Discuss the types of stainless steel corrosion

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Stainless steel is corrosion-resistant due to the formation of an extremely thin, highly adhesive, translucent film of chromium oxide on its surface. Once this film is damaged, the chromium in the steel reacts chemically with the oxygen in the atmosphere to rapidly restore this film; similarly, mechanical damage also allows a new protective film to form quickly. However, if subjected to chemical erosion by ions such as chloride ions, it may be difficult to resist this erosion, which can exacerbate corrosion due to unimpeded access of oxygen. The main types of corrosion are as follows: 1. Pitting: This is the common form of corrosion that affects stainless steel. It generally begins with needle-like corrosion, and due to this corrosion, the affected area turns black or dark brown. In most severe corrosion environments, the number and depth of pitting increase, giving the surface a corroded appearance. Under conditions of mild corrosion, pitting itself cannot significantly decrease on the surface; however, corrosion may occur on the surface to form a thin film, and when rust spots appear, it can cause the surrounding area to lose its luster. 　　 2. Crevice corrosion: It occurs in conditions of insufficient oxygen. For example, it can be caused by either metal cleaners or non-metal cleaners; an aqueous electrolyte formed from rainwater or condensation water can also lead to crevice corrosion. Low-alloy steel is more prone to this type of corrosion; crevice corrosion often occurs in areas where cracks are very small and oxygen has difficulty penetrating. The emergence of. If gaps are unavoidable, consideration should be given to using steel grades with higher corrosion resistance and a greater alloy content. Special attention should be paid in the design to minimizing crevice corrosion as much as possible. In areas where moisture is particularly likely to accumulate, efforts should be made to avoid gaps. 3. Electrochemical corrosion: This type of corrosion can occur when two metals with a large difference in electrochemical potential come into contact with each other. If water vapor connects these two metals, an electrical circuit is formed, and the resulting current will significantly increase the corrosion rate of the metal that is prone to chemical reactions. The potential energy difference between any two stainless steels is not sufficient to cause such corrosion; it only has a slight effect and does not lead to a significant increase in corrosion. However, when carbon steel and large areas of stainless steel are combined, the carbon steel will corrode rapidly; therefore, where different metals are joined together, it is necessary to prevent moisture from accumulating in those areas. If it is impossible to avoid water vapor, these two metals must be electrically insulated from each other. 　　 4. Stress corrosion cracking (SCC): Stress corrosion cracking can occur in two situations. Stainless steel may experience chloride stress corrosion cracking when in a chloride-containing aqueous environment. For example, in a sea fog environment, where steel is under high tensile stress and the temperature is above normal levels (usually over 60°C), its use in construction will inevitably have some impact, unless the steel has been subjected to the sensitization treatment described below. At lower temperatures, in normal harsh environments including organic chemicals, stress corrosion cracking can also occur, and such conditions are often unavoidable.