JP2011089872A - Method of estimating corrosion rate, and corrosion rate estimating device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of estimating corrosion rate and a corrosion rate estimating device for estimating the corrosion rate in a gap of a metal. <P>SOLUTION: The corrosion rate estimating device includes a test material 11, where two metal plates of which an insulation film on one surface has been removed are disposed at a prescribed interval so that one surface of one metal plate mutually faces that of the other metal plate; an electrochemical impedance measuring means 12 of acquiring the impedance of the test material from the response of an electric signal applied between the metal plates of the test material 11; and a calculation means 13 of calculating the polarization resistance of the metal plates, from the impedance of the test material 11 acquired by the electrochemical impedance measuring means 12. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a method and apparatus for estimating the corrosion rate of a metal.

  As a method for estimating the corrosion rate of a metal material, the metal is exposed to an aqueous solution as one electrode and kept at a constant potential difference with a reference electrode such as a silver / silver chloride electrode. ) (See Non-Patent Document 1, for example). This method is called electrochemical impedance measurement. As shown in FIG. 1, water molecules aggregate, align and adsorb at the interface between the metal and aqueous solution to form an electric double layer. When metal atoms become positive ions from the metal surface, the resistance (polarization resistance) is reduced. When positive ions move offshore, they are further subjected to resistance (solution resistance). It is known that when such a conduction mechanism is modeled, an equivalent circuit as shown in FIG. 2 is obtained (see, for example, Non-Patent Document 2).

  In the equivalent circuit of FIG. 2, the polarization resistance is a resistance when a metal is lost as an ion due to corrosion, and its inverse is proportional to the corrosion rate. If the valence of a metal ion is known, if the voltage to be initially applied is determined, the value of the current that flows when the metal becomes an ion can be calculated from the value of the polarization resistance and the voltage. This current is a current that flows when metal is lost as ions due to corrosion, and indicates the corrosion rate.

  Therefore, in order to estimate the corrosion rate of a metallic material, the metal is exposed as an electrode in an aqueous solution (for example, an aqueous sodium hydroxide solution), and a voltage is applied between any other electrode and the current is applied. 2 is measured, and the value of the polarization resistance in FIG. 2 may be obtained from the response. For example, the applied current is AC and the frequency is gradually changed. In a low frequency state, preferably a direct current, the capacitance component of the electric double layer does not contribute to the flow of current, as is known from the AC circuit theory of electrical engineering. On the other hand, when the frequency is sufficiently increased, the electric double layer capacitance causes an alternating current to flow. Therefore, the electric double layer capacitance C and the polarization resistance Rp can be specified from the equivalent circuit of FIG. 2 by analyzing the current response by changing the frequency and applying alternating current. Conventionally, the value of polarization resistance was obtained by such a method, and the corrosion rate was estimated.

Toyo Technica, Inc. “Electrochemical Measurement Overview” “http://www.toyo.co.jp/solartron/qa.html” (searched on August 10, 2009) Hiroyuki Saito “Durability degradation measures for reinforced concrete columns” 2008 weathering technology research results presentation text, pp. 75-85 (2008)

  As described above, the corrosion rate when the metal is in a general plane could be estimated. However, what has become a problem in recent years is a phenomenon in which metal partially corrodes prematurely due to gaps between papers, structures and bolts. FIG. 3 is a diagram for explaining that a water film is formed on the metal surface even if the relative humidity does not reach the dew point of 100% due to the size of the gap portion (gap radius or the like). In order to analyze the phenomenon in which metal partially corrodes at an early stage, it is necessary to clarify what corrosion rate is obtained at what humidity.

4 and 5 are diagrams for explaining the reason why the crevice corrosion of FIG. 3 occurs. When the vapor pressure in the region of the gap and P _r as in Figure 4, when a range where the magnitude of the surface tension of water can not be neglected is the action of capillary condensation would water rises the capillary only h. The balance of surface tension is as shown in FIG. So, if you calculate the balance of these relationships,
(Formula 1)
P _r / P ₀ = (1−V ₍₁₎ / RT) (2γ / r)
It becomes. Here, P ₀ is atmospheric pressure, V _(l) is the molar volume of water (liquid state), R is a gas constant, T is room temperature (measured in absolute temperature Kelvin K), γ is the surface tension of water, and r is a capillary tube Is the radius.

Among these, since the constants other than _Pr and r are constants, the vapor pressure in the gap portion is determined by the gap diameter r. FIG. 3 is a schematic representation of Equation 1.

  Referring to FIG. 3, it can be seen that a water film is formed even with steam having a relative humidity of about 98% when the clearance is about 1 μm, and a water film is formed even when the relative humidity is smaller when the clearance is smaller than that. That is, when such a small clearance exists, it is considered that the portion forms a water film below the dew point and causes an early corrosion failure.

  However, with such a small gap, it is difficult to examine the state of condensation, to install a reference electrode in that part to control the potential difference, and to insert a counter electrode to reduce the current. There was a problem that it was difficult to confirm the corrosion status of the part.

  In order to solve this problem, an object of the present invention is to provide a corrosion rate estimation method and a corrosion rate estimation apparatus capable of estimating a corrosion rate at a gap portion of a metal.

  In order to achieve the above object, a corrosion rate estimation method and a corrosion rate estimation apparatus according to the present invention are a pair of two metal plates with wiring for taking out voltage current to the outside, and an insulating spacer is inserted between them. The test material in which a desired gap was formed between the two metal plates was used. After this test material was exposed to a predetermined temperature and humidity for a predetermined time, an AC voltage was applied to measure the frequency characteristics of the current response, and the polarization resistance was obtained from the measurement result.

  Specifically, in the corrosion rate estimation method according to the present invention, the metal is applied to a test material in which two metal plates from which an insulating film on one surface is removed are arranged so that the one surface faces at a predetermined interval. After applying the electrochemical impedance measurement procedure to obtain the impedance of the test material from the response of the electrical signal applied between the plates, and the electrochemical impedance measurement procedure, the polarization resistance of the metal plate is calculated from the impedance of the test material And a calculation procedure.

  The corrosion rate estimation apparatus according to the present invention includes a test material in which two metal plates from which an insulating film on one surface is removed are arranged so that the one surface faces each other at a predetermined interval, and between the metal plates of the test material. An electrochemical impedance measuring means for obtaining an impedance of the test material from a response of an electrical signal applied to the electronic signal; a calculating means for calculating a polarization resistance of the metal plate from the impedance of the test material obtained by the electrochemical impedance measuring means; .

  A desired gap is produced using two identical metal plates, and an impedance which is a current response to a voltage applied between the two metal plates in an aqueous solution is measured.

  At this time, since the corrosion generated on each metal plate can be regarded as the same, the equivalent circuit is obtained by connecting the equivalent circuits of FIG. 2 in series as shown in FIG. That is, the circuit of FIG. 6 can be regarded as the polarization resistance Rp, solution resistance Rs, and electric double layer capacitance C of the circuit of FIG. The measured impedance result is Cole-Cole plotted, and the polarization resistance of FIG. 6 can be obtained by curve fitting.

  Therefore, the corrosion rate estimation method and the corrosion rate estimation apparatus according to the present invention can know the dew condensation state and the corrosion state in the gap portion of the metal, and can estimate the corrosion rate in the gap portion of the metal.

  In addition, the present corrosion rate estimation method and the present corrosion rate estimation device can calculate the polarization resistance by measuring the impedance by applying a voltage between the metal plates of the test material, which is necessary for the conventional electrochemical impedance measurement. The existing reference electrode can be omitted.

  The corrosion rate estimation method according to the present invention is characterized in that an exposure procedure for exposing the test material to a predetermined environment is performed before the electrochemical impedance measurement procedure.

  The corrosion rate estimation apparatus according to the present invention further includes exposure means for exposing the test material to a predetermined environment.

  By exposing the test material to a predetermined environment for a predetermined period, a change in corrosion rate with time can be estimated.

  The corrosion rate estimation method according to the present invention is characterized in that an interval between the two metal plates of the test material is 1 μm or less.

  The corrosion rate estimation apparatus according to the present invention is characterized in that an interval between the two metal plates of the test material is 1 μm or less.

  When the gap between the test materials is 1 μm or less, a water film is formed in the gap even at the dew point or lower. Therefore, a more accurate corrosion rate can be estimated.

  The present invention can provide a corrosion rate estimation method and a corrosion rate estimation apparatus capable of estimating a corrosion rate at a crevice portion of a metal.

It is a figure explaining the interface of a metal and aqueous solution. It is an equivalent circuit diagram explaining the interface of a metal and aqueous solution. It is a figure explaining the relationship between the clearance radius and relative humidity when a water film is formed on the metal surface. It is a figure explaining the reason for crevice corrosion. It is a figure explaining the reason for crevice corrosion. It is an equivalent circuit diagram explaining the interface of the test material and aqueous solution used with the corrosion rate estimation method and the corrosion rate estimation apparatus according to the present invention. It is the schematic explaining the test material used with the corrosion rate estimation method and corrosion rate estimation apparatus which concern on this invention. It is an example of the result of having measured the impedance of the test material by the electrochemical impedance measuring means of the corrosion rate estimating apparatus according to the present invention. It is the schematic explaining the corrosion rate estimation apparatus which concerns on this invention.

  Embodiments of the present invention will be described with reference to the accompanying drawings. The embodiments described below are examples of the present invention, and the present invention is not limited to the following embodiments. In the present specification and drawings, the same reference numerals denote the same components.

  FIG. 9 is a schematic diagram illustrating the corrosion rate estimation apparatus of the present embodiment. The corrosion rate estimation apparatus is applied between a test material 11 in which two metal plates from which an insulating film on one surface is removed are arranged so that the first surface faces at a predetermined interval, and the metal plate of the test material 11. An electrochemical impedance measuring means 12 for obtaining the impedance of the test material from the response of the electrical signal, and a calculating means 13 for calculating the polarization resistance of the metal plate from the impedance of the test material 11 obtained by the electrochemical impedance measuring means 12; .

  FIG. 7 is a schematic diagram illustrating the test material 11. In the test material 11, two metal plates 21 from which the insulating film on one surface is removed are arranged so that the one surface faces at a predetermined interval. Two metal plates 21 of metal for which the corrosion rate at the crevice portion is to be examined are prepared. The metal plates 21 have the same size. The lead wire 23 is attached to the two metal plates 21. The five surfaces of the metal plate 21 including the place where the lead wire 23 is attached are insulated. In other words, five surfaces except one facing each other are insulated. An insulating material such as an organic paint may be appropriately used for the insulation.

  The metal plate 21 is opposed so that a desired distance d is formed. The desired distance d can be formed by sandwiching an insulating spacer 22 between the metal plates 21 or by supporting the metal plate 21 from the outside and fixing it at a predetermined position. An example of the spacer 22 is a microtube. More desirably, it is more reliable to fix the two metal plates 21 with an elastic body such as a rubber band. The size of the metal plate 21 may be appropriately designed, but as an example, about 25 mm × 50 mm × 3 mm is easy to use.

  The desired distance d may be appropriately designed in accordance with the environment where the corrosion rate is actually estimated. According to FIG. 3, when the clearance is 0.0001 [cm] or less, a water film is formed between the metals even if the relative humidity is less than 100%. For this reason, for example, if the distance between the two metal plates 21 of the test material 11 is 1 μm or less (0.0001 [cm]) or less, a water film is formed between the metal plates 21 even when the relative humidity is less than 100%. . Furthermore, when it is desired to estimate the corrosion rate at a relative humidity of 95%, for example, by setting the distance between the metal plates 21 of the test material 11 to be 0.00001 [cm] or less, a water film is formed between the metal plates 21 at a relative humidity of 95%. Is formed.

  This corrosion rate estimation apparatus implements the following corrosion rate estimation method. In this corrosion rate estimation method, after the electrochemical impedance measurement procedure for obtaining the impedance of the test material 11 from the response of the electrical signal applied between the metal plates 21 of the test material 11 and the electrochemical impedance measurement procedure, the test material 11 And a calculation procedure for calculating the polarization resistance of the metal plate 21 from the impedance.

  Here, it is desirable that the corrosion rate estimation apparatus further includes an exposure unit 15 that exposes the test material 11 to a predetermined environment. In this corrosion rate estimation method, an exposure procedure for exposing the test material 11 to a predetermined environment is performed before the electrochemical impedance measurement procedure.

  In the exposure procedure, the test material 11 is exposed to a desired temperature and humidity by the exposure means 15. The exposure means 15 is a thermostat, for example. Moreover, the exposure means 15 may spray salt water as needed. Further, the aqueous solution 19 may be disposed in the exposure means 15 to replenish water vapor. The aqueous solution 19 is, for example, sulfuric acid or a sodium hydroxide aqueous solution. The exposure time may be designed as appropriate, but is preferably 24 hours or longer.

  Lead wires 23 from the respective metal plates 21 of the test material 11 are connected to the electrochemical impedance measuring means 12. Since a water film is formed between the metal plates 21 of the test material 11 after the exposure procedure, the corrosion rate estimation apparatus can continuously perform the electrochemical impedance measurement procedure. In the electrochemical impedance measurement procedure, the electrochemical impedance measurement means 12 applies a voltage between the metal plates 21 to measure the current response, that is, the impedance response. The voltage is easily measured when the peak-to-peak is about 10 mV. Moreover, the electrochemical impedance measuring means 12 is easy to obtain an impedance shape when the alternating voltage waveform is a sine wave and the frequency is several kHz to several mHz. For example, the electrochemical impedance measuring unit 12 performs measurement at about 10 kHz to 1 mHz.

  FIG. 8 is an example of a result measured by the electrochemical impedance measuring means 12. The metal plate 21 of the test material 11 has a size of 25 mm × 50 mm × 3 mm. The predetermined interval d is 1 to 3 μm. Three types of materials of the metal plate 21 were measured: Zn, Zn-55% Al, and Al. In the exposure procedure, the test material 11 was exposed at 25 ° C. and 98% relative humidity.

  The calculation means 13 performs a Cole-Cole plot of the impedance measurement result in the calculation procedure, and calculates the polarization resistance by curve fitting. The respective polarization resistances calculated by the calculation means 13 were approximately 10 kΩ, 5 kΩ, and 1 kΩ for Zn, Zn-55% Al, and Al, respectively. Since the electrochemical impedance measuring means 12 applied a voltage of 10 mV to the test material 11, each of Zn, Zn-55% Al, and Al can be estimated to have a corrosion rate of about 1 μA, 2 μA, and 10 μA.

11: Test material 12: Electrochemical impedance measuring means 13: Calculation means 15: Exposure means 19: Aqueous solution 21: Metal plate 22: Spacer 23: Lead wire

Claims (6)

Two metal plates, from which the insulating film on one surface is removed, are applied between the metal plates at a predetermined interval so that the one surface faces each other. Electrochemical impedance measurement procedure to obtain impedance;
After the electrochemical impedance measurement procedure, a calculation procedure for calculating the polarization resistance of the metal plate from the impedance of the test material,
Perform corrosion rate estimation method.   The corrosion rate estimation method according to claim 1, wherein an exposure procedure for exposing the test material to a predetermined environment is performed before the electrochemical impedance measurement procedure.   The corrosion rate estimation method according to claim 1 or 2, wherein an interval between the two metal plates of the test material is 1 µm or less. A test material in which two metal plates from which an insulating film on one surface is removed are arranged so that the one surface faces at a predetermined interval;
An electrochemical impedance measuring means for obtaining an impedance of the test material from a response of an electric signal applied between the metal plates of the test material;
Calculating means for calculating the polarization resistance of the metal plate from the impedance of the test material acquired by the electrochemical impedance measuring means;
Corrosion rate estimation device comprising:   The corrosion rate estimation apparatus according to claim 4, further comprising exposure means for exposing the test material to a predetermined environment.   The corrosion rate estimation apparatus according to claim 4 or 5, wherein a distance between the two metal plates of the test material is 1 µm or less.

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