Stainless steels are highly corrosion resistant—however, stainless steel applications can still remain at risk to surface damage. Oxidation, corrosion, rusting, or staining can occur over the long-term in harsh environments without routine cleaning and maintenance. Repeat mechanical damage also contributes to a faster degradation of the metal.
All stainless steels contain at least 10.5% chromium by weight. It is this chromium content that creates a shield called the passive layer, which protects stainless steel from corrosion—unlike other steels. The higher the chromium content, the greater the corrosion resistance. Stainless steel rusting occurs when the passive layer is damaged and there is not enough chromium for it to reform.
How does the passive layer work?
The passive layer is created as the chromium content in the stainless steel’s surface reacts to oxygen. The passive layer acts as a protective barrier, preventing further oxidation of the stainless steel. In comparison, ordinary carbon steel surfaces form ferric oxide when exposed to oxygen. Ferric oxide does not form a continuous layer, so it eventually spalls off, leaving raw steel exposed and prone to a destructive rusting cycle.
The passive layer of stainless steel is self-repairing. If it is damaged, chromium in the exposed stainless steel will react with oxygen to form new chromium oxide. As long as there is sufficient chromium present, the chromium oxide layer will continue to reform and protect the stainless steel surface.
Causes of stainless steel corrosion
Chromium can protect 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, hydrochloric acids, sulfuric acids, iron or carbon steel contact, and high temperatures. There are over 150 grades of stainless steel, and some are more prone to corrosion than others. The corrosion resistance and other useful properties of stainless steel are enhanced by increasing the chromium content, or by the addition of other elements such as molybdenum, nickel, and nitrogen.
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.
Read more: Cleaning Stainless Steel
