Corrosion can occur as a direct result of exposure to atmospheres such as air, process gases used in heat treatment (sulfurous gases or nitriding gas using ammonia) and environmental gases.
Corrosion can occur through intermetallic contact of different metals (galvanic). It can also occur by contact with other materials such as soils and acidic liquids. Corrosion can be classified as in Figure 1.
One has to consider the operating environment as part of the material selection decision. This means matching the corrosion resistance of the metal to the environmental operating conditions, which will, in turn, determine the rate of corrosion.
The heat treatment of the metal and the selection of the processing environment will play a significant part in the in the reduction (not elimination) of corrosion. For example, although the surface wear resistance of nitrided steels has been improved, the corrosion resistance has been reduced. Conversely, if plain-carbon/low-alloy steel is nitrided, its corrosion resistance has been improved.
The chromium content of a stainless steel will naturally form a chrome-oxide layer on the surface of the steel. This chrome-oxide layer will be very difficult to break through unless the surface oxide is reduced and the oxide removed.
The same applies to aluminum. Aluminum has a natural affinity for oxygen and will form aluminum oxide on the surface. This is the primary reason that aluminum has a resistance to corrosion. If the aluminum oxide surface is reduced to aluminum, however, surface corrosion can occur. It is necessary to have a very clean furnace when processing aluminum for the solutionization and precipitation treatment.
Changing the metallic phase and microstructure also has the potential to lead to corrosion. As an example, in the oil-drilling industry the primary agents that will contribute significantly to corrosion are:
- Hydrogen Sulfide
- Elemental sulfur
- Hydrochloric acids
- Hydrofluoric acids