Method for testing anticorrosion properties of a coating and use thereof

Abstract

The invention discloses a test method for the anti-corrosion performance of a coating and an application thereof, and relates to the technical field of detection. The test method for the anti-corrosion performance of the coating provided by the invention comprises the following steps: S1. scribing on the surface of the coating to be tested to form a scratch with an initial width of w1; S2. applying a corrosive solution to the surface of the coating to be tested with the scratch obtained in step S1 for corrosion, wherein the corrosive solution comprises an aqueous salt solution, the salts comprise sodium chloride and ammonium sulfate in a mass ratio of (10-50):1, and the concentration of the sodium chloride in the corrosive solution is 3-5 wt%; the ratio w2 / w1 of the scratch width w2 after 480-720 h of corrosion to the initial scratch width is used as the evaluation basis for the test result of the anti-corrosion performance of the coating: when w2 / w1 ≤ 2.8, the anti-corrosion performance of the coating is determined to be qualified; when w2 / w1 > 2.8, the anti-corrosion performance of the coating is determined to be unqualified. The method provided by the invention can realize substitution for the traditional CCT detection method.

Description

Technical Field

[0001] This invention relates to the field of testing technology, and more specifically, to a method for testing the anti-corrosion performance of a coating and its application. Background Technology

[0002] Automotive metal parts are constantly exposed to splash zones and high-humidity, high-salt environments, so their corrosion resistance directly affects the durability and safety of the entire vehicle. Currently, the automotive industry typically coats metal parts with cathodic electrophoretic coatings such as epoxy and polyurethane to extend their service life. After vehicle production, a full-vehicle corrosion test is required to simulate the vehicle's performance under various operating conditions; only those that pass the test are released. The industry currently primarily uses the Cyclic Corrosion Test (CCT) 1008h (42 days) method as the standard test method for evaluating the corrosion resistance of coatings on these parts. The procedure is typically as follows: first, continuous spraying with a 5wt% NaCl aqueous solution at 35℃ for 4 hours; then drying at 60℃ for 2 hours; followed by standing at 50℃ and relative humidity ≥95% for 2 hours; after 1008h (126 cycles) of corrosion, the corrosion width is measured, and a corrosion width ≤2mm is generally considered acceptable. While this method is universal, CCT testing suffers from long cycles and high equipment costs. To address this, practitioners have proposed alternative methods, such as the Prohesion cyclic corrosion salt spray test. However, this method uses 0.05wt% NaCl and 0.35wt% (NH4)2SO4 as reagents, resulting in an extremely slow corrosion rate. To obtain test results equivalent to a 1008-hour CCT test, an experiment of over 5000 hours is required. Other alternative methods, such as the standard neutral salt spray test, have significantly different corrosion mechanisms and products compared to the CCT test process, leading to significantly different results that are not reliable for comparison.

[0003] Therefore, a simpler and faster testing method is needed. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention provides a method for testing the anti-corrosion performance of coatings and its application.

[0005] The above-mentioned objective of this invention is achieved through the following technical solution: A method for testing the anti-corrosion performance of a coating includes the following steps: S1. Scratch is made on the surface of the coating to be tested to form a scratch with an initial width of w1; S2. Apply an etching solution to the surface of the scratched coating obtained in step S1. The etching solution includes an aqueous solution of salt, wherein the salt comprises sodium chloride and ammonium sulfate in a mass ratio of (10-50):1, and the concentration of sodium chloride in the etching solution is 3-5 wt%. The ratio of the scratch width w2 after 480-720 h of etching to the initial scratch width w2 / w1 is used as the evaluation basis for the coating's anti-corrosion performance test result. If w2 / w1≤2.8, the coating's anti-corrosion performance is deemed qualified; if w2 / w1>2.8, the coating's anti-corrosion performance is deemed unqualified.

[0006] In the automotive industry, cathodic electrophoretic coatings (such as epoxy and polyurethane cathodic electrophoretic paints) are commonly used to protect metal parts. However, the typical failure mode of these coatings during corrosion testing is cathodic disbondment and propagation at scratches. The corrosion performance testing method provided in this application utilizes the synergistic effect of high-concentration chloride and sulfate ions. This improves both corrosion and testing efficiency, while maintaining a corrosion mechanism and products consistent with the commonly used CCT testing process in this industry, thus allowing for a replacement of the traditional CCT testing process. It should be noted that this application uses sodium chloride and ammonium sulfate together to prepare the corrosion solution because chloride ions play a major corrosive role, while ammonium sulfate, through hydrolysis, lowers the pH of the system and promotes corrosion. Sulfate ions can also damage the coating structure and enhance penetration. Furthermore, the corrosion mechanism and products after the introduction of ammonium sulfate are consistent with the traditional CCT corrosion process. Under the combined effect of these factors, this application can improve corrosion efficiency, and the test results are comparable to those of the traditional CCT process.

[0007] The inventors discovered that after 480-720 hours of corrosion, the scratch width w2 formed by the diffusion of corrosion products is close to the result of CCT 1008h. w2 / w1 ≤ 2.8, which is equivalent to a passing CCT test result, thus indicating that the coating performance is acceptable. w2 / w1 > 2.8, which is equivalent to a failing CCT test result. Taking an initial scratch width w1 = 0.5mm as an example, the standard for CCT 1008h is unilateral diffusion ≤ 2mm. The total scratch width corresponding to the acceptable limit is 0.5 + 2 + 2 = 4.5mm, and the critical value of w2 / w1 is 4.5 / 0.5 = 9. This application uses 2.8 as the judgment threshold, leaving sufficient safety margin.

[0008] In this application, the scratch described in step S1 is "X" shaped, and the initial width of the scratch refers to the width of any one side of the X-shaped scratch. Preferably, the widths of the two sides of the X-shaped scratch are the same. Preferably, the scratch described in step S1 penetrates the coating to be tested.

[0009] For example, the mass ratio of sodium chloride to ammonium sulfate in the salt described in step S2 can be 10:1, 11:1, 12:1, 13:1, 14:1, 15:1, 16:1, 17:1, 18:1, 19:1, 20:1, 25:1, 30:1, 35:1, 40:1, 45:1, 50:1, or any two of these values.

[0010] Preferably, the thickness of the coating to be tested in step S1 is 20-30 μm.

[0011] Preferably, 0.4 ≤ w1 ≤ 0.6 mm.

[0012] Preferably, the coating to be tested in step S1 includes a cathodic electrophoretic coating.

[0013] More preferably, the coating to be tested in step S1 includes at least one of epoxy coating and polyurethane coating.

[0014] Preferably, the corrosion procedure in step S2 includes alternating spraying and drying, wherein each spraying session lasts for 1-3 hours and each drying session lasts for 1-3 hours.

[0015] The tests provided in this application primarily target the coatings on the surfaces of metal parts and the exposed metal surfaces at scratches that penetrate the coating. By simulating alternating wet and dry conditions through the aforementioned intermittent spraying process, this application can obtain needle-like α-FeOOH corrosion products from the exposed metal (mainly ferrous metals) surface at the scratches. These products are consistent with those produced by the traditional CCT corrosion process, thus resulting in more accurate test results.

[0016] Preferably, the amount of paint applied in each spraying is 0.1-0.3 L / h. -1 m -2 .

[0017] Preferably, each spraying is performed at a temperature of 30-40°C.

[0018] Preferably, each drying process is carried out at a temperature of 40-50°C.

[0019] Preferably, the pH of the corrosive solution in step S2 is 6.5-7.2.

[0020] Preferably, step S2 involves adjusting the pH by adding an acid or a base to the corrosive solution. More preferably, the acid includes HCl. More preferably, the base includes NaOH.

[0021] Preferably, the concentration of sodium chloride in the corrosive solution in step S2 is 3-4.8 wt%.

[0022] Preferably, the concentration of ammonium sulfate in the corrosive solution in step S2 is 0.1-0.3 wt%.

[0023] This application also protects the application of the test method in the quality inspection of metal parts of automobiles.

[0024] Preferably, the metal component includes at least one of a stamped and welded bracket, a suspension bracket, a brake bracket, and a steering knuckle.

[0025] Preferably, the metal parts are made of at least one of cold-rolled steel, cast iron, cast steel, aluminum alloy, and magnesium alloy.

[0026] Preferably, the material of the metal parts includes at least one of cold-rolled steel, cast iron, and cast steel.

[0027] Compared with the prior art, the present invention has the following beneficial effects: The method provided in this application can complete the corrosion resistance test of automotive coatings in a short time (≤720 h), and the test results are close to those of the conventional CCT 1008 h test, with a deviation of no more than 20%. Detailed Implementation

[0028] The present invention will be further described below with reference to specific embodiments, but the embodiments do not limit the present invention in any way. Unless otherwise stated, the raw materials and reagents used in the embodiments of the present invention are conventionally purchased raw materials and reagents. Information on the raw materials used in some embodiments and comparative examples is as follows: The coating to be tested was pretreated with silane on the surface of a 150×75mm SPCC cold-rolled sheet, and then formed by cathodic electrophoresis. The coating to be tested was an epoxy coating obtained by curing an electrophoretic coating purchased from Xiangjiang Kansai. The thickness of the coating to be tested was 25μm.

[0029] The CCT 1008h etch width test was performed on the coating to be tested, including the following steps: A1. Sample preparation: Make an "X" shaped scratch on the surface of the coating to be tested. The scratch penetrates the coating and the width of the two sides is equal. A2. Corrosion was carried out using a 5 wt% sodium chloride aqueous solution (pH 7.0) according to the following cyclic corrosion procedure: continuous spraying at 35°C for 4 hours (spray volume 0.18 L / h). -1 m -2 Then, it was dried at 60℃ for 2 hours, and then left to stand at 50℃ and 95% relative humidity for 2 hours. This cycle was repeated 126 times (1008 hours).

[0030] Example 1 A method for testing the anti-corrosion performance of a coating includes the following steps: S1. Scratch is made on the surface of the coating to be tested to form a scratch that penetrates the coating and has an initial width w1 = 0.5 mm; the scratch is "X" shaped, and the initial width of the scratch refers to the width of any one side of the X-shaped scratch; S2. An etching solution is applied to the scratched surface of the test coating obtained in step S1. The etching solution comprises an aqueous solution of salts, wherein the salts include sodium chloride and ammonium sulfate. The concentration of sodium chloride in the etching solution is 3.8 wt%, and the concentration of ammonium sulfate is 0.25 wt%. The pH of the etching solution is 6.8. The etching procedure includes alternating spraying and drying, wherein each spraying session lasts 2 hours at a temperature of 35°C, and the spraying amount is 0.18 L / h. -1 m -2 The drying time was 2 hours and the temperature was 45℃. The ratio of the scratch width w2 after 600 hours of corrosion to the initial scratch width w2 / w1 was used as the evaluation basis for the coating's anti-corrosion performance test results. We found that w2=1.17 mm and w2 / w1<2.8, so the coating's anti-corrosion performance was deemed qualified.

[0031] Examples 2-13 and Comparative Examples 1-2 The differences between Examples 2-13 and Comparative Examples 1-2 of the corrosion resistance performance testing method described in this application and Example 1 are shown in Table 1-2, while all other aspects are consistent with Example 1.

[0032] Table 1. Table 2. Note: The calculation method for "the deviation between the test results and CCT 1008h" mentioned in Table 1-2 above is D=(w3-w2) / w3, where w3 is the etch width of CCT 1008h.

[0033] Comparative Example 3 A method for testing the anti-corrosion performance of a coating, wherein the only difference from Example 1 is: Ammonium sulfate is not added to the corrosion solution in step S2.

[0034] The obtained w2 in this comparative example is 1.55mm. The test result shows that the anti-corrosion performance of the coating is unqualified. The deviation of this comparative example result from CCT1008h is 35%.

[0035] Comparative Example 4 A method for testing the anti-corrosion performance of a coating, wherein the only difference from Example 1 is: No sodium chloride is added to the corrosion solution in step S2.

[0036] The comparative example yielded a w2 of 1.57 mm, and the test result showed that the coating's anti-corrosion performance was unqualified. The deviation between the comparative example result and CCT1008h was 37%.

[0037] Comparative Example 5 A method for testing the anti-corrosion performance of a coating, wherein the only difference from Example 1 is: In step S2, an equal amount of sodium sulfate is used to replace ammonium sulfate in the corrosion solution.

[0038] The comparative example yielded a w2 of 1.68 mm, indicating that the coating's anti-corrosion performance was unqualified. The deviation between the comparative example result and CCT1008h was 46%.

[0039] As can be seen from the data of the above embodiments and comparative examples, the method provided in this application can complete the anti-corrosion performance test of automotive coatings in a short time (≤720h), and the test results are close to the conventional CCT 1008h effect, with a deviation of no more than 20%.

[0040] According to Comparative Examples 1-4, the composition of the corrosive solution was unsuitable, which affected the accuracy of the test results.

[0041] According to Comparative Example 5, replacing ammonium sulfate with an equal amount of sodium sulfate failed to produce corrosion products similar to those obtained in the CCT 1008h test, thus affecting the accuracy of the test results.

[0042] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A method for testing the anti-corrosion performance of a coating, characterized in that, Includes the following steps: S1. Scratch is made on the surface of the coating to be tested to form a scratch with an initial width of w1; S2. Apply an etching solution to the surface of the scratched coating obtained in step S1. The etching solution includes an aqueous solution of salt, wherein the salt comprises sodium chloride and ammonium sulfate in a mass ratio of (10-50):1, and the concentration of sodium chloride in the etching solution is 3-5 wt%. The ratio of the scratch width w2 after 480-720 h of etching to the initial scratch width w2 / w1 is used as the evaluation basis for the coating's anti-corrosion performance test result. If w2 / w1≤2.8, the coating's anti-corrosion performance is deemed qualified; if w2 / w1>2.8, the coating's anti-corrosion performance is deemed unqualified.

2. The test method for the anti-corrosion performance of the coating as described in claim 1, characterized in that, Includes at least one of the following (1)-(2): (1) The thickness of the coating to be tested in step S1 is 20-30 μm; (2) 0.4≤w1≤0.6 mm.

3. The test method for the anti-corrosion performance of the coating as described in claim 1 or 2, characterized in that, The coating to be tested in step S1 includes a cathodic electrophoretic coating.

4. The test method for the anti-corrosion performance of the coating as described in claim 1, characterized in that, The corrosion procedure described in step S2 includes alternating spraying and drying, wherein each spraying session lasts for 1-3 hours and each drying session lasts for 1-3 hours.

5. The test method for the anti-corrosion performance of the coating as described in claim 4, characterized in that, The spray settling rate for each application is 0.1-0.3 L / h. -1 m -2 .

6. The test method for the anti-corrosion performance of the coating as described in claim 4, characterized in that, Each spraying is carried out at a temperature of 30-40℃.

7. The test method for the anti-corrosion performance of the coating as described in claim 4, characterized in that, Each drying process is carried out at a temperature of 40-50°C.

8. The test method for the anti-corrosion performance of the coating as described in claim 1, characterized in that, The pH of the corrosive solution in step S2 is 6.5-7.

2.

9. The test method for the anti-corrosion performance of the coating as described in claim 1 or 8, characterized in that, Includes at least one of the following (1)-(2): (1) The concentration of sodium chloride in the corrosive solution in step S2 is 3-4.8 wt%; (2) The concentration of ammonium sulfate in the corrosive solution in step S2 is 0.1-0.3 wt%.

10. The application of the test method according to any one of claims 1-9 in the quality inspection of metal parts of automobiles.

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