictly or microscopically, the state of the metal interface and the corrosion environment are constantly changing, such as when substances in a specific corrosion environment concentrate on the corrosion interface, and corrosion products accumulate at the metal interface over time. It changes and the corrosion state of the metal is not constant.

Therefore, in order to accurately evaluate the corrosion rate i _corr , it is necessary to measure the polarization conductance Y _corr and the Tafel gradients β _a and β _c . The Tafel slopes β _a and β _c are important parameters in the anodic and cathodic reactions that determine the corrosion rate i _corr .

Furthermore, the corrosion inhibitor generally acts directly on the anodic reaction or the cathodic reaction to directly change the Tafel slopes β _a and β _c of the anodic reaction and the cathodic reaction, thereby suppressing the corrosion. It is necessary to measure the polarization conductance Y _corr and the Tafel slopes β _a and β _c in order to accurately measure the corrosion rate i _corr after adding the corrosion inhibitor electrochemically by capturing the effect of the agent It becomes.

Therefore, it is extremely important to simultaneously and accurately obtain the values of the polarization conductance Y _corr and the Tafel slopes β _a and β _c when evaluating the corrosion of metal materials or examining the anticorrosion method from the corrosion rate. .

Conventionally, a polarization resistance method is known as means for obtaining the polarization resistance R _p or the polarization conductance Y _corr . This method, which is known as a so-called linear polarization resistance method, is obtained from [Equation 9] from the potential change ΔE from the natural potential when a minute polarization is given and the current density Δi flowing at that time.

However, β _a and β _c cannot be obtained by this method.

Thus, for the polarization resistance method in which only the value of R _p can be measured and β _a and β _c cannot be measured, | ΔE | and | Δi | 11], there is a method for obtaining β _a , β _c , and i _corr from actually measured data of ΔE−log | Δi |.

  However, in this method, it is indispensable to apply a large ΔE or Δi from the outside so as to greatly change the corrosion state of the metal, and this application has a problem that the corrosion state of the metal changes greatly.

Also, it is judged that it is difficult to measure β _a and β _c simultaneously with the polarization resistance R _p or the polarization conductance Y _corr, and simulates an environment close to the target environment and target metal combination, A method is used in which β _a · β _c /2.3303 (β _a + β _c ) is determined from the measured data of ΔE-log | Δi |, and this is treated as _a constant, but the measurement accuracy is ± 200%. There was a problem that the measurement error was extremely large.

Although the value of apparent β _a · β _c /2.303 (β _a + β _c ) can be set, β _a and β _c cannot be determined as individual parameters, and the obtained speed parameters are There is a limit.

Furthermore, the corrosion inhibitor generally acts directly on the anodic reaction or the cathodic reaction to directly change the Tafel slopes β _a and β _c of the anodic reaction and the cathodic reaction described above, thereby reacting either or both of these reactions. The speed is decreased, and as a result, corrosion is suppressed. However, the above parameters vary depending on the addition concentration and type of the corrosion inhibitor.

  Therefore, it is difficult to quantitatively evaluate the corrosion inhibitory effect of the corrosion inhibitor quantitatively with the above-described method that is not real-time measurement.

On the other hand, in Patent Document 2, a constant charge amount is applied to a metal, and the polarization resistance R _p or the polarization conductance Y _corr and the Tafel slope β _{a are} determined from the time response change of the polarization value | ΔE | observed at that time. And β _c are disclosed.

  This method is excellent in principle, but on the other hand, a dedicated device is required to give a certain amount of charge to the target metal, and measurement is not possible with the commonly used potentiostats and galvanostats. There was a problem that it was possible.

JP 61-235580 A JP 54-054092 A Akira Fujishima et al .: Electrochemical measurement method, Hakuhodo Publishing (1984)

The present invention has been made in view of such circumstances, and an object of the present invention is to easily perform polarization conductance in a metal corrosion reaction even in a noisy environment using an electrochemical measurement apparatus that has been widely distributed. Y _corr and Tafel slopes β _a and β _c can be determined simultaneously, thereby providing a method for accurately and quickly measuring the corrosion rate of a metal, and further, the corrosion rate of the metal Control the anticorrosion potential of the electrocorrosion according to the corrosion rate set as a target, calculated using the measurement method, or control the addition amount of the corrosion inhibitor so as to be in the desired corrosion rate range, An object of the present invention is to provide a metal corrosion prevention method capable of accurately preventing metal corrosion.

  The gist of the present invention is as follows.

(1) direct polarization current density is measured in a state that gave fine DC polarization to the extent that does not impair the corrosion state of the target metal i _s the polarization conductance Y _p, and is measured in a state that does not give a direct polarization From the polarization conductance Y _corr , the anode Tafel slope β _a and the cathode Tafel slope β _c satisfying the relation of [Formula 1] are calculated, and then the corrosion rate i _corr is calculated by [Formula 2]. , A method of measuring the corrosion rate of metals.

(2) In the method for preventing corrosion of a metal by electrocorrosion, the anode Tafel slope β _a and the corrosion rate i _corr are calculated by the method for measuring the corrosion rate of the metal according to claim 1, and the measured corrosion potential E _corr from so as to satisfy the relationship of [expression 3], characterized in that cathodic protection by anticorrosion potential E which is calculated according to the target corrosion rate i _a set arbitrarily, anticorrosive method of the metal.

(3) A corrosion prevention method for metals in an environment to which a corrosion inhibitor is added, wherein the corrosion rate i _corr is calculated by the metal corrosion rate measurement method according to claim 1 and the corrosion rate is monitored quantitatively. In addition, the metal corrosion prevention method is characterized in that, based on the calculated corrosion rate i _corr , the addition amount of the corrosion inhibitor is controlled so as to be within a desired corrosion rate range.

According to the present invention, it is possible to easily and easily determine the polarization conductance Y _corr and the Tafel slopes β _a and β _c in _a metal corrosion reaction simultaneously and easily using a generally distributed electrochemical measuring device. Therefore, when the corrosion state is measured, the metal corrosion rate can be measured quickly and accurately.

Also, the metal using the measuring method of the corrosion rate of the calculated quantitatively, by controlling the anticorrosion potential E of cathodic protection in accordance with the corrosion rate i _a to be set as a target, or a desired corrosion rate range By controlling the amount of such a corrosion inhibitor to be added, it is possible to provide a metal anticorrosion method capable of accurately preventing metal corrosion.

The present inventor studied to simultaneously and easily obtain the polarization conductance Y _corr and the Tafel gradients β _a and β _c in the corrosion reaction of the metal. As a result, the above [Formula 1] was newly derived. Thus, it has been newly found that the polarization conductance Y _corr and the Tafel gradients β _a and β _c can be obtained simultaneously and rapidly, and the metal corrosion rate can be calculated with high accuracy.

For cathodic protection, it is possible to quantitatively calculate a relationship between the anode reaction corrosion rate and the potential of the metal, thereby, in accordance with the target corrosion rate i _a, the anticorrosion potential E at the time of cathodic protection of metal It is possible to determine quantitatively.

Here, [Formula 1] is based on the Butler-Volmer formula relating to the reaction kinetics of an electrochemical reaction based on a conventionally known composite electrode reaction theory, as described in Non-Patent Document 1 and the like. , the polarization conductance Y _p in the presence of a steady DC current density i _s, and determined by analyzing the mutual relationship of the polarization conductance Y _corr measured in a state do not give a direct polarization, mutual specific relationships Has succeeded in finding out.

As a result, the corrosion rate i _corr can be measured quickly and easily with high accuracy by [Equation 2].

Further, the anticorrosive method of the metal by cathodic protection, made it possible according to the target corrosion rate i _a, quantitatively determining the corrosion potential E at the time of cathodic protection of metal by [Equation 3].

  In addition, the corrosion rate is discussed over a long period of several weeks to several years, and it is necessary to grasp the change in the corrosion state and the presence or absence of the corrosion state within that period. [Equation 3] can be grasped within a few minutes to several tens of minutes, and the corrosion state can be evaluated almost in real time and the required anticorrosion potential can be determined.

That is, when applying the anti-corrosion, the target corrosion rate i _a (target anode reaction rate current density) under the anti-corrosion state is specifically determined, and the target of the anti-corrosion target potential required to achieve the target is [ It is possible to quickly determine by Equation 3].

In addition, in the metal corrosion prevention method using a corrosion inhibitor, the corrosion rate i _corr is measured by this method after appropriately adding the corrosion inhibitor, and whether or not a desired inhibition effect is exhibited as a specific corrosion rate. Can be confirmed.

  If the desired corrosion inhibitory effect is not obtained, a corrosion inhibitor can be added as appropriate to control the amount of addition of the corrosion inhibitor until a sufficient desired corrosion inhibitory effect is obtained. It becomes possible.

  The following explains in detail the method for measuring the corrosion rate and the method for setting the anticorrosion potential based on it, and the method for measuring the corrosion rate of the target metal in the corrosion state in the environment where the corrosion inhibitor is added. .

First, the corrosion potential E _{corr in a} state where no DC polarization is applied to the metal subject to the investigation of the corrosion state and the polarization conductance Y _corr at the corrosion potential are measured. The same applies to measurement in an environment to which a corrosion inhibitor is added.

On the other hand, any corrosion potential, under slight DC polarization state so as not to impair the corrosion state, measuring a DC polarization current density i _s, the polarization conductance Y _p under a DC polarization state.

  Here, the DC polarization state that is so fine as not to impair the corrosion state may be any DC polarization potential that does not significantly change the corrosion state of the target metal. Although the range is not specified, depending on the amount of noise in the measurement environment, it is generally about ± 100 mV at most in a measurement environment without noise, and about ± 1 to ± 2 V at most in a measurement environment with noise. It is.

  On the other hand, the lower limit value is arbitrarily determined depending on the measurement accuracy of the measuring instrument and the presence state of noise, but 1 mV to several mV is a general lower limit value in a situation where noise can be ignored. .

  A commercially available electrometer can be used for measuring the corrosion potential, and a commercially available power supply device such as a potentiostat or a galvanostat can be used for applying the DC polarization.

  In the present invention, a two-electrode method can be used in addition to the three-electrode method, and in this case, the potential difference between the sample and the counter electrode can be substituted as the corrosion potential.

  In addition, the polarization conductance is measured by a DC polarization method, an impedance method using a sine wave, a measurement method using a rectangular wave, etc., and an analysis method in a frequency domain such as FFT (Fast Fourier Transfer). Can be used in combination.

In order to accurately measure the corrosion rate i _corr , it is preferable to perform measurement by changing the DC polarization to at least three levels with respect to the corrosion potential. Further, as described above, the range of direct current polarization is preferably small enough not to impair the corrosion state, but the range can be arbitrarily set depending on the combination of the metal and the corrosive environment.

Also, in the measurement of the polarization conductances Y _corr and Y _p , it is necessary to give a polarization superimposed on the DC polarization, but the range of polarization for measuring the polarization conductance is arbitrarily determined depending on the combination of the metal and the corrosive environment. It is possible to set.

  However, since the polarization conductance is defined as the reciprocal of the slope of the tangent at the direct current polarization potential of the polarization curve, the smaller the degree of polarization, the more faithful to the definition, and preferably 200 mV or less.

  Further, at 20 mV or less, when the relational expression between the DC polarization potential E and the DC polarization current i: E = a × ln (bi) (where a and b are constants) is i≈0, E≈abi. Since a simple approximate expression is established, it is more preferably set to 20 mV or less.

As described above, the corrosion potential E _corr, polarization conductance Y _corr in corrosion potential, the DC-DC polarization in polarization state current density i _s (at least three levels or more DC polarization state), and measures the Y _p in each DC polarization state . For direct polarization current density i _s, as described above, the accuracy of the measurement the more number of levels is improved.

Next, the anode and cathode Tafel gradients β _a and β _c satisfying [Equation 1] are obtained from the above measurement results by calculation.

[Expression 1], beta _a, in order to be implicit function with respect to beta _c, analytically beta _a, calculating the beta _c is generally a difficult, the calculation of the solution, numerical by computer analysis Can be used.

Also, as an approximation of [Equation 1], [Equation 12] can be used. Since [Equation 12] is also an implicit function with respect to β _a and β _c , β _Although it is generally difficult to calculate _a and β _c , it is possible to use numerical analysis by a computer to calculate the solution.

As a result of the above calculation, the corrosion rate i _corr is calculated by [Expression 2] using the calculated β _a and β _c and the actually measured Y _corr .

  According to this method, in order to grasp the corrosion rate, it is possible to accurately measure within several minutes to several tens of hours. Therefore, since the corrosion rate can be measured in a short time with respect to the measurement point by the method of the present invention for the target metal, the corrosion state can be evaluated almost in real time. Therefore, various examinations and responses can be quickly performed according to the corrosion state.

When applying cathodic protection, further using measured corrosion potential E _corr, the cathode reaction rate current density i _c of the anode reaction rate current density (i.e., corrosion rate) i _a and the metal of the metal, [Equation 13] It becomes possible to calculate by.

Therefore, based on these quantitative relationship, the anode reaction rate current density as a target at the time of applying the cathodic protection, i.e., to arbitrarily set the corrosion rate i _a to the targeted potential necessary to achieve this E is derived from [Equation 13], so that the anticorrosion potential when the metal is anticorrosive can be determined.

  Here, the anticorrosion potential can be determined as the potential E derived by [Equation 13], but a potential lower than the derived potential E may be determined as the anticorrosion potential.

  In recent years, titanium lining, etc., used for the purpose of anti-corrosion and high-strength steel has been used, but hydrogen generated by the application of electro-corrosion may cause embrittlement of these materials. The amount should be as low as possible.

Thus, when hydrogen generation becomes a problem, the hydrogen generation rate can be quantitatively grasped as _{ic in} [Equation 13], and according to this method, hydrogen generation _ic and anticorrosion can be obtained. in view of both the degree i _a, it is also possible to determine the corrosion potential at the time of sacrificial metal.

As for the corrosion rate in the environment to which the corrosion inhibitor is added, the specific corrosion rate i _corr finally calculated from [Equation 2] by the above procedure can be measured almost in real time from the measurement time point. Whether or not the anticorrosion state achieves the target anticorrosion state can be quantitatively determined.

  Therefore, after adding the corrosion inhibitor, the time-dependent transition of the anticorrosion state can be quantitatively monitored by performing the same measurement at appropriate intervals.

Furthermore, as described above, by quantitatively monitoring the time course of the anticorrosion state, the state exceeding the range of the desired corrosion rate based on the calculated corrosion rate i _corr , that is, the corrosion inhibitor. If the effect of the corrosion inhibitor becomes insufficient, the addition amount of the corrosion inhibitor is controlled, and if necessary, the above-mentioned measurement is repeated to manage the addition amount of the corrosion inhibitor. It can be adjusted to be within the speed range.

  Here, the control method of the addition amount of the corrosion inhibitor is not particularly specified, and may be adjusted by one addition amount, or each addition amount may be adjusted in a plurality of times. The concentration of the corrosion inhibitor may be adjusted, and these may be arbitrarily combined.

  Further, the range of the desired corrosion rate is not particularly specified, and may be set as appropriate in consideration of the service life, usage environment, cost constraints, and the like.

  In addition, the above measurement can be performed automatically by computer control or dedicated equipment, and the monitoring of the effect of the corrosion inhibitor and the management of the amount of addition of the corrosion inhibitor can be performed unattended. is there.

  Specifically, a corrosion inhibitor or a solution obtained by diluting it to an appropriate concentration is appropriately added to the target environment according to the effectiveness of the corrosion inhibitor used. It is possible to obtain the optimum addition amount by stopping the charging when a sufficient effect is obtained while monitoring the effect after the charging amount quickly by the method of the present invention.

  Furthermore, the introduction of corrosion inhibitors and the monitoring of their effects can be automated by computer control of monitor measurements, and can be automated by existing corrosion-controlled sequence control of corrosion inhibitors or control of the amount of corrosion inhibitor input by computer control. It is possible, and by combining both, it becomes possible to manage the addition amount of the corrosion inhibitor unattended.

  Examples of the present invention are shown below.

  Steel (JIS SS400 steel), aluminum, and zinc were taken up as examples of the target metal.

  The test piece was obtained by cutting a commercially available rolled material to 30 mm × 50 mm, polishing the test surface with # 800 emery paper, and connecting the lead wire for electrochemical measurement to this test piece, All surfaces other than the test surface were coated with resin and used for the test.

  For the test solution, NaCl 3.0% by mass and 0.03% by mass are used, and the pH before the corrosion test is set to three levels of 2, 7, and 13, and the test piece is immersed in these solutions. And measured. In addition, all the tests were implemented on the conditions of an open air environment and room temperature (about 20-30 degreeC).

  Although the corrosion period differs depending on the combination of the metal and the solution, the corrosion period was set to 2 to 30 days so that the corrosion mass reduction measurement of the test piece can be stably performed.

  For electrochemical measurement, a potentiostat (Model 2000 manufactured by Toho Giken) was used in combination with a function generator (FG-8 manufactured by Toho Giken).

  All electrochemical measurements in the examples were carried out by the three-electrode method. A platinum plate was used as a counter electrode required at that time, and a self-made Ag / AgCl electrode was used as a reference electrode.

  In general, electrochemical measurement is performed in a low noise environment with few disturbance factors, but in this example, ± 80 mV is used for the purpose of performing evaluation including practicality in a practical environment where disturbance factors exist. Measurements were performed in an environment where there was commercial frequency noise.

  Using the above measurement configuration, calculation of corrosion rate parameters and corrosion rate based on various measurements and measurement results were performed according to the following procedure.

(I) The corrosion potential E _corr is measured by an electrometer built in the potentiostat. Hold constant potential at the observed corrosion potential.

(II) A polarization conductance Y _corr is obtained by a linear polarization method, impedance measurement, and measurement using a rectangular wave constant potential with potential control at a constant potential held potential.

  In the impedance measurement and the rectangular wave constant potential measurement, the polarization from the corrosion potential was ± 20 mV, the frequency was 0.01 Hz, and the resistance obtained here was the total resistance.

In addition, in order to correct the solution resistance, a waveform of ± 20 mV and 10 kHz is applied by impedance measurement and measurement using a rectangular wave constant potential to obtain a solution resistance, which is subtracted from the total resistance observed in the low frequency measurement. The polarization resistance R _corr was evaluated and the inverse thereof was evaluated as Y _corr .

Since the polarization conductance Y _corr measured and evaluated by the linear polarization method, impedance measurement, and measurement using a rectangular wave gives almost the same result, the final evaluation value is a rectangular wave constant potential that can be measured most stably. The measured value was adopted.

From the potential state (III) subsequently held at a constant potential, it performs a DC polarization of -15 mV, to measure the direct polarization current density i _s required for DC polarization observed after the polarization holding 3 minutes. Further, the polarization conductance Y _p under direct current polarization is measured by the same procedure as (II).

The measurements of (IV) and (II) and (III) are repeated at least three times, and Y _corr when i _s = 0 and the measured values of _s and Y _p are set as a set to obtain three or more points. Immediately after the measurement is completed, any external polarization such as direct current polarization is stopped and the natural corrosion state is continued.

Using the numerical values obtained by measurement, β _a and β _c satisfying [Equation 1] are obtained by numerical analysis by a computer.

Next, the corrosion rate i _corr is calculated by [Equation 2] from the obtained β _a and β _c .

As described above, the measurements (I) to (IV) were repeated at intervals of 1 to 2.5 hours, and the change over time in the corrosion rate i _corr was measured. The result of the change in the measured corrosion rate with time was integrated over time, and the average corrosion rate until the corrosion test piece was taken out was obtained. All measurements, calculations and instrument control were performed by a microcomputer.

  On the other hand, the coating and corrosion products of the sample taken out after the corrosion test were removed, and the mass change before and after the corrosion test was measured. Based on this mass change, the average corrosion rate due to mass reduction during the corrosion test period was calculated.

  Correspondence between the average corrosion rate calculated according to the present invention and the average corrosion rate obtained by actual measurement of mass reduction is shown in FIGS.

  As is clear from these figures, the average corrosion rate calculated by the present invention is in a very good correspondence with the average corrosion rate obtained by mass reduction, and is excellent in quantitative response within ± 15% under all conditions. I found out.

  In the conventional polarization resistance method, a double logarithmic diagram is used when discussing the correspondence relationship equivalent to that in FIGS. 1 to 3, and if the accuracy is ± 200%, it is judged to be excellent. However, according to the present invention, the corrosion rate can be measured quickly with extremely high accuracy.

  Next, an example in the case where the anticorrosion is applied will be described.

  First, the test target metal, test conditions, and electrochemical measurement conditions were the same as in Example 1, and the calculation of the corrosion rate parameter and the calculation of the corrosion rate were performed in the same manner as in Example 1.

Further, in Expression 3], the target corrosion rate i _a metal subjected to electrolytic protection by setting a 0.01 .mu.A / cm ^2, was determined cathodic protection potential E required for this.

  The test maintained at the anticorrosion potential according to the combination of the metal and the solution determined by the above measurement was carried out for 3 years for each combination of each metal and each solution.

  After conducting corrosion protection for 3 years, the coating and deposits on the test pieces subject to protection were removed and the mass change before and after the test was determined. The mass change result was less than 0.1 mg, and it was confirmed that the combination of all the metals and the solution was in a good cathodic protection state.

  Next, an example relating to corrosion prevention of a metal by a corrosion inhibitor will be described.

  Steel (JIS SS400 steel) was taken up as an example of the target metal. The test piece was obtained by cutting a commercially available rolled material to 30 mm x 50 mm, polishing the test surface with # 800 emery paper, connecting lead wires for electrochemical measurement to this test piece, and testing All surfaces other than the surface were coated with resin and subjected to the test.

  As a test solution, 3% by mass, 0.3% by mass, and 0.03% by mass NaCl solution were used, and the pH was set to 7 without adding a corrosion inhibitor. The test was carried out by immersing the test piece in the above solution prepared with an initial concentration of 0.05 ppm of sodium polyphosphate.

  In addition, the test was implemented on the conditions of an open air environment and room temperature (about 20-30 degreeC). The corrosion period was 1 year.

  The calculation from the electrochemical measurement and the calculation of the corrosion rate parameter to the calculation of the corrosion rate in the natural corrosion state (I) to (IV) was performed in the same manner as in Example 1. Furthermore, in this example, in addition to the above (I) to (IV), the following procedure (V) for adding a corrosion inhibitor was added.

(V) If the measured corrosion rate i _corr exceeds the set 0.01 mm / year with the desired amount of corrosion in this test as the speed at the time of measurement, it is automatically controlled by computer control. The amount of sodium polyphosphate added in the solution was increased by about 0.05 ppm at the total solution concentration.

Within 5 minutes after the addition of the corrosion inhibitor, the procedures (I) to (IV) of Example 1 and the addition of the corrosion inhibitor (V) were repeated. This procedure was a control configuration that was repeated until the corrosion rate i _corr at the time of measurement was less than 0.01 mm / year.

A series of procedures for repeatedly measuring the corrosion rate i _corr and controlling the addition amount of the corrosion inhibitor was performed at intervals of one day, and the change with time of the corrosion rate i _corr was measured. The results of the temporal change of the corrosion rate i _corr measured in this way were integrated over time, and the average corrosion rate until the corrosion test piece was taken out was obtained. As a result, the corrosion amount was 0.006 mm / y or less for all the test pieces.

  On the other hand, the coating and surface deposits of the sample taken out after this year-long corrosion test were removed, and the mass change before and after the corrosion test was measured. Based on this mass change, the average corrosion rate due to mass reduction during the corrosion test period was calculated. As a result, the corrosion amount was 0.005 mm / y or less for all the test pieces.

  From the above, by measuring the corrosion rate in the environment containing the corrosion inhibitor by the method of the present invention, it is possible to accurately monitor the corrosion situation, and further, by managing the addition amount of the corrosion inhibitor It was found that the metal can be stably maintained in a desired anticorrosion state.

  According to the present invention, as described above, the corrosion rate of a metal can be measured quickly and accurately, and when applying anticorrosion, the optimum anticorrosion potential can be set quantitatively. In the case of anticorrosion with an inhibitor, it is possible to confirm the effect and appropriately manage the amount of addition of the corrosion inhibitor. Therefore, the present invention can be effectively used for corrosion evaluation and corrosion prevention of metals.

The average contrast of corrosion rates were evaluated by the average corrosion rate and mass loss was evaluated by the present invention, the vertical axis 0~8000μA / cm ^2, a diagram displayed as a horizontal axis 0~8000μA / cm ^2. The average contrast of corrosion rates were evaluated by the average corrosion rate and mass loss was evaluated by the present invention, the vertical axis 0~1000μA / cm ^2, a diagram displayed as a horizontal axis 0~1000μA / cm ^2. The average contrast of corrosion rates were evaluated by the average corrosion rate and mass loss was evaluated by the present invention, the vertical axis 0~100μA / cm ^2, a diagram displayed as a horizontal axis 0~100μA / cm ^2.

Claims (3)

It is measured in a state that gave fine DC polarization to the extent that does not impair the corrosion state of the target metal DC polarization current density i _s the polarization conductance Y _p, and the polarization conductance Y measured in a state do not give a direct polarization from _corr, is calculated the anode Tafel slope beta _a and cathode Tafel slope beta _c satisfy the relationship of [expression 1], then the corrosion of metals and calculates the corrosion rate i _corr by [equation 2] Speed measurement method.

In the metal anticorrosion method, the anode Tafel slope β _a and the corrosion rate i _corr are calculated by the method for measuring the corrosion rate of the metal according to claim 1, and further, from the measured corrosion potential E _corr , [ so as to satisfy the relationship of formula 3], anticorrosion method of metals characterized in that cathodic protection by anticorrosion potential E which is calculated according to the target corrosion rate i _a set arbitrarily.

A method for preventing corrosion of a metal in an environment to which a corrosion inhibitor is added, wherein the corrosion rate i _corr is calculated by the method for measuring a corrosion rate of a metal according to claim 1 and the corrosion rate is quantitatively monitored. A method for preventing corrosion of a metal, characterized in that the addition amount of a corrosion inhibitor is controlled based on the calculated corrosion rate i _{corr so as} to be in a desired corrosion rate range.

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