Corrosion evaluation terminal and corrosion evaluation method

Abstract

A corrosion evaluation terminal 10 comprises: an insulating water absorption member 11 that has a gap 11A which is not in contact with a metal member 100; and a counter electrode 12 that is disposed so as to be in contact with the water absorption member 11 or so as to be embedded in the water absorption member 11. The water absorption member 11 is impregnated with a solvent, the water absorption member 11 is brought into contact with the metal member 100, the counter electrode 12 and the metal member 100 are connected to an electrochemical measurement device 50 to apply an alternating-current signal, a voltage and a current between the counter electrode 12 and the metal member 100 are measured to calculate an impedance, and the charge transfer resistance is determined to evaluate the corrosion rate.

Description

Corrosion evaluation terminal and corrosion evaluation method

[0001] The present disclosure relates to a corrosion evaluation terminal and a corrosion evaluation method.

[0002] When metals come into contact with water, they dissolve as ions, causing corrosion reactions to occur. Metal dissolution is an oxidation reaction from the metal's perspective, and a corresponding reduction reaction is also required. Hydrogen predominates in reduction reactions in acidic solutions, while oxygen predominates in neutral to alkaline aqueous solutions. When the metal surface is uniformly wet, the oxidation-reduction reaction proceeds uniformly on the metal surface, causing the metal to corrode uniformly.

[0003] On the other hand, when there are structural gaps that restrict material transfer or when the metal surface is uneven, severe localized corrosion can occur. Such severe corrosion can shorten the life of metal products and cause accidents. Therefore, it is a phenomenon that requires particular countermeasures, but because the corrosion behavior differs depending on the characteristics of each individual structure, individual evaluation is required.

[0004] Corrosion evaluation methods include measuring the mass loss due to corrosion and electrochemically measuring the target sample. The Japanese Industrial Standards (JIS) standardizes the pitting potential measurement method and the ferric chloride corrosion test method as methods for evaluating the corrosion resistance of stainless steel. Additionally, a method using a crevice-forming material with a toothed structure has been investigated (Non-Patent Document 1).

[0005] Electrochemical measurement methods can characterize detailed behavior throughout the process leading up to mass loss. Electrochemical impedance spectroscopy (EIS), in particular, is an effective method for understanding corrosion phenomena because it allows for the analysis of metal surfaces by modeling them as electrical circuits. However, electrochemical methods require the aqueous solution to act as an electrical conductor, making it difficult to measure conditions in which the aqueous solution decreases as the solution changes from wet to dry. One example of applying EIS to such conditions involves creating a sample in which a pair of steel members are embedded in resin, and using the exposed steel surface as a reaction surface (Non-Patent Documents 2 and 3). This method allows for EIS between the exposed steel members, using the thin film of water on the sample surface as a conductor, making it possible to evaluate the drying process.

[0006] Hachiro Imai, "Origin and Issues of Standardized Corrosion Test Methods," Materials, The Society of Materials Science, Japan, Nov. 1996, Vol. 45, No. 11, pp. 1248-1253; Atsushi Nishikata and Toshiaki Otsuka, "Fundamentals and Measurement Methods of Corrosion and Protection (4) Electrochemical Measurements in Atmospheric Corrosion Environments," Electrochemistry, The Electrochemical Society, 2015, Vol. 83, No. 6, pp. 483-488; Yanzhuo Shi, Eiji Tada, and Atsushi Nishikata, "A Method for Determining the Corrosion Rate of a Metal under a Thin Electrolyte Film," Journal of The Electrochemical Society, The Electrochemical Society, 2015, Vol. 162, No. 4, C135-C139 Masuo Aizawa, "Physicochemical Study of Water in Polymer Gels," Tokyo Institute of Technology Research Repository, <https: / / t2r2.star.titech.ac.jp / rrws / file / CTT100595307 / ATD100000413 / > Shinichi Makaino, "DC Polarization Method and AC Impedance Method," Surface Technology, Surface Finishing Society of Japan, 1994, Vol. 45, No. 10, pp. 973-978

[0007] With conventional technology, it was difficult to apply EIS to narrow spaces where crevice corrosion and pitting corrosion occur. The method using a sample in which multiple steel materials are embedded in resin requires the sample surface to be smooth, which requires surface polishing. This makes it difficult to evaluate practical steel materials such as plated materials.

[0008] Furthermore, as the solution resistance increases due to the thinning of the water film during the drying process, the current distribution on the steel sample becomes uneven, and analysis must assume a complex distributed constant circuit, making it difficult to take into account the effects of a narrow space.

[0009] The present disclosure has been made in view of the above, and aims to evaluate corrosion in a space where limitations on mass transport and spatial bias in substance concentration occur.

[0010] A corrosion evaluation terminal according to one embodiment of the present disclosure is a corrosion evaluation terminal that is brought into contact with a metal member to be evaluated and used to evaluate localized corrosion of the metal member, and has an insulating water-absorbing member with a gap that does not contact the metal member, and a counter electrode that is arranged so as to contact or be buried in the water-absorbing member. The water-absorbing member is impregnated with a solvent, and a voltage is applied between the counter electrode and the metal member while the water-absorbing member and the metal member are in contact with each other.

[0011] In one embodiment of the corrosion evaluation method of the present disclosure, a solvent is impregnated into an insulating water-absorbent member having a gap that does not contact a metal member, the water-absorbent member is brought into contact with the metal member, a counter electrode arranged so as to be in contact with or embedded in the water-absorbent member and the metal member are connected to an electrochemical measurement device, an AC signal is applied, the voltage applied between the counter electrode and the metal member and the corresponding current are measured, and the charge transfer resistance is calculated to evaluate the corrosion rate. The charge transfer resistance can be derived using electrochemical impedance spectroscopy or electrochemical techniques such as DC polarization.

[0012] According to the present disclosure, corrosion in a space where limitations on mass transport and spatial bias in substance concentration occur can be evaluated.

[0013] Fig. 1 is a diagram showing an example of the configuration of a corrosion evaluation terminal. Fig. 2 is a diagram showing an example of the configuration of a corrosion evaluation terminal. Fig. 3 is a diagram showing an example of the configuration of a corrosion evaluation terminal. Fig. 4 is a diagram for explaining an example of a corrosion evaluation method. Fig. 5 is a schematic diagram showing an example of a measurement result by electrochemical impedance spectroscopy, illustrated as a Nyquist plot.

[0014] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings.

[0015] An example of the configuration of a corrosion evaluation terminal 10 of this embodiment will be described with reference to Figures 1 to 3. The corrosion evaluation terminal 10 of this embodiment includes a water-absorbing member 11, a counter electrode 12, and a lead wire 13.

[0016] The water-absorbent member 11 is made of a water-absorbent material and is in contact with the metal member 100. The material for the water-absorbent member 11 may be a porous material that retains moisture within the structure through capillary action, such as string or sponge, or a material that is inherently water-absorbent, such as a polymeric material or gel. To prevent a battery reaction between the water-absorbent member 11 and the metal member 100, an insulating material is used. For example, the water-absorbent member 11 may be made of any material, such as cotton, linen, silk, polypropylene (PP), polyvinyl alcohol, nylon, paper, or rubber. When using a material that is inherently water-absorbent, such as a polymeric material or gel, the conductivity of the water-absorbent member 11 may decrease, potentially resulting in a decrease in the current value or a phase shift in the frequency response (see Non-Patent Document 4). Therefore, it is preferable to use a material that retains moisture within the structure through capillary action for the water-absorbent member 11. Furthermore, using a porous material that retains moisture within the structure through capillary action for the water-absorbent member 11 makes it possible to evaluate corrosion during the drying process.

[0017] The water-absorbent member 11 has a gap 11A that does not contact the metal member 100 when it is brought into contact with the metal member 100. This gap 11A allows for the reproduction of a space in which mass transfer is restricted and a spatial bias in the substance concentration occurs. In the example of FIG. 1, a portion of the contact portion of the water-absorbent member 11 with the metal member 100 is cut away to provide the gap 11A. In the example of FIG. 2, the gap 11A is provided inside the contact portion of the water-absorbent member 11 with the metal member 100. In the example of FIG. 3, a rope-shaped water-absorbent member 11 is used. In the cross section of the rope-shaped water-absorbent member 11, the gap 11A is generated in the portion that is not in contact with the metal member 100. Under conditions that simulate corrosion in a fastening tie string in a communication facility, a 4 mm diameter rope made of polyvinyl alcohol may be used as the water-absorbent member 11.

[0018] The counter electrode 12 is disposed so as to contact or be embedded in the water-absorbent member 11. The counter electrode 12 is connected to an electrochemical measurement device via a lead wire 13. When the water-absorbent member 11 is impregnated with a solvent and a voltage is applied between the counter electrode 12 and the metal member 100, an oxidation-reduction reaction occurs on the surface of the metal member 100, and charge is transferred between the counter electrode 12 and the metal member 100. The progress of the reaction on the surface of the metal member 100 can be determined by measuring the current value with the electrochemical measurement device. The counter electrode 12 can be made of a corrosion-resistant material such as platinum, gold, or carbon. When a less corrosive solvent is used, the counter electrode 12 can be made of an inexpensive material such as iron or nickel. Alternatively, a semiconductor material can be used for the counter electrode 12. The solvent impregnated into the water-absorbent member 11 can be a solvent equivalent to the environment in which the metal member 100 is expected to be used.

[0019] Since the absorbent member 11 retains moisture to maintain electrical conductivity between the counter electrode 12 and the metal member 100, the entire absorbent member 11 does not need to be absorbent, and the surface of the member may be covered with an absorbent material.

[0020] An example of a corrosion evaluation method will be described with reference to FIG.

[0021] As shown in Figure 4, during measurement, the water-absorbent member 11 is impregnated with a solvent and brought into contact with the metal member 100. At this time, there is a gap 11A where the water-absorbent member 11 and the metal member 100 are not in contact. The counter electrode 12 is connected to the electrochemical measurement apparatus 50, and the metal member 100 is connected to the electrochemical measurement apparatus 50 as a working electrode. In the case of the three-electrode method, a reference electrode 51 is placed in contact with or embedded in the water-absorbent member 11 and connected to the electrochemical measurement apparatus 50. A common reference electrode made of silver or silver chloride is used as the reference electrode 51.

[0022] An AC signal is applied using the electrochemical measurement device 50 while changing the frequency in the range of 1 MHz to 1 Hz, and the voltage and current between the counter electrode 12 and the metal member 100 are measured to calculate the impedance. The measurement results are plotted using a Nyquist plot, the charge transfer resistance is determined, and the corrosion rate is evaluated using the reciprocal of the charge transfer resistance.

[0023] Figure 5 shows a schematic diagram of an example of measurement results when EIS is used to scan frequencies from 1 MHz to 10 mHz, illustrated as a Nyquist plot. In the Nyquist plot, two incomplete semicircles are obtained due to corrosion reactions at the contact point with the water-absorbing member 11. Of these, the semicircle on the left (high-frequency side) is due to the corrosion reaction, and the semicircle on the right (low-frequency side) is due to an accompanying reaction. The diameter of the semicircle on the high-frequency side, i.e., the charge transfer resistance, is determined analytically. The reciprocal of the charge transfer resistance can be considered to correspond to the corrosion rate (Non-Patent Document 5). Since it is sufficient to determine the charge transfer resistance, measurements in the high-frequency range up to about 1 Hz are sufficient. This allows for shorter measurement times compared to conventional EIS. Furthermore, by assuming in advance that the high-frequency measurement results will form semicircles on the Nyquist plot, measurement time can be further reduced by narrowing the applied voltage frequency to about three points, such as 1 MHz, 1 kHz, and 1 Hz.

[0024] To convert from charge transfer resistance to corrosion rate, it is necessary to derive a constant for the conversion individually, but for relative evaluation, charge transfer resistance can be used.

[0025] As described above, the corrosion evaluation terminal 10 of this embodiment includes an insulating water-absorbent member 11 with a gap 11A that does not contact the metal member 100, and a counter electrode 12 that is disposed so as to contact or be embedded in the water-absorbent member 11. The water-absorbent member 11 is impregnated with a solvent, and a voltage is applied between the counter electrode 12 and the metal member 100 while the water-absorbent member 11 and the metal member 100 are in contact with each other. This allows corrosion to be evaluated in a space where material transfer is restricted and spatial bias in material concentration occurs. Furthermore, because no processing of the metal member 100 is required, the method can be applied to a variety of practical products.

[0026] In the corrosion evaluation method of this embodiment, the water-absorbent member 11 is impregnated with a solvent, the water-absorbent member 11 is brought into contact with the metal member 100, the counter electrode 12 and the metal member 100 are connected to the electrochemical measurement device 50, a voltage is applied, and the voltage and current between the counter electrode 12 and the metal member 100 are measured to calculate the impedance and determine the charge transfer resistance. This makes it possible to evaluate the corrosion rate and to estimate the lifespan and derive the useful life of the metal member 100.

[0027] REFERENCE SIGNS LIST 10 corrosion evaluation terminal 11 water-absorbing member 11A gap 12 counter electrode 13 lead wire 50 electrochemical measurement device 51 reference electrode 100 metal member

Claims

1. A corrosion evaluation terminal that is brought into contact with a metal component to be evaluated and used to evaluate localized corrosion of the metal component, the corrosion evaluation terminal having an insulating water-absorbing member with a gap that does not contact the metal component, and a counter electrode that is arranged so as to contact or be embedded in the water-absorbing member, wherein the water-absorbing member is impregnated with a solvent, and a voltage is applied between the counter electrode and the metal component while the water-absorbing member and the metal component are in contact with each other.

2. A corrosion evaluation terminal according to claim 1, wherein the water-absorbing member is a rope-shaped polyvinyl alcohol.

3. A corrosion evaluation method comprising the steps of: impregnating an insulating water-absorbing member with a solvent, the water-absorbing member having a gap that does not contact the metal member; bringing the water-absorbing member into contact with the metal member; connecting a counter electrode arranged so as to contact or be embedded in the water-absorbing member and the metal member to an electrochemical measurement device and applying an AC signal; measuring the voltage applied between the counter electrode and the metal member and the corresponding current; determining the charge transfer resistance and evaluating the corrosion rate.

4. The corrosion evaluation method according to claim 3, wherein charge transfer resistance is determined from the measurement results in the frequency band from 1 MHz to 1 Hz.

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