The stainless steel plates of the heat exchanger are only resistant to weakly corrosive media such as air, steam, and water, and steel that is chemically corrosive media such as acids, alkalis, and salts. In practical applications, steel resistant to weakly corrosive media is often called stainless steel, and steel resistant to chemical media is called acid-resistant steel. There is a certain difference in chemical composition between the two, and the former is not necessarily resistant to corrosion by chemical media. The corrosion resistance of stainless steel depends on the alloying elements contained in the steel.
The corrosion resistance mechanism of stainless steel plates: Chromium is the basic element for stainless steel to obtain corrosion resistance. When the chromium content in the steel reaches about 12%, the chromium will interact with the oxygen in the corrosive medium to form a very thin layer on the surface of the steel. Oxide film (Cr2O3 self-passivation film) is extremely difficult to dissolve in water and can further prevent oxygen and iron corrosion. Destroying the passivation film Cr2O3 means destroying its resistance to oxygen corrosion.
The halogen elements in the corrosive medium (like the common chloride ions in water) can also replace Cr under certain conditions, so stainless steel will rust under certain conditions. It will also be corroded in media containing acid, alkali, and salt. Therefore, the size of the corrosion resistance of stainless steel is changed with the chemical composition of the steel itself, the state of mutual addition, the use conditions and the type of environmental media. The corrosion of stainless steel in water is mainly caused by chloride ions in the water. Other additional elements in stainless steel also play different roles. Mo will offset the corrosion caused by chloride ions to a certain extent, but it also has an applicable range. 304, 316 belong to austenitic stainless steel. The American grade is the AISI standard of the American Iron and Steel Institute, and the Japanese is the Japanese Industrial Association standard SUS.