Long-term exposure of the product to the air may cause damage or deterioration of materials or properties under the action of natural conditions, which is a natural corrosive effect. The causes of corrosion are often related to corrosive components such as oxygen, temperature and humidity, and pollutants in the environment. Among them, salt spray corrosion is the most common and destructive corrosion.
How does salt spray corrode metal materials? This is because salt spray contains chloride ions, which can penetrate the oxide layer and protective layer on the metal surface, and directly electrochemically react with the metal layer, causing corrosion. Moreover, chloride ion contains a certain amount of hydration energy, which is easily adsorbed by the voids on the metal surface, and replaces the oxygen in the oxide layer, turning the insoluble oxide into soluble chloride, making the surface layer with protective properties active. . Since the product may be corroded by salt spray, it is necessary to evaluate its corrosion resistance and service life before leaving the factory.
The salt spray test can be conducted by using natural environmental exposure or artificial acceleration to simulate the salt spray environment. The natural environment exposure is the method closest to the real use situation, and the artificial accelerated simulated salt spray environment test is to use the salt spray test box, place the product in the test box, and simulate the salt spray environment to evaluate the product's resistance to salt spray corrosion. . Compared with the natural environment, the artificial salt fog test can control the salt spray concentration, greatly improve the corrosion rate and shorten the ort reliability testing time. With the current technical conditions, products that need to be tested for more than one year in the natural environment can get close results in just 24 hours under the artificial simulated salt spray environment.
The salt spray test mainly includes four methods, neutral salt spray test, copper salt accelerated acetate salt spray test, acetate salt spray test and alternating salt spray test.
(1) Neutral salt spray test, also known as NSS test, is currently the most widely used and earliest accelerated corrosion test method. Generally, a 5% sodium chloride salt solution is used, and the pH value of the solution is adjusted to the neutral range (6.5-7.2), which is used as a spray. The test temperature is controlled at 35°C, and the sedimentation rate of the salt spray should be 1-2ml/80cm/h.
(2) Copper salt accelerated acetic acid salt spray test, also known as CASS test, is a rapid salt spray corrosion test developed in recent years. The test temperature is 50 ℃, and a small amount of copper salt-copper chloride is added to the salt solution, which has a strong ability to induce corrosion, and the corrosion rate is about 8 times that of the NSS test.
(3) Acetic acid salt spray test, also known as ASS test, is developed on the basis of neutral salt spray test. Add glacial acetic acid to the 5% sodium chloride solution to reduce the pH value of the solution to about 3, the solution becomes acidic, and the final salt spray also changes from neutral to acidic. The corrosion rate is about 3 times that of the NSS test.
(4) The alternating salt spray test is a comprehensive salt spray test, and its principle is to add a constant damp heat test on the basis of the neutral salt spray test. Mainly used for cavity-type complete machine products, through the penetration of humid environment, salt spray corrosion occurs not only on the surface of the product, but also inside the product. The alternating salt spray test continuously replaces the product environment under two conditions of salt spray and damp heat, and finally evaluates the degree of change in the electrical and mechanical properties of the whole product.