Corrosion is the damage or deterioration of materials or their properties caused by the environment. Most of the corrosion occurs in the atmosphere, which contains corrosive components and factors such as oxygen, humidity, temperature changes, and pollutants.
Salt spray corrosion is a common and most destructive atmospheric corrosion. The corrosion of salt spray on the surface of metal materials is caused by the electrochemical reaction between the contained chloride ions penetrating the oxide layer and the protective layer of the metal surface and the internal metal. At the same time, chloride ions contain a certain amount of hydration energy and are easily displaced by the pores and cracks adsorbed on the metal surface to replace the oxygen in the oxide layer, turning insoluble oxides into soluble chlorides, and turning the passivated surface into an active surface.
A salt spray test is an environmental test that mainly uses artificially simulated salt spray environmental conditions created by salt spray test equipment to assess the corrosion resistance of products or metal materials. It is divided into two categories, one is the natural environment exposure test, and the other is the artificially accelerated simulated salt spray environment test.
The artificial simulation salt spray environment test is to use test equipment with a certain volume space – the salt spray test chamber (as shown in the figure), and uses artificial methods in its volume space to create a salt spray environment to test the salt spray corrosion resistance of the product. performance quality assessment.
Compared with the natural environment, the salt concentration of chloride in the salt spray environment can be several times or dozens of times that of the general natural environment salt spray content, which greatly increases the corrosion rate. The salt spray test is carried out on the product and the results are obtained. The time is also greatly shortened. For example, if a product sample is tested in a natural exposure environment, it may take 1 year for it to corrode, but it only takes 24 hours to obtain similar results in an artificially simulated salt spray environment.
Laboratory-simulated salt spray can be divided into four categories:
⑴Neutral salt spray test (NSS test) is an accelerated corrosion test method that appeared the earliest and is currently the most widely used. It uses a 5% sodium chloride saline solution, and the pH value of the solution is adjusted in the neutral range (6.5-7.2) as a solution for spraying. The test temperature is 35°C, and the sedimentation rate of salt mist is required to be 1-2ml/80cm/h.
⑵Acetic salt spray test (ASS test) is developed based on a neutral salt spray test. It is to add some glacial acetic acid to the 5% sodium chloride solution so that the pH value of the solution is reduced to about 3, the solution becomes acidic, and the final salt spray is also changed from neutral salt spray to acidic. Its corrosion rate is about 3 times faster than that of the NSS test.
(3) Copper salt accelerated acetic acid salt spray test (CASS test) is a rapid salt spray corrosion test recently developed abroad. 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.
⑷Alternating salt spray test is a comprehensive salt spray test, which is a neutral salt spray test plus a constant damp heat test. It is mainly used for cavity-type complete machine products. Through the penetration of the humid environment, salt spray corrosion not only occurs on the surface of the product but also inside the product. It is to alternately switch the product under two environmental conditions of salt spray and humid heat, and finally assess whether the electrical and mechanical properties of the whole product have changed.
