Estimation method

Abstract

In a drying process under a condition that a non-metal member 40 is in contact with a metal member 100, changes in impedance over time are analyzed using EIS, a local corrosion period is estimated from the rate of the changes in impedance in a high-frequency region, and the progress conditions of local corrosion over the local corrosion period are evaluated.

Description

Estimation method

[0001] The present disclosure relates to an estimation method.

[0002] Methods for evaluating the progress of metal corrosion include visual comparison, corrosion weight loss measurement (weighing method), and electrochemical measurement. Methods for electrochemical measurement include the Tafel extrapolation method, the polarization resistance method, and electrochemical impedance spectroscopy (EIS).

[0003] In EIS, when analyzing measurement data, an equivalent circuit model in which an electric double layer capacitance and a charge transfer resistance are connected in parallel, and a solution resistance is connected in series therewith is generally used. By expressing the electrochemical behavior at the interface between a solution and a metal surface with this equivalent circuit model, EIS becomes a method effective for analyzing corrosion phenomena and understanding corrosion mechanisms.

[0004] "Test Methods for Corrosion Resistance of Platings", JIS H 8502 Kazuhiko Noda, Satoru Saito, "II. Fundamentals of Electrochemical Measurement of Corrosion - Polarization Curves (Current-Potential Curves) -", Zairyo to Kankyo (Corrosion Engineering), 2018, Vol. 67, No. 7, pp. 9-16 Yoshinao Hoshi, Noriyuki Itagaki, "II. Fundamentals of Electrochemical Measurement of Corrosion - Electrochemical Impedance -", Zairyo to Kankyo (Corrosion Engineering), 2018, Vol. 67, No. 7, pp. 55-58

[0005] In EIS, it is premised that the solution uniformly contacts the entire surface of the metal to be measured. When the contact area between the metal and the solution (effective electrode area) decreases, the solution resistance and the charge transfer resistance apparently increase. Therefore, when EIS is applied to the drying process of a solution, it is necessary to consider the effective electrode area.

[0006] When a non-metallic member is in contact with a metal, moisture remains at the contact portion during the drying process, and localized corrosion progresses. At this time, while moisture remains at the contact portion, the effective electrode area can be considered to be constant. However, it is difficult to visually confirm a thinned solution, and as a result, it has been difficult to accurately estimate the period during which moisture remains at the contact portion (hereinafter referred to as the localized corrosion period).

[0007] This disclosure has been made in view of the above, and aims to evaluate local corrosion at locations where a non-metallic component comes into contact with a metal component during the drying process.

[0008] An estimation method according to one aspect of the present disclosure is an estimation method for estimating local corrosion during a drying process when a contact object is present with respect to a metal, comprising: analyzing the change in impedance over time using electrochemical impedance spectroscopy; estimating the local corrosion period from the rate of change in impedance in the high-frequency region; and deriving the corrosion rate from the reciprocal of the charge transfer resistance for the local corrosion period.

[0009] According to this disclosure, localized corrosion at the point where a non-metallic component comes into contact with a metal component during the drying process can be evaluated.

[0010] Figure 1 shows an example of the configuration of a local corrosion estimation system. Figure 2 shows an example of the configuration of a local corrosion estimation system. Figure 3 shows an example of the drying process. Figure 4 shows an example of the change in impedance during the drying process in the frequency range of 1 mHz to 1 MHz. Figure 5 shows an example of the change in impedance over time during the drying process at a frequency of 1 MHz. Figure 6 shows an example of the result after normalizing and fitting both time and impedance. Figure 7 is a flowchart showing an example of the process flow for estimating local corrosion.

[0011] Referring to Figure 1, an example of the configuration of the local corrosion estimation system of this embodiment will be described. The local corrosion estimation system evaluates the progress of local corrosion at the point where the non-metallic member 40 comes into contact with the metal member 100 during the drying process.

[0012] The local corrosion estimation system comprises a measuring device 10 and a computer 20. An electrode 30 is connected to the measuring device 10 as the counter electrode, and the metal member 100 to be measured is connected as the working electrode. An insulating non-metallic member 40 is positioned in contact with the metal member 100.

[0013] The material of the non-metallic member 40 may be a porous material that retains moisture inside its structure by capillary action, such as a string or a sponge, or it may be a material that has water absorption properties, such as a polymer material or a gel. The non-metallic member 40 is preferably a material that retains water, and it is preferable to bring the non-metallic member 40 into contact with the electrode 30. By making the non-metallic member 40 contain water, electrical conductivity between the electrode 30 and the metal member 100 can be maintained during the drying process.

[0014] In the example shown in Figure 1, a case is assumed where the metal member 100 is fixed with a rope, and a rope-shaped polyvinyl alcohol is used as the non-metallic member 40. Figure 1 shows a cross-section of the rope-shaped non-metallic member 40. As shown in Figure 2, a linear electrode 30 may be placed inside the non-metallic member 40. Note that the shape of the non-metallic member 40 is not limited to a rope, and a non-metallic member 40 of any shape can be used.

[0015] In the measurement using the measuring device 10, the two-electrode EIS method is applied, and as shown in Figure 3, water 200 is brought into contact with the electrode 30 and the metal member 100, and the frequency response during the drying process of the water is measured. More specifically, the measuring device 10 applies a low voltage of 30 mV or less, sets the frequency range to 10 mHz to 1 MHz, and performs interval measurements at appropriate time intervals (for example, every hour). When an AC voltage is applied between the electrode 30 and the metal member 100, an oxidation-reduction reaction occurs on the surface of the metal member 100, and charge transfer occurs between the electrode 30 and the metal member 100. The measuring device 10 measures the current flowing in accordance with the applied AC voltage and calculates the impedance for each frequency based on the measured voltage, current amplitude, and phase difference.

[0016] The computer 20 estimates the local corrosion period from the change in impedance over time obtained from the measurement results, and evaluates the progress of local corrosion during the local corrosion period.

[0017] Figure 4 shows an example of the results of measuring the drying process using EIS with a frequency range of 10 mHz to 1 MHz and an interval of 1 hour. The horizontal axis is plotted as frequency and the vertical axis as impedance. Figure 4 shows the measurement results for the 1st, 4th, 5th, and 12th trials. As shown in Figure 4, the impedance in the high-frequency region asymptotically approaches a constant value during the drying process. In this embodiment, the timing at which the impedance in the high-frequency region asymptotically approaches a constant value is considered to be the timing at which the effective electrode area near the contact portion of the non-metallic member 40 asymptotically approaches a constant value.

[0018] Figure 5 shows an example of the time variation of impedance at a frequency of 1 MHz. The horizontal axis represents elapsed time, and the vertical axis represents impedance. The figure shows the results of 12 measurements taken at 1-hour intervals. During the drying process, the contact area between the metal member 100 and the water 200 decreases, but the decrease in contact area is suppressed because water 200 remains in the area where the metal member 100 and the non-metallic member 40 are in contact. Therefore, the point at which the impedance is above a certain value and the difference from the previous measurement has decreased is considered to be the period during which local corrosion progresses.

[0019] To derive the local corrosion period, the formula obtained by fitting is used after normalizing both time and impedance. Figure 6 shows an example of the result of fitting after normalizing both time and impedance.

[0020] In the example in Figure 6, the value of the X axis is X' = (X - μ x ) / σ x The values ​​on the Y-axis were normalized by Y' = Y / Ymax. x σ is the mean value of X. x is the standard deviation of X. The X-axis can also be normalized by applying X' = (X - X min) / (X max - X min).

[0021] The normalized measured values ​​were fitted using the tanh function. Other fitting methods besides the tanh function may also be used, such as the sigmoid function, logarithmic function, Gompertz curve, or polynomial. Normalizing the measured values ​​allows the method to be applied to various types of metal components 100 under various conditions.

[0022] Since the local corrosion period is defined as the period after most of the moisture has evaporated due to drying, the local corrosion period is defined as the period in which Y'≧0.2 and dY'≦0.1 for the normalized Y'. To focus on the later stages of drying, the local corrosion period may also be defined as the period in which Y'≧0.5 and dY'≦0.1. Note that dY' is the difference between consecutive Y' values. The rate of change of Y' (also called the rate of change) may also be used.

[0023] Computer 20 normalizes the measured values ​​to derive the local corrosion period, and then evaluates the progress of local corrosion during the local corrosion period. For example, for the local corrosion period, Computer 20 derives the corrosion rate from the reciprocal of the charge transfer resistance obtained from the measurement results and estimates the local corrosion. The local corrosion period can be assumed to be constant in terms of the effective electrode area. The charge transfer resistance is obtained from the diameter of the semicircle on the high-frequency side when the measurement results are plotted on a Nyquist plot. The reciprocal of the charge transfer resistance can be considered to correspond to the corrosion rate.

[0024] Referring to the flowchart in Figure 7, an example of the process flow for estimating local corrosion using a local corrosion estimation system will be explained.

[0025] In step S1, the measuring device 10 measures the frequency response during the drying process of water by performing interval measurements at predetermined time intervals using EIS.

[0026] In step S2, the computer 20 normalizes the impedance in the high-frequency range (e.g., 1 MHz).

[0027] In step S3, the computer 20 determines the fitting method, fits the normalized measurement data, and derives the period during which the rate of change in impedance after normalization is less than or equal to a predetermined value as the local corrosion period.

[0028] In step S4, the computer 20 assumes that the effective electrode area is constant during the localized corrosion period and derives the corrosion rate from the reciprocal of the measured charge transfer resistance.

[0029] As described above, during the drying process in which the non-metallic member 40 is in contact with the metal member 100, the change in impedance over time is analyzed using EIS, the local corrosion period is estimated from the rate of change in impedance in the high-frequency region, and the progress of local corrosion during the local corrosion period is evaluated. This makes it possible to estimate the period during which moisture accumulates at the point where the non-metallic member 40 is in contact with the metal member 100 and local corrosion progresses, and to evaluate the progress of local corrosion.

[0030] 10 Measuring device 20 Calculator 30 Electrode 40 Non-metallic component 100 Metallic component 200 Water

Claims

1. An estimation method for estimating local corrosion during the drying process when a material is in contact with a metal, comprising: analyzing the change in impedance over time using electrochemical impedance spectroscopy; estimating the local corrosion period from the rate of change in impedance in the high-frequency region; and evaluating the progress of local corrosion during the local corrosion period.

2. An estimation method according to claim 1, wherein the local corrosion period is estimated by normalizing the impedance.

3. An estimation method according to claim 2, wherein the local corrosion period is estimated by fitting the normalized impedance with one of the following: a tanh function, a sigmoid function, a logarithmic function, a Gompertz curve, and a polynomial.

4. Estimation method according to claim 1, wherein the contact object is a rope-shaped, insulating, water-absorbing member.

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