Corrosion resistance evaluation method

Abstract

The present invention uses the absolute value |Z| of impedance Z extracted at frequencies lower than or equal to the frequency at the apex of a semicircle (Nyquist plot) in the GEIS method or the PEIS method and evaluates the corrosion resistance of a constituent material of a working electrode of interest on the basis of the change over time in the absolute value |Z|.

Description

Corrosion resistance evaluation method

[0001] The present invention relates to a corrosion resistance evaluation method for calculating the effectiveness of the corrosion resistance of a surface treatment applied to a substrate surface in an environmental test such as a salt spray test, and for quickly determining the superiority or inferiority of the corrosion resistance. This application claims priority to international application PCT / JP2024 / 024007 filed on July 3, 2024, the contents of which are incorporated herein by reference.

[0002] Various products offered on the market are required to have a certain level of durability. Among these durability requirements, resistance to corrosion, or corrosion resistance, is a major indicator of the durability required for surface treatment. In recent years, there have been increasing demands from the market for products with long-term reliability in order to realize a sustainable society. In order to ensure this long-term reliability, it is necessary to extend the environmental testing period before supplying products to the market and thoroughly confirm the quality of the product in advance.

[0003] However, when products require long-term reliability over several decades, even accelerated testing requires several months of testing, which poses a problem of extending the time from the start of testing to product delivery. Furthermore, if rework is required, delivery delays are likely to occur. To date, methods using electrochemical measurements have been considered to speed up and quantify corrosion resistance evaluation.

[0004] For example, Patent Document 1 describes that the capacitance determined by an AC impedance method, which is a type of electrochemical measurement for measuring the degree of corrosion of reinforcing bars in concrete, correlates with the corrosion of the reinforcing bars.

[0005] Furthermore, Patent Document 2 also describes that the AC impedance method can be used to determine the type of deterioration or damage to a material, and that corrosion resistance can be quantitatively evaluated.

[0006] Furthermore, Patent Document 3 describes that the durability of an anode electrode material in an aqueous electrolyte solution can be evaluated by applying a constant current for a certain period of time, followed by sweeping to a predetermined voltage, and then applying the predetermined voltage for a certain period of time.

[0007] Furthermore, Patent Document 4 describes that passing an electric current through a surface treatment film promotes the penetration of corrosion factors into the film, and that corrosion resistance can be determined by observing a certain voltage or current.

[0008] On the other hand, Non-Patent Document 1 reports that the self-repair function of a surface-treated surface can be monitored by using a 3D impedance method, which is a type of AC impedance method.

[0009] JP 2007-17405 A JP 2605109 A JP 2022-81975 A JP 7156481 A

[0010] Surface Technology Vol. 72 No. 9 2021 p. 462-466.

[0011] However, the evaluation methods described in Patent Documents 1 and 2 and Non-Patent Document 1 quantitatively detect the amount of corrosion and the self-repair process when a surface-treated material is already corroded or damaged, and are not capable of evaluating the corrosion resistance of an initial material that is not corroded or damaged.

[0012] Furthermore, the evaluation methods described in Patent Documents 3 and 4 require the corrosion test to be repeated for 100 cycles or more, or require a long test to be continuously carried out for several tens of days, making it impossible to evaluate corrosion resistance in a short period of time.

[0013] Furthermore, the evaluation method described in Patent Document 3 is an accelerated test in which a high voltage and a high current are applied to observe the deterioration of a catalyst supported on an electrode. Therefore, if the test is performed under the conditions disclosed in Patent Document 3, the surface treatment film will dissolve, making it difficult to apply the evaluation method described in Patent Document 3 to a short-term evaluation of corrosion resistance.

[0014] On the other hand, Patent Document 3 also proposes a determination using the CV method (cyclic voltammetry method). However, this method involves repeating oxidation (anodic reaction) and reduction (cathodic reaction) on the same surface, which not only makes it impossible to compare with the corrosion reaction (anodic reaction) that occurs in environmental testing, but also causes a reduction reaction, making it difficult to evaluate the corrosion resistance of surface treatment films in a short period of time, even when the current and voltage are reduced.

[0015] The present invention has been proposed in view of the above-mentioned problems, and aims to provide a corrosion resistance evaluation method that can quantitatively evaluate the corrosion resistance of a test material in its initial state in a short period of time based on its impedance value, using an electrolyte solution composed of components similar to those of the solution used in environmental testing.

[0016] One aspect of the corrosion resistance evaluation method according to the present disclosure is a corrosion resistance evaluation method that evaluates the corrosion resistance of a constituent material of the working electrode against an electrolyte solution by a three-electrode method using a working electrode, a counter electrode, and a reference electrode, each of which is in contact with the electrolyte solution. The corrosion resistance evaluation method includes an initial natural potential measurement step of measuring the natural potential when no current flows from the working electrode to the counter electrode, a GEIS measurement step of applying a reference current between the working electrode and the counter electrode and varying the frequency to perform measurement by a constant current alternating current impedance method (GEIS method), and a constant current measurement step of performing constant current measurement by a chronopotentiometry method (CP method). The GEIS measurement step and the constant current measurement step are alternately repeated multiple times, and the absolute value |Z| of impedance Z extracted at a frequency equal to or lower than the apex frequency of a semicircle (Nyquist plot) in the GEIS method is used to evaluate the corrosion resistance of the constituent material of the working electrode from the change over time in the absolute value |Z|.

[0017] Another aspect of the corrosion resistance evaluation method according to the present disclosure is a corrosion resistance evaluation method for evaluating the corrosion resistance of a constituent material of a working electrode against an electrolyte solution by a three-electrode method using a working electrode, a counter electrode, and a reference electrode, each of which is in contact with the electrolyte solution, the method including: an initial rest potential measurement step for measuring an initial rest potential when no current flows from the working electrode to the counter electrode; a PEIS measurement step for performing measurement by a constant voltage alternating current impedance method (PEIS method) while varying a frequency at a first applied voltage between the working electrode and the counter electrode; a first rest potential measurement step for measuring a first rest potential after the PEIS measurement step; a constant voltage measurement step for performing a constant voltage measurement by a chronoamperometry method (CA method) at a second applied voltage; and a second natural potential measurement step of measuring a second natural potential after the constant voltage measurement step, wherein the first applied voltage is a voltage obtained by adding an additional voltage in the range of 1 mV to 200 mV relative to the initial natural potential or the second natural potential, and the second applied voltage is a voltage obtained by adding an additional voltage in the range of 1 mV to 200 mV relative to the first natural potential, and the PEIS measurement step, the first natural potential measurement step, the constant voltage measurement step, and the second natural potential measurement step are repeated multiple times in this order, and the absolute value |Z| of impedance Z extracted at a frequency equal to or lower than the apex frequency of a semicircle (Nyquist plot) in the PEIS method is used to evaluate the corrosion resistance of the constituent material of the working electrode from the change over time of the absolute value |Z|.

[0018] According to the present invention, it is possible to provide a corrosion resistance evaluation method that can quantitatively evaluate the corrosion resistance of a test material in its initial state in a short period of time based on its impedance value, using an electrolyte solution composed of components similar to those of the solution used in environmental testing.

[0019] FIG. 1 is a schematic configuration diagram showing an example of an evaluation device suitable for the corrosion resistance evaluation method of one embodiment of the present invention. FIG. 2 is a schematic cross-sectional view showing an example of a test material (working electrode). FIG. 3 is a schematic cross-sectional view showing another example of a test material (working electrode). FIG. 4 is a flowchart showing a corrosion resistance evaluation method of a first embodiment in stages. FIG. 5 is a graph showing the relationship between elapsed time and change in current in the first embodiment. FIG. 6 is a schematic diagram showing an analysis method of the first embodiment. FIG. 7 is a schematic diagram showing an analysis method of the first embodiment. FIG. 8 is a graph showing the relationship between elapsed time and change in current in a modified example of the first embodiment. FIG. 9 is a flowchart showing a corrosion resistance evaluation method of a second embodiment in stages. FIG. 10 is a graph showing the relationship between elapsed time and change in potential in a modified example of the second embodiment. FIG. 11 is a graph showing the relationship between elapsed time and change in potential in a modified example of the second embodiment. FIG. 12 is a graph showing the evaluation results of corrosion resistance in Example 1. FIG. 13 is an explanatory diagram showing the results of appearance observation versus elapsed time in a salt spray test in Example 1. FIG. 14 is a graph showing the evaluation results of corrosion resistance in Example 2. FIG. 15 is an explanatory diagram showing the results of appearance observation versus elapsed time in a salt spray test in Example 2. FIG. 16 is a graph showing the evaluation results of corrosion resistance in Example 3. FIG. 10 is an explanatory diagram showing the elapsed time in a salt spray test and the results of appearance observation in Example 3. FIG. 11 is an explanatory diagram in which corrosion resistance determination by machine learning is added to a flowchart showing the steps of a corrosion resistance evaluation method in the first embodiment. FIG. 12 is an explanatory diagram in which corrosion resistance determination by machine learning is added to a flowchart showing the steps of a corrosion resistance evaluation method in the second embodiment. FIG. 13 is a schematic diagram of acquiring state variables in the third embodiment. FIG. 14 is an explanatory diagram of corrosion resistance determination using machine learning in Example 4.

[0020] Hereinafter, a corrosion resistance evaluation method according to one embodiment of the present invention will be described with reference to the drawings. Note that the embodiment shown below is specifically described to provide a better understanding of the gist of the invention, and does not limit the present invention unless otherwise specified. Furthermore, the drawings used in the following description may show essential parts enlarged for convenience in order to make the features of the present invention easier to understand, and the dimensional ratios of each component may not necessarily be the same as those in reality.

[0021] 1 is a schematic diagram showing an example of an evaluation device suitable for a corrosion resistance evaluation method according to one embodiment of the present invention. The evaluation device 10 includes a liquid tank 6 containing an electrolytic solution 2, a reference electrode 3 and a counter electrode 4 immersed in the electrolytic solution 2, a working electrode terminal 7 in contact with the working electrode 1, and a potentiostat device 8 connected to the reference electrode 3, the counter electrode 4, and the working electrode 1.

[0022] A working electrode, which is a surface-treated test material 1, is placed in the lower part of the liquid tank 6 and is in contact with the electrolyte 2 through the opening 5. A working electrode terminal 7 is connected to the working electrode (object to be evaluated) 1. As shown in FIG. 2 , the test material (working electrode) 1 only needs to have a base material 11 whose surface is covered with plating 12. Alternatively, as shown in FIG. 3 , the test material (working electrode) 1 may have a base material 11 whose surface is covered with a coating film 13.

[0023] The base material 11 of the test material 1 may be any material having electrical conductivity, and is preferably a metal material. The plating 12, which is the surface treatment applied to the base material 11, may be Ni, Cu, Zn, Au, Ag, or a chemical conversion coating. ２ The plating may be a composite plating including an insulating material such as ZnNi, ZnSn, ZnFe, Cr, or Cd. In this embodiment, the characteristics of the plating are particularly pronounced with highly corrosion-resistant plating, and therefore, the corrosion resistance can be further improved with highly corrosion-resistant plating such as ZnNi, ZnSn, ZnFe, Cr, or Cd.

[0024] The potentiostat device 8 connected to the test material 1 may be a potentio-galvanostat device incorporating an FRA (Frequency Response Analyzer).

[0025] When connecting the working electrode terminal 7 of this potentiostat device 8, the working electrode terminal 7 may be electrically connected to a portion of the test material 1 that is covered with plating 12, or the working electrode terminal 7 may be connected to a portion where the base material 11 is exposed.

[0026] On the other hand, when a coating film 13 is applied as a surface treatment to the base 11 of the test material 1, the coating film 13 may generally be a urethane-based, epoxy-based, acrylic-based, or silicone-based film containing a pigment. When connecting the working electrode terminal 7 of the potentiostat device 8 to this test material 1, if the coating film 13 is made of an insulating material, it is preferable to connect it directly to the base 11.

[0027] Furthermore, a surface treatment using a silicon-based inorganic binder may be used as a top coat on the plating 12, and is referred to as a coating film to distinguish it from the chromate film applied on the plating 12. The thicker the coating film, the better the corrosion resistance. However, when applying a coating film 13 on the plating 12 to compare it with a chromate film, it is desirable from the viewpoint of product application that the film thickness be 10 μm or less.

[0028] In this way, the test material 1, which has been subjected to surface treatment such as plating 12 or coating 13, is electrically connected to the reference electrode 3 and counter electrode 4 via the electrolyte 2. A liquid with a pH of 1 to 14 can be used as the electrolyte 2, but since the plating 12 may dissolve when immersed in an acid or alkali, it is preferable to carry out the treatment at a pH of 3 to 11.

[0029] Furthermore, when conducting tests assuming salt-damaged regions, the pH of electrolyte 2 is preferably 5 to 9. In this case, electrolyte 2 may contain at least one or more of sodium chloride, magnesium chloride, strontium chloride, potassium chloride, sodium bicarbonate, potassium bromide, boric acid, sodium fluoride, and sodium sulfate, or electrolyte 2 containing all of them may be selected. In particular, when evaluating salt-damaged regions, it is preferable to use electrolyte 2 made of natural seawater or artificial seawater prepared to have the same components as seawater.

[0030] The reference electrode 3 can be an Ag / AgCl electrode, an Hg / HgCl electrode, or a hydrogen electrode. The counter electrode 4 is preferably made of a metal insoluble in the electrolyte 2, such as Pt, Ti, or carbon. The counter electrode 4 may be in the shape of a mesh, in addition to a plate. However, the surface area of ​​the counter electrode 4, or the electrode area defined by the outermost periphery if the counter electrode is mesh-shaped, must be larger than the opening area of ​​the opening 5 of the liquid tank 6 that brings the electrolyte 2 and the test material 1 into contact.

[0031] The corrosion resistance evaluation method of this embodiment is performed using an electrochemical measurement system including the evaluation device 10 in which the sample is set as described above. FIG. 4 is a flowchart showing the steps of the corrosion resistance evaluation method of this embodiment. FIG. 5 is a graph showing the relationship between elapsed time and current change in this embodiment. First, the natural potential is measured when no current flows from the test material (working electrode) 1 to the counter electrode 4 (initial natural potential measurement step S1). The natural potential measurement time is set to 60 min or less, and for high-speed evaluation technology, it is desirable to set it to 10 min or less.

[0032] Next, a voltage of +100 mV or less from the natural potential is applied to the test material 1, and an AC impedance measurement (GEIS (Galvano Electrochemical Impedance Spectroscopy) measurement) is performed (GEIS measurement step S2). At this time, the current flowing through the test material (working electrode) 1 is 100 μA / cm ２ It is preferable to set the current amplitude to ±10 μA / cm or less. ２ The current amplitude in the GEIS measurement step S2 may be controlled by the voltage amplitude, in which case it is sufficient if it is ±100 mV or less, and it is preferable to set it to ±10 mV or less.

[0033] Next, constant current measurement of CP (Chronopotentiometry) is carried out (constant current measurement step S3). The current at this time is preferably the same as or less than the reference current used in the AC impedance measurement (GEIS measurement step S2). In addition, to avoid exposing the base material 11 of the test material 1, the applied current density is 10 μA / cm.２ It is preferable to set it as follows:

[0034] The GEIS measurement step S2 and the constant current measurement step S3 are alternately repeated multiple times to advance the corrosion of the surface treatment film on the test material 1, and time-dependent data can be obtained by measuring the AC impedance in the GEIS measurement step S2. The number of repetitions at this time is preferably, for example, four or more times in order to quantitatively evaluate the change over time due to the correction function.

[0035] Regarding the AC impedance measurement results obtained through the above-mentioned steps in the GEIS measurement step S2, a frequency is selected to analyze the change over time in the absolute value |Z| of the impedance Z. When configured as an equivalent circuit model such as a Randles circuit, the frequency selected at this time is the charge transfer resistance R of the Randles circuit as shown in FIG. ｃｔ and the electric double layer capacitance C ｄｌ The value of the peak frequency obtained by 1 / (2πR ｃｔ C ｄｌ ) or lower, and the change over time in the absolute value |Z| of the impedance Z at that time is observed.

[0036] For example, in the schematic diagram shown in Figure 7, the right side shows the impedance |Z| at a selected frequency on the vertical axis and time on the horizontal axis. In this case, the frequency band to be selected is preferably a low frequency band of 10 Hz to 100 mHz, because the effect of the electric double layer capacitance Cdl is smaller than in a high frequency band above 10 Hz, and evaluation results closer to those of actual environmental testing can be obtained. On the other hand, changes in the high frequency band are used to check whether irregular changes have occurred during measurement.

[0037] As described above, according to the corrosion resistance evaluation method of this embodiment, it is possible to quantitatively measure the corrosion resistance of an initial state sample before it is put into a corrosion resistance test, for example, within 2 to 3 hours from the start of measurement.

[0038] 8, which shows the relationship between elapsed time and current change, a modified example of the corrosion resistance evaluation method of the first embodiment involves performing a natural potential measurement step, a GEIS measurement step, a constant current measurement step, and then a natural potential measurement step again. Corrosion resistance can also be evaluated by repeating this GEIS measurement step, natural potential measurement step, constant current measurement step, and natural potential measurement step multiple times.

[0039] Second Embodiment Fig. 9 is a flowchart showing the steps of the corrosion resistance evaluation method of this embodiment. Fig. 10 is a graph showing the relationship between the elapsed time and the change in potential of this embodiment. In the following explanation, only the parts that are different from the first embodiment will be explained, and a duplicate explanation of the same configuration will be omitted. In the corrosion resistance evaluation method of the second embodiment, the natural potential is measured when no current flows from the test material (working electrode) 1 to the counter electrode 4 (initial natural potential measurement step S11).

[0040] Next, using this initial natural potential or the second natural potential described later as a reference, a voltage of +100 mV or less from these natural potentials is applied to the test material 1, and AC impedance measurement (PEIS (Potential Electrochemical Impedance Spectroscopy) measurement) is performed (PEIS measurement step S12).

[0041] The voltage amplitude in this PEIS measurement step S12 may be set to ±100 mV or less, and is preferably set to ±10 mV or less. The measurement frequency is preferably in the range of 10 MHz to 1 mHz, and the measurement is preferably performed in the range of 3 MHz to 100 mHz. The frequency is preferably varied from high to low.

[0042] After the AC impedance of the test material (working electrode) 1 is measured in this PEIS measurement step S12, the spontaneous potential (first spontaneous potential) is measured again (first spontaneous potential measurement step S13). Then, a voltage of +100 mV or less based on this voltage is applied to the test material 1, and a constant voltage measurement of CA (chronoamperometry) is performed (constant voltage measurement step S14).

[0043] In this constant voltage measurement step S14, the voltage must also be set so that the plating 12 and coating 13 of the test material 1 do not dissolve in the electrolyte 2, and the entire substrate 11 is not exposed during measurement. Therefore, it is preferable to set the voltage to +50 mV or less from the natural potential rather than +100 mV or less.

[0044] After the constant voltage measurement of the test material (working electrode) 1 is performed in this constant voltage measurement step S14, the natural potential (second natural potential) is measured again (second natural potential measurement step S15).

[0045] The above-described PEIS measurement step S12, first natural potential measurement step S13, constant voltage measurement step S14, and second natural potential measurement step S15 are repeated multiple times in order to advance corrosion of the surface treatment film (plating 12 or coating film 13) on the test material 1, and time-dependent data is obtained by measuring the AC impedance in the PEIS measurement step S12. The number of repetitions is preferably, for example, four or more times, in order to quantitatively evaluate the change over time due to the correction function.

[0046] Based on the AC impedance measurement results obtained through the above-described steps in the PEIS measurement step S12, by observing the change over time in the absolute value |Z| of the impedance Z, as in the first embodiment described above, it is possible to quantitatively measure the corrosion resistance of the initial state sample before it is put into the corrosion resistance test within, for example, 2 to 3 hours from the start of the measurement.

[0047] 11, which shows the relationship between elapsed time and change in potential, a modified example of the corrosion resistance evaluation method of the second embodiment involves performing a natural-state potential measurement step, a constant-current measurement step, a natural-state potential measurement step again, a PEIS measurement step, and then a natural-state potential measurement step again. Corrosion resistance can also be evaluated by repeating this cycle of the constant-current measurement step, natural-state potential measurement step, PEIS measurement step, and natural-state potential measurement step multiple times.

[0048] Third Embodiment In a third embodiment, a corrosion resistance evaluation is performed using a learning unit that learns corrosion resistance. The learning unit uses the frequency measured by the GEIS or PEIS method, the number of repetitions of the GEIS or PEIS measurement and the CP or CA measurement, and the impedance value as state variables, and learns the relationship between the state variables and corrosion resistance according to a training data set created based on the state variables. The learning unit can evaluate the corrosion resistance of a new sample based on the above learning results.

[0049] 18 and 19 are flowcharts showing the steps of the corrosion resistance evaluation method of this embodiment. FIG. 18 is a flowchart of the corrosion resistance evaluation method based on the first embodiment. In FIG. 18, the frequency, the number of measurement repetitions, and the impedance value measured by the GEIS method obtained in the first embodiment are used as state variables. FIG. 19 is a flowchart of the corrosion resistance evaluation method based on the second embodiment. In FIG. 19, the frequency, the number of measurement repetitions, and the impedance value measured by the PEIS method obtained in the second embodiment are used as state variables. FIG. 20 shows the range in which data is acquired as the state variables of machine learning in this embodiment. Impedance values ​​from high frequencies to low frequencies may be acquired as data for each repetition. By acquiring impedance values ​​from high frequencies to low frequencies as data, it is also possible to determine the change over time in the absolute value |Z| of the impedance Z at a predetermined frequency.

[0050] In the following description, only the parts that are different from the first and second embodiments will be described, and a duplicate description of the same configuration will be omitted.

[0051] In the corrosion resistance evaluation method of the third embodiment, the impedance value or its change for each repetition is obtained at each of a plurality of frequencies, not at a single frequency, and these values ​​are used as state variables for machine learning. The learning unit learns the relationship between the state variables and corrosion resistance through machine learning.

[0052] For example, in the first or second embodiment, the measured impedance values ​​are recorded as state variables for each repetition from high frequency to low frequency, and the recorded data is used for learning. For example, by having the learning unit perform machine learning using data obtained by similar measurements as training data, the learning unit can use this training data to evaluate the corrosion resistance of newly acquired data.

[0053] In the first and second embodiments, the charge transfer resistance R ｃｔ and the electric double layer capacitance C ｄｌ The value of the peak frequency obtained by 1 / (2πR ｃｔ C ｄｌ ) or less is preferably selected. In contrast, in the third embodiment, unlike the first and second embodiments, it is preferable to perform machine learning on data of a wide frequency band. For example, it is preferable to perform machine learning on the impedance value or its change in each of the low frequency band and the high frequency band. The frequency at which the impedance change is obtained in each of the low frequency band and the high frequency band is not limited to one, but may be multiple. If a low frequency band of 500 kHz or less is selected and the analyzed impedance is used as a state variable for machine learning, there is a high correlation between the state variable and corrosion resistance, and corrosion resistance can be evaluated with high accuracy.

[0054] As shown in the third embodiment, by performing machine learning on the state variables obtained in this manner, it is possible to quantitatively obtain a tendency for high corrosion resistance.

[0055] Although one embodiment of the present invention has been described above, this embodiment is presented as an example and is not intended to limit the scope of the invention. This embodiment can be embodied in various other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. These embodiments and their modifications are included within the scope and spirit of the invention, as well as the inventions described in the claims and their equivalents.

[0056] The corrosion resistance of a test material serving as a working electrode was actually measured using the corrosion resistance evaluation method of the present invention. Example 1: A base material 11 was made of iron, and a ZnNi topcoat was applied as a plating 12 to the surface of the iron, which is a highly corrosion-resistant plating that clearly shows the progression of corrosion over a salt spray test time. The results of evaluating the corrosion resistance provided by the topcoat are shown in Figure 12.

[0057] The top coat in Example 1 refers to a chromate coating that is often used in Zn-based plating. The reference electrode 3 was made of Ag / AgCl, the counter electrode 4 was made of Pt, and the electrolyte 2 was artificial seawater similar to that used in salt spray tests. Data was acquired according to the flowchart of the second embodiment shown in FIG.

[0058] The AC impedance measurement conditions for the PEIS measurement step S12 were an applied voltage of +50 mV from the natural potential with a voltage amplitude of ±10 mV, and measurements were taken from 3 MHz to 100 mHz. The CA measurement conditions for the constant voltage measurement step S14 were a current flow at +50 mV from the natural potential for 5 minutes, allowing corrosion. These steps were repeated 10 times, and the impedance |Z| at a frequency of 1 Hz was extracted, and its change over time was then analyzed.

[0059] 12, Sample 1 was ZnNi plated only, Sample 2 was ZnNi plated with a top coat treatment, and Sample 3 was ZnNi plated with a highly corrosion-resistant top coat treatment. Comparing the impedance |Z| after 7000 seconds, Sample 1 was 65 Ω, Sample 2 was 2323 Ω, and Sample 3 was 16759 Ω, suggesting that Sample 3 had the highest corrosion resistance.

[0060] Figure 13 shows the results of an actual salt spray test performed on each of the above samples 1 to 3, and visual observations of the elapsed time. As shown in Figure 13, white rust appeared on sample 1 after 120 hours, and red rust appeared after 1000 hours. White rust appeared on sample 2 after 120 hours, but red rust did not appear until 3000 hours. White rust first appeared on sample 3 after 3000 hours. It was confirmed that the relationship between the impedance |Z| values ​​after 7000 seconds, obtained in the initial state by electrochemical measurement, can quantitatively indicate the superiority or inferiority of the corrosion resistance obtained in the salt spray test. Furthermore, this prediction was completed in just 7000 seconds.

[0061] Example 2 The substrate 11 was made of iron, and the surface thereof was coated with a ZnNi plating as the plating 12, which is a highly corrosion-resistant plating that clearly shows the progression of corrosion over a salt spray test time. The corrosion resistance provided by the top coat was evaluated, and the results are shown in FIG. 14 .

[0062] In Example 2, data was acquired according to the flowchart of Fig. 4, which is the first embodiment described above. As in Example 1, ZnNi plating was used for the plating 12, Ag / AgCl was used for the reference electrode 3, Pt was used for the counter electrode 4, and artificial seawater used in the salt spray test was used for the electrolyte 2.

[0063] Sample 4 was ZnNi plated, Sample 5 was ZnNi plated + top coat, Sample 6 was ZnNi plated with an inorganic silica coat as a top coat (paint film), and Sample 7 was Zn plated + highly corrosion-resistant top coat. The top coats of Samples 5 and 7 were chromate films, which are often used in Zn-based materials.

[0064] The AC impedance measurement in the GEIS measurement step S2 is performed at a reference current of 0 μA / cm ２ and the current amplitude was ±0.5 μA / cm ２ Then, CP measurement was performed as a constant current measurement step S3, where a current of 3 μA was applied for a fixed period of time. These steps S2 to S3 were repeated 10 times, and the change over time in impedance |Z| obtained at a frequency of 1 Hz was plotted ( FIG. 14 ).

[0065] According to the results shown in Fig. 14, the impedance |Z| after 7000 seconds was 701 Ω for sample 4, 6160 Ω for sample 5, 8337 Ω for sample 6, and 13781 Ω for sample 7. Fig. 15 shows the results of the salt spray test, and when the superiority or inferiority of corrosion resistance was determined based on the time until the appearance of white rust and red rust, results suggesting the results shown in Fig. 14 were obtained.

[0066] Furthermore, in the process flow of the second embodiment, the magnitude of the impedance |Z| immediately after the start of AC impedance measurement (the value on the far left in FIG. 14 ) reflects the influence of the presence or absence and type of top coat. Sample 4 has no top coat, Samples 5 and 7 have chromate coatings, and Sample 6 has a paint film, and the differences can be determined by the magnitude of the impedance |Z|.

[0067] Example 3 The substrate 11 was made of iron, and a ZnNi plating, which is a highly corrosion-resistant plating that can clearly show the progression of corrosion over a salt spray test time, was applied to the surface of the substrate 11 as the plating 12. The corrosion resistance provided by the top coat was evaluated, and the results are shown in FIG. 16 .

[0068] In this Example 2, the reference current for AC impedance measurement is 0 μA / cm ２ Although the current value was different from that during the CP measurement in the constant current measurement step S3, it was performed at the same current value. Specifically, the conditions for the AC impedance measurement in the GEIS measurement step S2 were a reference current of 3 μA and a current amplitude of ±0.5 μA, and the constant current value for the CP measurement conditions was also 3 μA. These GEIS measurement steps S2 to S3 were repeated 10 times, and the change over time in impedance |Z| obtained at a frequency of 1 Hz was plotted ( FIG. 16 ).

[0069] Sample 8 was ZnNi plated, Sample 9 was ZnNi plated and then chromate-treated, Sample 10 was ZnNi plated and then coated, and Sample 11 was ZnNi plated and then highly corrosion-resistant chromate-treated. As a result of the measurement, the final impedance |Z| was 730 Ω for Sample 8, 4315 Ω for Sample 9, 6285 Ω for Sample 10, and 9460 Ω for Sample 11.

[0070] FIG. 17 shows the results of the salt spray test, and a correlation was obtained in which the higher the impedance |Z|, the higher the corrosion resistance.

[0071] Example 4 In Example 4, the substrate 11 was made of iron, and the surface thereof was coated with a metal plating 12, which allows corrosion progression to be clearly determined over a salt spray test time. The data obtained in the first embodiment was used as a state variable, and corrosion resistance was analyzed using a trained machine learning model. Fig. 21 is a corrosion resistance map based on the training data. Fig. 21 shows the training data and the results of the measurement sample.

[0072] The metal plating near the origin in Figure 21 has the lowest corrosion resistance in the salt spray test, and corrosion resistance increases when Corrosion Resistance Learning Axis 1 and Corrosion Resistance Learning Axis 2 each show larger positive values. The open circles represent training data, and the measured samples were classified using a machine learning model based on this data (8.92, 2.09). This point is shown as a black circle. When this measured sample was subjected to a salt spray test, it was revealed that it still had good corrosion resistance even after 3,000 hours, making it possible to use machine learning to identify samples with high corrosion resistance in advance.

[0073] According to Examples 1 to 4 described above, the impedance |Z| obtained at the final stage of the repetitions could be obtained within three hours under any of the conditions. The obtained results correlate with the results obtained after 3,000 hours in a salt spray test. According to the Accelerated Exposure Test Handbook (published by the Japan Weathering Test Center Foundation), a conventional salt spray test is considered to be an accelerated test equivalent to one year in 240 hours. Therefore, in order to guarantee a durability for several decades, corrosion resistance could only be assessed qualitatively unless environmental testing of several thousand hours was conducted. However, according to the corrosion resistance evaluation method of the present invention, results equivalent to 3,000 hours in a conventional salt spray test could be obtained in a short time of, for example, three hours, confirming that corrosion resistance can be quantitatively and efficiently determined in a short time.

[0074] According to the corrosion resistance evaluation method of the present invention, the corrosion resistance of the constituent material of the working electrode can be evaluated from the change over time in the absolute value |Z| of the impedance Z extracted at a frequency equal to or lower than the apex frequency of the semicircle (Nyquist plot) in the GEIS method or the PEIS method, in about 1 / 1000 hours of the conventional salt spray test. This allows the corrosion resistance of various products offered on the market to be evaluated in a short time, greatly improving the efficiency of product development. Therefore, the method has industrial applicability.

[0075] REFERENCE SIGNS LIST 1... Test material (working electrode) 2... Electrolyte 3... Reference electrode 4... Counter electrode 6... Liquid tank 7... Working electrode terminal 8... Potentiostat device 10... Evaluation device 11... Base material 12... Plating 13... Coating film

Claims

1. A corrosion resistance evaluation method for evaluating the corrosion resistance of a constituent material of a working electrode against an electrolyte by a three-electrode method using a working electrode, a counter electrode, and a reference electrode, each of which is in contact with the electrolyte, the method comprising: an initial natural potential measurement step for measuring the natural potential when no current flows from the working electrode to the counter electrode; a constant current alternating current impedance measurement step for applying a reference current between the working electrode and the counter electrode and varying the frequency to perform measurement by a constant current alternating current impedance measurement method (GEIS method); and a constant current measurement step for performing constant current measurement by a chronopotentiometry method (CP method), wherein the GEIS measurement step and the constant current measurement step are alternately repeated multiple times, and the method evaluates the corrosion resistance of a constituent material of the working electrode from the change over time in the absolute value |Z| of impedance Z extracted at a frequency equal to or lower than the apex frequency of a semicircle (Nyquist plot) in the GEIS method.

2. A corrosion resistance evaluation method for evaluating the corrosion resistance of a material constituting a working electrode against an electrolyte by a three-electrode method using a working electrode, a counter electrode, and a reference electrode, each of which is in contact with the electrolyte, the method comprising: an initial rest potential measurement step for measuring an initial rest potential when no current flows from the working electrode to the counter electrode; a PEIS measurement step for performing measurement by a constant voltage alternating current impedance method (PEIS method) by varying the frequency at a first applied voltage between the working electrode and the counter electrode; a first rest potential measurement step for measuring a first rest potential after the PEIS measurement step; a constant voltage measurement step for performing constant voltage measurement by a chronoamperometry method (CA method) at a second applied voltage; and a second rest potential measurement step for measuring a second rest potential after the constant voltage measurement step, wherein the first applied voltage is a voltage obtained by adding an additional voltage in the range of 1 mV to 200 mV to the initial rest potential or the second rest potential, the second applied voltage is a voltage obtained by adding an additional voltage in the range of 1 mV to 200 mV to the first natural potential, the PEIS measurement step, the first natural potential measurement step, the constant voltage measurement step, and the second natural potential measurement step are repeated multiple times in this order, and the method for evaluating corrosion resistance evaluates the corrosion resistance of a constituent material of the working electrode from the change over time in the absolute value |Z| of impedance Z extracted at a frequency equal to or lower than the apex frequency of a semicircle (Nyquist plot) in the PEIS method.

3. In the GEIS method and the CP method, the current density flowing through the working electrode is 100 μA / cm ２ The corrosion resistance evaluation method according to claim 1, wherein:

4. In the PEIS method and the CA method, the current density flowing through the working electrode is 100 μA / cm ２ The corrosion resistance evaluation method according to claim 2, wherein:

5. The corrosion resistance evaluation method according to claim 1 or 2, wherein the electrolyte is natural seawater or artificial seawater.

6. The frequency in the GEIS method is determined by the charge transfer resistance R ｃｔ and the electric double layer capacitance C ｄｌ 1 / (2πR ｃｔ C ｄｌ 2. The corrosion resistance evaluation method according to claim 1, wherein the frequency is lower than that of the reference frequency.

7. The frequency in the PEIS method is determined by the charge transfer resistance R ｃｔ and the electric double layer capacitance C ｄｌ 1 / (2πR ｃｔ C ｄｌ 3. The corrosion resistance evaluation method according to claim 2, wherein the frequency is lower than that of the reference frequency.

8. The corrosion resistance evaluation method according to claim 1, wherein the GEIS measurement step and the constant current measurement step are alternately repeated four or more times.

9. The corrosion resistance evaluation method according to claim 2, wherein the PEIS measurement step, the first natural potential measurement step, the constant voltage measurement step, and the second natural potential measurement step are repeated four or more times in this order.

10. The corrosion resistance evaluation method according to claim 1, wherein in the GEIS method, the frequency is varied from a high frequency side to a low frequency side.

11. The corrosion resistance evaluation method according to claim 2, wherein in the PEIS method, the frequency is varied from a high frequency side to a low frequency side.

12. The corrosion resistance evaluation method according to claim 1, comprising a step of learning the corrosion resistance according to a training data set created based on the state variables, the frequency used in the GEIS measurement, the number of times the GEIS measurement process and the constant current measurement process are repeated, and the impedance values ​​or their changes at multiple frequencies as state variables.

13. The corrosion resistance evaluation method according to claim 2, comprising a step of learning the corrosion resistance according to a training data set created based on the state variables, the frequency used in the PEIS measurement, the number of repetitions of the PEIS measurement step, the first natural potential measurement step, the constant voltage measurement step, and the second natural potential measurement step, and the impedance value being state variables.

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