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1. Corrosion of so-called salt spray
Corrosion is the destruction or deterioration of a material or its properties caused by the environment. Most of the corrosion occurs in the atmosphere, which contains corrosive components and corrosive factors such as oxygen, humidity, temperature changes, and pollutants. Salt spray corrosion is a common and most destructive atmospheric corrosion. The salt spray mentioned here refers to the atmosphere of chloride. Its main corrosive component is the chloride salt in the ocean, sodium chloride, which is mainly derived from the ocean and the saline-alkali areas in the interior.
The corrosion of the surface of the metal material by the salt spray is caused by the electrochemical reaction between the oxide layer containing the chloride ions penetrating the metal surface and the protective layer and the internal metal. At the same time, chloride ions contain a certain hydration energy, which is easily adsorbed on the pores of the metal surface, and the cracks are displaced and replace the oxygen in the chlorinated layer, turning the insoluble oxide into a soluble chloride, making the passivated surface active. surface. Causes a bad reaction to the product.
2. Salt spray test and its connection with the actual
The salt spray test is an environmental test that uses artificial simulated salt spray environmental conditions created by salt spray test equipment to evaluate the corrosion resistance of products or metal materials. It is divided into two categories, one is the natural environmental exposure test, and the other is the artificial accelerated simulated salt spray environment test. The artificial simulated salt spray environment test uses a test equipment with a certain volume space—the salt spray test chamber, to artificially use the salt spray environment in the volume space to assess the salt spray corrosion resistance quality of the product. .
Compared with the natural environment, the salt concentration of the chloride in the salt spray environment can be several times or several times higher than the salt spray content of the general natural environment, so that the corrosion rate is greatly improved, and the product is subjected to a salt spray test to obtain a result. The time is also greatly shortened. If a product sample is tested in a natural exposure environment, it may take 1 year to corrode, and in a simulated simulated salt spray environment, as long as 24 hours, similar results can be obtained.
3. Scope of application:
1) Copper plating on the surface of steel + nickel + chromium or nickel + chromium;
2) The surface of copper or copper alloy is plated with nickel + chromium;
3) The surface of 300 series or 400 series stainless steel is plated with nickel + chromium;
4) Copper or nickel + chromium on the surface of aluminum or aluminum alloy;
5) The surface of the zinc alloy is plated with copper + nickel + chromium;
6) Plastic plating parts
4. Artificial simulated salt spray test includes
1) Neutral salt spray test (NSS test) is one of the earliest applications in the field of accelerated corrosion test. It uses a 5% aqueous solution of sodium chloride solution, and the pH of the solution is adjusted to a neutral range (6-7) as a solution for spraying. The test temperature is taken at 35 ° C, and the sedimentation rate of the salt spray is required to be between 1 and 2 ml / 80 cm 2 · h.
2) The acetate spray test (ASS test) was developed on the basis of a neutral salt spray test. It is to add some glacial acetic acid to the 5% sodium chloride solution, the pH value of the solution is reduced to about 3, the solution becomes acidic, and the salt spray finally formed is also changed from neutral salt spray to acid. Its corrosion rate is about 3 times faster than the NSS test.
3) Copper salt accelerated acetate spray test (CASS test) is a rapid salt spray corrosion test developed recently in foreign countries. The test temperature is 50 ° C, and a small amount of copper salt - copper chloride is added to the salt solution to strongly induce corrosion. Its corrosion rate is about 8 times that of the NSS test.
5. Determination of salt spray test standards and test results
A. Standards are uniform rules for repetitive things and overviews. The salt spray test standard specifies the salt spray test conditions, such as temperature, humidity, sodium chloride solution concentration and pH value, and also sets technical requirements for the performance of the salt spray test chamber. The salt spray test standard for the same product is selected according to the characteristics of the salt spray test, the corrosion rate of the metal, and the sensitivity to salt spray.
B. The purpose of the salt spray test is to assess the salt spray corrosion quality of the product or metal material, and the salt spray test result is the judgment of the product quality. Whether the judgment result is correct and reasonable is the correct measurement of the product or metal resistance. The key to the quality of salt spray corrosion.
C. The method for determining the salt spray test results includes: rating determination method, weighing determination method, corrosion occurrence determination method, and corrosion data statistical analysis method.
1) The rating method is to divide the percentage of the ratio of the corrosion area to the total area into several levels according to a certain method, and use one level as the basis for the qualification judgment, which is suitable for the evaluation of the flat sample;
2) Weighing determination method is to evaluate the corrosion resistance quality of the sample by weighing the weight of the sample before and after the corrosion test, and it is especially suitable for assessing the corrosion resistance quality of a certain metal. ;
3) The corrosion occurrence determination method is a qualitative determination method. It determines the sample after the salt spray corrosion test, whether the product is corroded or not. Most of the general product standards use this method; the statistical analysis method of corrosion data provides A method of designing corrosion tests, analyzing corrosion data, and determining the confidence of corrosion data. It is mainly used to analyze and count corrosion conditions, rather than specifically for the quality determination of a specific product.
It is clear to the surface treatment that the coating primarily serves to decorate the appearance and protect the substrate from corrosion.
November 30, 2022