Stainless steels are not natural noble materials in same way as gold or platinum, which are more or less inert to most environments.
The corrosion resistance of a stainless steel is dependent on a thin invisible film on the steel surface, the passive film. The passive film consists mainly of a chromium oxide that forms in reaction with oxygen in the air.
For spontaneous development, even after a damage of the passive film on the steel surface, the steel has to contain at least 12% chromium.
Other alloy elements, for example molybdenum and nitrogen, improve the corrosion resistance in corrosive environments. The passive film can be broken down completely or partly with corrosion as a result. However, the passive layer has the ability of repassivation in an environment containing oxygen, such as air or aerated solutions. This environment is sufficient for both the creation and maintenance of the passsive layer of stainless steels. There are, nevertheless, environments that cause permanent breakdown of the passive layer. Under circumstances where the passive layer cannot be rebuilt, corrosion occurs on the unprotected surface.
Different media can cause different types of corrosion attack that may vary in nature and appearance, and several forms of corrosion can occur on stainless steels.
- Uniform Corrosion
- Galvanic Corrosion
- Pitting Corrosion
- Crevice Corrosion
- Atmospheric Corrosion
- Intergranular Corrosion
- Stress Corrosion Cracking
- Corrosion Fatigue