The excellent corrosion resistance of stainless steel is due to the formation of an invisible oxide film on the surface of the steel, making it passive. This passive film is formed as a result of steel reacting with oxygen when exposed to the atmosphere, or as a result of contact with other oxygen-containing environments. If the passivation film is destroyed, stainless steel will continue to corrode. In many cases, the passivation film is only destroyed on the metal surface and in local areas, and the effect of corrosion is to form tiny holes or pits, resulting in irregularly distributed small pit-like corrosion on the surface of the material.
The occurrence of pitting corrosion is likely to be due to the presence of chloride ions combined with depolarizers. The pitting corrosion of passive metals such as stainless steel is often caused by the local damage of certain aggressive anions to the passive film, protecting the passive state with high corrosion resistance. Usually an oxidizing environment is required, but this is exactly the condition under which pitting corrosion occurs. The medium for pitting corrosion is the presence of heavy metal ions such as FE3+, Cu2+, Hg2+ in C1-, Br-, I-, Cl04-solutions or chloride solutions of Na+, Ca2+ alkali and alkaline earth metal ions containing H2O2, O2, etc.
The pitting rate increases with increasing temperature. For example, in a solution with a concentration of 4%-10% sodium chloride, the maximum weight loss due to pitting corrosion is reached at 90°C; for a more dilute solution, the maximum occurs at a higher temperature.
Post time: Feb-24-2023