Inspection method of separator for fuel cell and ferroxyl test liquid

Abstract

This method for inspecting a fuel cell separator (10) comprises a ferroxyl test step for detecting a defective portion on the surface of a separator (10) in which a corrosion-resistant film (14) containing titanium and a conductive film (16) are formed on a base material (12) made of stainless steel. An aqueous solution containing potassium ferrocyanide, potassium ferricyanide, chloride ions, hydrogen fluoride, and an acid and having a pH of 3 or less is used as a ferroxyl test liquid (26).

Description

Method for Inspecting Fuel Cell Separator and Ferroxyl Test Solution

[0001] The present disclosure relates to a method for inspecting a fuel cell separator and a ferroxyl test solution.

[0002] In recent years, research and development on fuel cells that contribute to energy efficiency have been conducted to enable more people to secure access to affordable, reliable, sustainable and advanced energy (for example, Japanese Patent Application Laid-Open No. 2012-59485 (Document 1)).

[0003] Incidentally, in the technology related to fuel cells, a fuel cell separator (hereinafter referred to as a separator) is used inside a power generation cell. Since separators are required to have both conductivity and corrosion resistance, surface treatment for forming a corrosion-resistant film and a conductive film is performed on the surface of a base material such as stainless steel constituting the separator.

[0004] Corrosion-resistant films and conductive films inevitably have fragile portions such as film thickness variations, cracks, and pinholes. Such fragile portions serve as starting points for peeling of the corrosion-resistant film or conductive film and dissolution of the base material, and cause deterioration of the characteristics of the power generation cell due to iron ion contamination. Therefore, quality control of corrosion-resistant films and conductive films is important for separators based on stainless steel.

[0005] As one of the methods available for quality control of such separator coatings, an elution amount evaluation test has been proposed. Details of the elution amount evaluation test are described in "NEDO PEFC Cell Evaluation and Analysis Protocol 2023 Edition" (hereinafter referred to as Document 2), pp. 42 to 43, New Energy and Industrial Technology Development Organization (NEDO), published in July 2023, URL: https: / / www.nedo.go.jp / content / 100963953.pdf. In the elution amount evaluation test, a test solution for evaluation is brought into contact with a region of a predetermined size of the separator for a predetermined time, and the elution amount of iron eluted into the test solution is quantified by ICP analysis.

[0006] However, the above-mentioned elution evaluation test has limitations in the size that can be tested, and can only be performed on a portion of the separator. In addition, the elution evaluation test is time-consuming to obtain results and is costly.

[0007] This invention aims to solve the above-mentioned problems by achieving shorter inspection times and lower costs for corrosion-resistant coatings. Furthermore, it contributes to energy efficiency through improved fuel cell quality.

[0008] A first aspect of the present disclosure is a method for inspecting a fuel cell separator, comprising the steps of: preparing a fuel cell separator having a base material made of stainless steel, a corrosion-resistant coating containing titanium formed on the base material, and a conductive coating formed on the corrosion-resistant coating; and a ferroxyl test step of contacting a test paper containing a ferroxyl test solution with the surface of the fuel cell separator and detecting defective portions of the corrosion-resistant coating and the conductive coating on the surface of the fuel cell separator from discoloration spots on the test paper, wherein the ferroxyl test solution is an aqueous solution containing potassium ferrocyanide, potassium ferricyanide, chloride ions, hydrogen fluoride, and an acid, and has a pH of 3 or less.

[0009] A second aspect of the present disclosure is a ferroxyl test solution used for ferroxyl testing of a metallic material having a titanium-containing corrosion-resistant film formed on the surface of a substrate made of iron or an iron alloy, the ferroxyl test solution being an aqueous solution containing potassium ferrocyanide, potassium ferricyanide, chloride ions, hydrogen fluoride, and sulfuric acid, wherein the concentration of chloride ions is 1 to 2 mol / L, the concentration of hydrogen fluoride is 50 to 90 ppm, and the pH is 2 to 3.

[0010] The fuel cell separator inspection method and ferroxyl test solution disclosed herein can inspect the quality of corrosion-resistant coatings and conductive coatings in a short time and at low cost.

[0011] Figure 1 is a cross-sectional view of a separator for a fuel cell. Figure 2 is an explanatory diagram illustrating the problems of conventional ferroxyl test solutions. Figure 3A is an explanatory diagram of the step of placing a test paper on the separator in the separator test method according to the embodiment, and Figure 3B is an explanatory diagram of the step of dropping the ferroxyl test solution onto the test paper in Figure 3A. Figure 4 is an explanatory diagram of the reaction between the ferroxyl test solution and the separator according to the embodiment. Figure 5 is a photograph showing an example of a blue discoloration spot that occurs on the test paper. Figure 6 is a table showing the content of some components of the ferroxyl test solution according to Comparative Examples 1 to 10 and Examples 1 to 6, the stability of the ferroxyl test solution, and the evaluation results of its suitability for quality control of the separator. Figure 7 is a graph showing the correlation between the amount of iron eluted from the separator based on the elution amount evaluation test for Example 7 and the number of discoloration spots detected by the method of the embodiment. Figure 8 is a graph showing the correlation between the amount of iron leached from a separator that has undergone a degradation acceleration process, based on an elution evaluation test, and the number of discolored spots detected by the method of the embodiment, for Example 8.

[0012] A fuel cell stack using a solid polymer electrolyte membrane includes multiple stacked power generation cells. Each power generation cell is separated from each other by a pair of conductive fuel cell separators (hereinafter referred to as separator 10). Since the separator 10 also serves as a path for the current generated by the electrode reaction of the fuel cell, it is required to be both corrosion-resistant and conductive.

[0013] As shown in Figure 1, the separator 10 to be inspected in this embodiment comprises a base material 12 mainly made of highly corrosion-resistant stainless steel (iron alloy). Furthermore, a corrosion-resistant film 14 made of titanium is formed on the surface of the base material 12, and a conductive film 16 is formed on the corrosion-resistant film 14.

[0014] Because these corrosion-resistant coatings 14 and conductive coatings 16 are thin, only a few hundred nanometers thick, pinhole-like, crack-like, or island-like defects inevitably occur. Figure 1 shows an example of a defect that, due to long-term use of the fuel cell, becomes the starting point for corrosion of the substrate 12 of the separator 10 and causes iron leaching. The through-defect 18 is a defect that penetrates from the surface of the conductive coating 16 to the upper surface of the substrate 12. The through-defect 18 becomes the starting point for corrosion of the substrate 12, and is a part where peeling and dissolution of the corrosion-resistant coating 14 and conductive coating 16 are likely to progress.

[0015] The micro-defects 20 are defects that penetrate from the surface of the conductive film 16 to the upper surface of the substrate 12, but have an even finer width or diameter than the through-defects 18. Over time, such defects may increase in width or diameter and transform into through-defects 18, becoming a source of iron leaching. The areas with reduced film thickness 22 are areas where the corrosion-resistant film 14 or the conductive film 16 has become thin, or where the conductive film 16 has peeled off. In the initial state, the substrate 12 is covered with the corrosion-resistant film 14 in these areas with reduced film thickness 22, but with prolonged use, the corrosion-resistant film 14 erodes, exposing the substrate 12 and becoming a source of iron leaching.

[0016] In quality control of the separator 10, detection of through-defects 18, micro-defects 20, and areas of reduced film thickness 22 is important. The inventors of this application investigated a ferroxyl test method in order to detect defective areas more quickly and at lower cost across the entire surface of the separator 10.

[0017] The ferroxyl test method involves immersing a test strip 24 (see Figure 3A) in a mixed solution of potassium ferrocyanide, potassium ferricyanide, and sodium chloride (ferroxyl test solution 26), then attaching it to the plated surface of a steel substrate to examine pinholes in the plating. After a few minutes, the test strip 24 is peeled off and dried, and the degree of plating defects is determined based on the number of blue discoloration spots 28 (blue spots, see Figure 5) formed by the reaction between the iron substrate and the reagents.

[0018] A method for testing ferroxyl content in nickel-plated films or nickel / chromium-plated films applied to the surface of a steel substrate is known, as described in the Japanese Industrial Standard JIS H-8617:1999.

[0019] However, it was found that with conventional ferroxyl testing methods, only through-defects 18 can be detected in fuel cell separators 10 covered with a titanium-containing corrosion-resistant coating 14, and micro-defects 20 and areas with reduced film thickness 22 cannot be detected. In other words, micro-defects 20 shown in Figure 2 are likely to be overlooked because the degree of discoloration of the colored spots in the ferroxyl test is faint. Also, areas with reduced film thickness 22 cannot be detected because iron does not dissolve, and therefore no discolored spots 28 appear.

[0020] Therefore, the number (and size) of discolored spots 28 determined by the conventional ferroxyl test method does not correlate with the amount of iron leached in the leaching evaluation test (Reference 2), which presents a problem as it is not suitable for quality control of the separator 10.

[0021] (Embodiment) As shown in Figure 3A, the inspection method for the separator 10 of this embodiment involves placing a test paper 24 on its surface. White filter paper is preferably used as the test paper 24. Then, as shown in Figure 3B, ferroxyl test solution 26 is dropped onto the test paper 24, allowing the ferroxyl test solution 26 to permeate the test paper 24. This causes the test paper 24 containing the ferroxyl test solution 26 to come into contact with the separator 10. Alternatively, a test paper 24 that has been pre-soaked in the ferroxyl test solution 26 may be placed on the surface of the separator 10.

[0022] The ferroxyl test solution 26 of this embodiment is an aqueous solution obtained by dissolving 10 g of potassium hexacyanoferrate(II) trihydrate (potassium ferrocyanide), 10 g of potassium hexacyanoferrate(III) (potassium ferricyanide), a predetermined amount of chloride ions, an acid, and hydrogen fluoride in pure water to make 1000 ml.

[0023] Chloride ions are added to dissolve the passivation film formed on the surface of stainless steel and to promote the elution of iron ions from the stainless steel. Chloride ions are supplied from water-soluble chlorides such as sodium chloride, potassium chloride, calcium chloride, and magnesium chloride. Sodium chloride is suitably used in terms of cost and ease of handling. Chloride ions (sodium chloride) are preferably added at a concentration of, for example, 1 to 2 mol / L.

[0024] The acid is added to lower the pH of the ferroxyl test solution 26, thereby increasing the reactivity of the ferroxyl test solution 26 with the substrate 12 and the corrosion-resistant coating 14. The addition of the acid enables the detection of through-defects 18, micro-defects 20, and areas of reduced film thickness 22 in a short time.

[0025] In this embodiment, it is preferable that the acid be added at a concentration such that the pH is 3 or lower. Furthermore, if the pH of the ferroxyl test solution 26 drops too low, the stability and shelf life of the ferroxyl test solution 26 will decrease. For example, if it is desired to ensure stability of the ferroxyl test solution 26 for more than 15 hours after preparation, it is preferable to keep the amount of acid added within a range above pH 2. However, if the ferroxyl test solution 26 is to be used immediately, the amount of acid added may be slightly below pH 2.

[0026] In this embodiment, hydrochloric acid or sulfuric acid can be used as the acid added to the ferroxyl test solution 26. Sulfuric acid is preferable because it does not affect the amount of chloride ions, thus simplifying the preparation of the ferroxyl test solution 26.

[0027] As shown in Figure 4, hydrogen fluoride is added to corrode the titanium-containing corrosion-resistant coating 14, enabling the detection of micro-defects 20 and areas of reduced film thickness 22. The addition of hydrogen fluoride widens the diameter or width of the micro-defects 20 and dissolves the corrosion-resistant coating 14 in the areas of reduced film thickness 22, thereby exposing the substrate 12.

[0028] Hydrogen fluoride is preferably added at a concentration of 50 ppm or higher. If the concentration is lower than 50 ppm, the number of discolored spots 28 will not be consistent with the results of the elution amount evaluation test (Reference 2), and the suitability for quality control will decrease. If the concentration of hydrogen fluoride is too high, the stability of the ferroxyl test solution 26 will decrease, and its storage life will be reduced. If the ferroxyl test solution 26 deteriorates, it will become difficult to detect the discolored spots 28. For example, if it is desired to ensure stability of the ferroxyl test solution 26 for more than 15 hours after preparation, it is preferable to add hydrogen fluoride to 90 ppm or less. However, if the ferroxyl test solution 26 is to be used immediately, the amount of hydrogen fluoride added may slightly exceed 90 ppm.

[0029] Hydrogen fluoride may be added to the ferroxyl test solution 26 by directly adding hydrofluoric acid as a reagent. Alternatively, the hydrogen fluoride may be supplied in the ferroxyl test solution 26 by a reaction between a fluoride salt and an acid (sulfuric acid). Examples of such fluoride salts include sodium fluoride, potassium fluoride, ammonium fluoride, aluminum fluoride, and lithium fluoride.

[0030] After dropping the ferroxyl test solution 26, the reaction with the separator 10 is completed in a few minutes (e.g., 5 minutes) to about 10 minutes. As a result, as shown in Figure 4, blue discoloration spots 28 are formed on the test paper 24 at the defect locations of the corrosion-resistant film 14 and the conductive film 16 due to a color reaction with the eluted iron ions. The quality of the separator 10 is evaluated based on the number of discoloration spots 28 formed on the test paper 24. Furthermore, as shown in Figures 7 and 8, the number of discoloration spots 28 correlates with the amount of iron eluted according to the elution amount evaluation test (Reference 2), so the amount of iron eluted from the separator 10 can be estimated from the number of discoloration spots 28.

[0031] The following description of the ferroxyl test solution 26 in the comparative examples and examples will be given with reference to Figure 6. Note that since the concentrations of potassium ferrocyanide and potassium ferricyanide are the same in all comparative examples and examples, the concentration figures for these components are omitted in Figure 6.

[0032] (Comparative Example 1) In Comparative Example 1, the separator 10 was evaluated using the ferroxyl test solution 26 described in Japanese Industrial Standard JIS H-8617 as the ferroxyl test solution 26. The ferroxyl test solution 26 in Comparative Example 1 is an aqueous solution prepared by dissolving 10 g of potassium hexacyanoferrate(II) trihydrate (potassium ferrocyanide), 10 g of potassium hexacyanoferrate(III) (potassium ferricyanide), and 58.4 g (1 mol) of sodium chloride in pure water to make 1000 ml. The ferroxyl test solution 26 in Comparative Example 1 contains 1 mol / L of chloride ions. Note that the ferroxyl test solution 26 in Comparative Example 1 does not contain hydrogen fluoride or sulfuric acid.

[0033] The ferroxyl test solution 26 of Comparative Example 1 was confirmed to maintain its stability without deterioration even after 15 hours from preparation. However, when the ferroxyl test solution 26 of Comparative Example 1 was used, the number of discolored spots 28 did not match the results of the elution amount evaluation test (Reference 2), and the result was unsuitable for quality control of the separator 10.

[0034] (Comparative Example 2) Comparative Example 2 is the same as Comparative Example 1 except that the concentration of sodium chloride is 2 mol / L, which is twice that of Comparative Example 1. It was confirmed that the ferroxyl test solution 26 of Comparative Example 2 did not deteriorate and maintained its stability even after 15 hours had elapsed since preparation. However, even when using the ferroxyl test solution 26 of Comparative Example 2, the number of discolored spots 28 that were consistent with the results of the elution amount evaluation test (Reference 2) was not obtained, and the result was not suitable for quality control of the separator 10.

[0035] (Comparative Example 3) Comparative Example 3 was evaluated using a ferroxyl test solution 26 prepared by adding hydrogen fluoride at a concentration of 50 ppm to the ferroxyl test solution 26 of Comparative Example 1. It was confirmed that the ferroxyl test solution 26 of Comparative Example 3 remained stable without deterioration even after 15 hours from preparation. When using the ferroxyl test solution 26 of Comparative Example 3, there was a tendency for the number of discolored spots 28 to increase compared to Comparative Examples 1 and 2. However, the number of discolored spots 28 did not sufficiently match the results of the elution amount evaluation test (Reference 2), and the result was not suitable for quality control of the separator 10.

[0036] (Comparative Example 4) Comparative Example 4 was evaluated using a ferroxyl test solution 26 prepared by adding hydrogen fluoride at a concentration of 90 ppm to the ferroxyl test solution 26 of Comparative Example 1. It was confirmed that the ferroxyl test solution 26 of Comparative Example 4 remained stable without deterioration even after 15 hours from preparation. When using the ferroxyl test solution 26 of Comparative Example 4, there was a tendency for the number of discolored spots 28 to increase compared to Comparative Examples 1 and 2. However, the number of discolored spots 28 did not sufficiently match the results of the elution amount evaluation test (Reference 2), and the result was not suitable for quality control of the separator 10.

[0037] (Comparative Example 5) Comparative Example 5 was evaluated using a ferroxyl test solution 26 prepared by adding hydrogen fluoride at a concentration of 180 ppm to the ferroxyl test solution 26 of Comparative Example 1. The ferroxyl test solution 26 of Comparative Example 5 was intended to improve the reactivity with the titanium-containing corrosion-resistant coating 14 by increasing the concentration of hydrogen fluoride. However, the ferroxyl test solution 26 of Comparative Example 5 deteriorated within 15 hours of preparation and did not show a color change reaction with respect to iron ions. Therefore, the ferroxyl test solution 26 of Comparative Example 5 lacked stability and was not suitable for quality control of the separator 10.

[0038] (Comparative Example 6) Comparative Example 6 was evaluated using a ferroxyl test solution 26 prepared by adding hydrogen fluoride at a concentration of 360 ppm to the ferroxyl test solution 26 of Comparative Example 1. The ferroxyl test solution 26 of Comparative Example 6 was intended to improve the reactivity with the titanium-containing corrosion-resistant coating 14 by increasing the concentration of hydrogen fluoride. However, the ferroxyl test solution 26 of Comparative Example 6 deteriorated within 15 hours of preparation and did not show a color change reaction to iron ions. Therefore, the ferroxyl test solution 26 of Comparative Example 6 lacked stability and was not suitable for quality control of the separator 10.

[0039] (Comparative Example 7) Comparative Example 7 was evaluated using a ferroxyl test solution 26 prepared by adding sulfuric acid to the ferroxyl test solution 26 of Comparative Example 1. Sulfuric acid was added to the ferroxyl test solution 26 of Comparative Example 7 so that the pH was 0.5. The ferroxyl test solution 26 of Comparative Example 7 deteriorated within 15 hours of preparation and did not show a color change reaction to iron ions. Therefore, the ferroxyl test solution 26 of Comparative Example 7 lacked stability and was not suitable for quality control of the separator 10. From this result, it became clear that an excessively low pH value also adversely affects the stability of the ferroxyl test solution 26.

[0040] (Comparative Example 8) Comparative Example 8 was evaluated using a ferroxyl test solution 26 prepared by adding sulfuric acid to the ferroxyl test solution 26 of Comparative Example 1. Sulfuric acid was added to the ferroxyl test solution 26 of Comparative Example 8 so that the pH was 1. The ferroxyl test solution 26 of Comparative Example 8 deteriorated within 15 hours of preparation and did not show a color change reaction to iron ions. Therefore, the ferroxyl test solution 26 of Comparative Example 8 lacked stability and was not suitable for quality control of the separator 10. From this result, it was found that the stability of the ferroxyl test solution 26 was insufficient even at a pH of around 1.

[0041] (Comparative Example 9) Comparative Example 9 was evaluated using a ferroxyl test solution 26 prepared by adding sulfuric acid to the ferroxyl test solution 26 of Comparative Example 1. Sulfuric acid was added to the ferroxyl test solution 26 of Comparative Example 9 so that the pH was 2. It was confirmed that the ferroxyl test solution 26 of Comparative Example 9 remained stable without deterioration even after 15 hours from preparation. When using the ferroxyl test solution 26 of Comparative Example 9, there was a tendency for the number of discolored spots 28 to increase compared to Comparative Example 1. However, it was not possible to generate discolored spots 28 in the areas with reduced film thickness 22 where a thin corrosion-resistant film 14 remained.

[0042] (Comparative Example 10) In Comparative Example 10, evaluation was performed using the ferroxyl test solution 26 obtained by adding sulfuric acid to the ferroxyl test solution 26 of Comparative Example 1. Sulfuric acid was added to the ferroxyl test solution 26 of Comparative Example 10 so that the pH thereof became 3. It was confirmed that the ferroxyl test solution 26 of Comparative Example 10 did not deteriorate and maintained stability even after 15 hours elapsed from preparation. When the ferroxyl test solution 26 of Comparative Example 10 was used, a tendency of an increase in the number of discolored spots 28 was observed compared to Comparative Example 1. However, as a result, no discolored spots 28 could be formed in the reduced film thickness portion 22 where the thin corrosion-resistant film 14 remained.

[0043] (Example 1) In Example 1, evaluation was performed using the ferroxyl test solution 26 obtained by adding sulfuric acid and hydrogen fluoride to the ferroxyl test solution 26 of Comparative Example 1. The ferroxyl test solution 26 of Example 1 contains hydrogen fluoride at a concentration of 50 ppm, and is adjusted to pH 2 by adding sulfuric acid. It was confirmed that the ferroxyl test solution 26 of Example 1 did not deteriorate and maintained stability even after 15 hours elapsed from preparation. Furthermore, when the ferroxyl test solution 26 of Example 1 was used, the number of discolored spots 28 consistent with the results of the elution amount evaluation test (Reference 2) was obtained, which was a result suitable for quality control of the separator 10.

[0044] (Example 2) In Example 2, evaluation was performed using the ferroxyl test solution 26 obtained by adding sulfuric acid and hydrogen fluoride to the ferroxyl test solution 26 of Comparative Example 1. The ferroxyl test solution 26 of Example 2 contains hydrogen fluoride at a concentration of 50 ppm, and is adjusted to pH 3 by adding sulfuric acid. It was confirmed that the ferroxyl test solution 26 of Example 2 did not deteriorate and maintained stability even after 15 hours elapsed from preparation. Furthermore, according to the ferroxyl test solution 26 of Example 2, the number of discolored spots 28 consistent with the results of the elution amount evaluation test (Reference 2) was obtained, which was a result suitable for quality control of the separator 10.

[0045] (Example 3) In Example 3, evaluation was performed using a ferroxyl test solution 26 prepared by adding sulfuric acid and hydrogen fluoride to the ferroxyl test solution 26 of Comparative Example 1. The ferroxyl test solution 26 of Example 3 contained sodium chloride at a concentration of 2 mol / L as a source of chloride ions, hydrogen fluoride at a concentration of 50 ppm, and the pH was adjusted to 2 by adding sulfuric acid. It was confirmed that the ferroxyl test solution 26 of Example 3 maintained its stability without deterioration even after 15 hours from preparation. Furthermore, the ferroxyl test solution 26 of Example 3 produced a number of discolored spots 28 consistent with the results of the elution amount evaluation test (Reference 2), resulting in a quality control suitable for the separator 10.

[0046] (Example 4) In Example 4, evaluation was performed using a ferroxyl test solution 26 prepared by adding sulfuric acid and hydrogen fluoride to the ferroxyl test solution 26 of Comparative Example 1. The ferroxyl test solution 26 of Example 4 contained 2 mol / L of sodium chloride and 50 ppm of hydrogen fluoride, and was adjusted to pH 3 by the addition of sulfuric acid. It was confirmed that the ferroxyl test solution 26 of Example 4 maintained its stability without deterioration even after 15 hours from preparation. Furthermore, the ferroxyl test solution 26 of Example 4 produced a number of discolored spots 28 consistent with the results of the elution amount evaluation test (Reference 2), resulting in a quality control suitable for the separator 10.

[0047] (Example 5) In Example 5, evaluation was performed using a ferroxyl test solution 26 prepared by adding sulfuric acid and hydrogen fluoride to the ferroxyl test solution 26 of Comparative Example 1. The ferroxyl test solution 26 of Example 5 contained 2 mol / L of sodium chloride and 90 ppm of hydrogen fluoride, and was adjusted to pH 2 by the addition of sulfuric acid. It was confirmed that the ferroxyl test solution 26 of Example 5 maintained its stability without deterioration even after 15 hours from preparation. Furthermore, the ferroxyl test solution 26 of Example 5 produced a number of discolored spots 28 consistent with the results of the elution amount evaluation test (Reference 2), resulting in a quality control suitable for the separator 10.

[0048] (Example 6) In Example 6, evaluation was performed using a ferroxyl test solution 26 obtained by adding sulfuric acid and hydrogen fluoride to the ferroxyl test solution 26 of Comparative Example 1. The ferroxyl test solution 26 of Example 6 contains sodium chloride at 2 mol / L, hydrogen fluoride at a concentration of 90 ppm, and is adjusted to pH 3 by adding sulfuric acid. It was confirmed that the ferroxyl test solution 26 of Example 6 did not deteriorate and maintained stability even after 15 hours had elapsed from preparation. Furthermore, with the ferroxyl test solution 26 of Example 6, the number of discolored spots 28 that matched the results of the elution amount evaluation test (Reference 2) was obtained, providing results suitable for quality control of the separator 10.

[0049] (Example 7) In Example 7, detection of discolored spots 28 was performed on various separators 10 by the method of bringing the ferroxyl test solution 26 into contact with the test paper 24 described with reference to FIGS. 3A to 4. In this example, the ferroxyl test solution 26 of Example 1 was used. In addition, measurement of the iron elution amount by an elution amount evaluation test (Reference 2) was performed on another portion cut out from the same separator 10.

[0050] As shown in FIG. 7, a linear correlation was observed between the number of detected discolored spots 28 and the measurement results of the iron elution amount obtained by the elution amount evaluation test (Reference 2). In this example, an offset occurs where the iron elution amount does not become zero even when the number of detected discolored spots 28 is near zero. Therefore, although there is a possibility that leakage occurs in the detection of discolored spots 28, a generally linear correlation was obtained.

[0051] Therefore, it was confirmed by this example that the iron elution amount can be estimated from the number of discolored spots 28 detected by the ferroxyl test method. That is, the number of discolored spots 28 detected in this example can be evaluated in association with the quantitative measurement results of the iron elution amount, enabling rapid and low-cost quality control of the separator 10.

[0052] (Example 8) In Example 8, prior to the detection of the discolored spots 28, the separator 10 was subjected to a degradation acceleration process that simulated the degradation that occurs in a fuel cell that has been in operation for a long period of time. In this degradation acceleration process, the surface of the separator 10 was brought into contact with an aqueous solution containing hydrogen fluoride (second acid).

[0053] Subsequently, the discolored spots 28 were detected by contacting the test paper 24, as described with reference to Figures 3A to 4, with the ferroxyl test solution 26. In this example, the ferroxyl test solution 26 from Example 1 was used. In addition, the amount of iron leached was measured from another section cut from the same separator 10 using an elution evaluation test (Reference 2).

[0054] As shown in Figure 8, when the degradation acceleration process was performed, a linear correlation was observed between the number of discolored spots 28 and the amount of iron leached. In particular, as the number of discolored spots 28 approached zero, the amount of iron leached according to the leaching amount evaluation test (Reference 2) also approached zero. Therefore, according to this embodiment, it is possible to detect discolored spots 28 that affect the amount of iron leached without missing any. Furthermore, because of the excellent correlation between the number of discolored spots 28 and the amount of iron leached, more accurate quality control of the separator 10 becomes possible.

[0055] With regard to the above embodiments, the following additional information is disclosed.

[0056] (Note 1) The inspection method for a fuel cell separator (10) of the present disclosure comprises the steps of: preparing a fuel cell separator having a base material (12) made of stainless steel, a corrosion-resistant coating (14) containing titanium formed on the base material, and a conductive coating (16) formed on the corrosion-resistant coating; and a ferroxyl test step of contacting a test paper (24) containing a ferroxyl test solution (26) with the surface of the fuel cell separator and detecting defective portions of the corrosion-resistant coating and the conductive coating on the surface of the fuel cell separator from discoloration spots (28) on the test paper, wherein the ferroxyl test solution is an aqueous solution containing potassium ferrocyanide, potassium ferricyanide, chloride ions, hydrogen fluoride, and an acid, and has a pH of 3 or less.

[0057] The above inspection method for fuel cell separators can detect defects that could cause stainless steel corrosion over time, even in separators covered with a corrosion-resistant coating containing titanium, by identifying discolored spots in the ferroxyl test solution. This inspection method allows for rapid and low-cost quality control of fuel cell separators.

[0058] (Note 2) The fuel cell separator inspection method described in Note 1, wherein the ferroxyl test solution may contain a fluoride salt that reacts with the acid to generate hydrogen fluoride. This fuel cell separator inspection method allows for the preparation of the ferroxyl test solution using a fluoride salt that is relatively safe and easy to handle.

[0059] (Note 3) The fuel cell separator inspection method described in Note 1, wherein the ferroxyl test solution may have a hydrogen fluoride concentration of 50 ppm or more. This fuel cell separator inspection method facilitates the detection of areas where iron may leach out due to aging of the separator by promoting the reaction of deteriorated areas of the titanium-containing corrosion-resistant coating.

[0060] (Note 4) The fuel cell separator inspection method described in Note 3, wherein the ferroxyl test solution may have a hydrogen fluoride concentration of 90 ppm or less. This fuel cell separator inspection method offers excellent stability of the ferroxyl test solution.

[0061] (Note 5) The inspection method for fuel cell separators described in Note 1, wherein the acid may be hydrochloric acid or sulfuric acid. This inspection method for fuel cell separators allows for more reliable detection of defects by accelerating the leaching of iron from stainless steel.

[0062] (Note 6) The inspection method for fuel cell separators described in Note 1, wherein the ferroxyl test solution may contain chloride ions at a concentration of 1 to 2 mol / L. This inspection method for fuel cell separators can accelerate the dissolution of iron by removing the passivation from the surface of the stainless steel, thereby enabling more reliable detection of defective parts.

[0063] (Note 7) The inspection method for fuel cell separators described in Note 1, wherein the conductive film may contain carbon.

[0064] (Note 8) A method for inspecting a fuel cell separator as described in any one of Notes 1 to 7, which may include a degradation acceleration step prior to the ferroxyl test step, in which the fuel cell separator is brought into contact with a second acid capable of corroding the corrosion-resistant coating to degrade the corrosion-resistant coating. This method for inspecting a fuel cell separator exhibits excellent correlation between the number of discolored spots produced by the reaction with the ferroxyl test solution and the amount of iron eluted, and enables quantitative evaluation of the corrosion-resistant coating from the number of discolored spots.

[0065] (Note 9) The ferroxyl test solution of the present disclosure is a ferroxyl test solution used for ferroxyl testing of a metallic material having a corrosion-resistant film containing titanium formed on the surface of a substrate made of iron or an iron alloy, and is an aqueous solution containing potassium ferrocyanide, potassium ferricyanide, chloride ions, hydrogen fluoride, and sulfuric acid, wherein the concentration of chloride ions is 1 to 2 mol / L, the concentration of hydrogen fluoride is 50 to 90 ppm, and the pH is 2 to 3.

[0066] This ferroxyl test solution can be suitably used for ferroxyl testing on various steel substrates covered with a corrosion-resistant coating containing titanium.

[0067] While this disclosure has been described in detail, it is not limited to the individual embodiments described above. These embodiments can be added, replaced, modified, partially deleted, etc., in any way that does not depart from the gist of this disclosure or from the spirit of this disclosure derived from the claims and their equivalents. These embodiments can also be implemented in combination. For example, the order of operations and processes in the embodiments described above are given as examples only and are not limited thereto. The same applies when numerical values ​​or mathematical formulas are used in the description of the embodiments described above.

[0068] 10...Separator 12...Substrate 14...Corrosion-resistant coating 16...Conductive coating 18...Through-hole defect 20...Micro-defect 22...Thickness reduction area 24...Test paper 26...Ferroxyl test solution 28...Discolored spot

Claims

1. A method for inspecting a fuel cell separator, comprising the steps of: preparing a fuel cell separator (10) having a base material (12) made of stainless steel, a corrosion-resistant coating (14) containing titanium formed on the base material, and a conductive coating (16) formed on the corrosion-resistant coating; and a ferroxyl test step of contacting a test paper (24) containing a ferroxyl test solution (26) with the surface of the fuel cell separator, and detecting defective portions of the corrosion-resistant coating and the conductive coating on the surface of the fuel cell separator from discoloration spots (28) on the test paper, wherein the ferroxyl test solution is an aqueous solution containing potassium ferrocyanide, potassium ferricyanide, chloride ions, hydrogen fluoride, and an acid, and has a pH of 3 or less.

2. A method for inspecting a fuel cell separator according to claim 1, wherein the ferroxyl test solution contains a fluoride salt that reacts with the acid to generate hydrogen fluoride.

3. A method for inspecting a fuel cell separator according to claim 1, wherein the ferroxyl test solution has a hydrogen fluoride concentration of 50 ppm or more.

4. A method for inspecting a fuel cell separator according to claim 3, wherein the ferroxyl test solution has a hydrogen fluoride concentration of 90 ppm or less.

5. A method for inspecting a fuel cell separator according to claim 1, wherein the acid is hydrochloric acid or sulfuric acid.

6. A method for inspecting a fuel cell separator according to claim 1, wherein the ferroxyl test solution contains chloride ions at a concentration of 1 to 2 mol / L.

7. A method for inspecting a fuel cell separator according to claim 1, wherein the conductive film contains carbon.

8. A method for inspecting a fuel cell separator according to any one of claims 1 to 7, comprising a degradation acceleration step in which, prior to the ferroxyl test step, the fuel cell separator is brought into contact with a second acid capable of corrosive the corrosion-resistant coating to degrade the corrosion-resistant coating.

9. A ferroxyl test solution used for ferroxyl testing of a metallic material having a titanium-containing corrosion-resistant film formed on the surface of a substrate made of iron or an iron alloy, wherein the solution is an aqueous solution containing potassium ferrocyanide, potassium ferricyanide, chloride ions, hydrogen fluoride, and sulfuric acid, the concentration of the chloride ions being 1 to 2 mol / L, the concentration of the hydrogen fluoride being 50 to 90 ppm, and the pH being 2 to 3.

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