While there is a common misconception that stainless steel is impervious to corrosion, it is still possible due to various factors. Chromium protects stainless steel if the localized concentration is 12% or higher. Anything reducing the localized chromium concentration below the 12% threshold will cause staining or rust. Common causes of stainless steel corrosion include:
Chlorides: Chromium oxide is particularly vulnerable to chlorides. Corrosion is accelerated in coastal areas with salt-spray exposure and in areas where de-icing salts are used during winter. Components for the chemical and food industries have high chromium content to compensate for regular exposure to chlorine, salt, and other corrosive substances.
Acids: Strong acids destabilize the passive layer. Hydrochloric and sulfuric acids can cause general surface corrosion. Hydrochloric acid is used in plastic production and descaling agents, while sulfuric acid is used in mineral processing, oil refining, and wastewater processing.
Iron and Carbon Steel Particles: Stainless steel corrosion can also be triggered by contact with iron or carbon steel particles. Trace particles from iron or carbon steel will rust stainless steel surfaces. If left unattended, rust spots may compromise the chromium oxide surface and evolve into localized corrosion such as pitting. Contamination occurs when stainless steel is subject to sparks or particles from nearby welding, cutting, drilling, or grinding of carbon steel.
High Temperatures: A less common form of rusting occurs after the stainless steel has been exposed to very high temperatures (750–1550°F). During processes such as welding, sensitization can occur, where the carbon and chromium bond to form carbides, causing the grain boundaries to become chromium deficient and the passive layer to become discontinuous. However, the damage can sometimes be mitigated with heat treating.
