Three commonly used methods for salt spray testing

In the fields of materials science and engineering, salt spray testing is a commonly used testing method to evaluate the corrosion resistance of materials. By simulating salt spray corrosion in marine environments, salt spray testing can help engineers and researchers understand the performance of materials in harsh environments. We can accelerate the corrosion rate of the sample by increasing the concentration of chloride in the salt spray environment, which can greatly save time in obtaining results.

The three commonly used methods for salt spray testing are: neutral salt spray (NSS) test, cyclic salt spray test, and copper ion accelerated salt spray test.

1、 Neutral salt spray test

The commonly used method for testing the corrosion resistance of coatings is the neutral salt spray test. This is the earliest and most widely used accelerated corrosion testing method, suitable for testing the corrosion resistance of various metal materials and coatings.

During testing, place the sample in the salt spray test chamber as specified, and then spray the atomization test solution. The fine mist settles evenly on the test surface under its own weight. It uses 5% sodium chloride saline solution, the solution PH value is adjusted in the neutral range (6~7) as the solution for spray, and the test temperature is 35 ℃.

The sedimentation rate of salt spray is required to be between 1-2ml/80cm every 2 hours. Place the sample in a salt spray box so that its exposed surface forms a 15-30 ° angle with the vertical direction. The spacing between the samples should allow the salt spray to freely settle onto all the samples, and the saline solution on the surface of the samples should not drip onto any other samples.

The samples do not form a shielding effect on the space, do not come into contact, and maintain electrical insulation between each other. The sample and bracket must also maintain electrical insulation and there should be no gaps in the structure. The position and angle of the nozzle determine the size and uniformity of the nozzle, and are determined by the salt content collected by the salt spray collector. It is generally stipulated that 1-2ml saline water shall be collected on an average of 80 square centimeters of horizontal area every hour after 24h of spray, in which the concentration of NaCl shall be within the range of 5 ± 1%.

Due to the different types of products, materials, and coatings tested, the total test time can be selected within the range of 8-3000 hours. According to the national standard, the test should be conducted in a 24-hour continuous spray mode, which sometimes needs to be adjusted according to the actual situation.

The corresponding standards for neutral salt spray test include GB6458-86 and ASTMB117.

2、 Cyclic salt spray test

The cyclic salt spray test is a simple test in which a sample is exposed to a series of different environments under repeated cycles, and can have two cyclic compositions. Spray cycle and drying cycle, more complex experimental steps (especially automobile salt spray test) require multi-step cycle including wet, salt spray or salt spray function and drying.

The goal of cyclic corrosion testing (CCT) is to reproduce the corrosion performance of materials in outdoor corrosive environments. CCT testing involves conducting corrosion cycling salt spray tests on samples exposed to salt spray test chambers under different environmental conditions. As in the Prohesion test, the sample is exposed to a cycle composed of salt spray and drying conditions.

3、 Copper ion accelerated salt spray test

The copper ion accelerated salt spray test is a rapid salt spray corrosion test conducted abroad. The test temperature is 50 ℃, and a small amount of copper salt copper chloride is added to the salt solution, causing strong corrosion. Its corrosion rate is approximately 8 times that of the neutral salt spray test. This method is suitable for rapid testing of corrosion resistance on zinc die castings and decorative chromium plating on the surface of steel parts under extremely harsh working conditions. It is also suitable for testing aluminum materials that have undergone anodizing, phosphating, or chromate treatment. The reliability, reproducibility, and accuracy of the method depend on strict control of some testing factors. The experimental period of this method is generally 6-720 hours. The testing method and other requirements are the same as the neutral salt spray test.

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