What Does Immunity Mean?
Immunity is the lack of any noticeable attack on a metal when exposed to a given operating environment. The metal properties remain the same even after exposing the metal to the particular environment for long periods. The main properties that remain unchanged are:
- Surface finish
Corrosionpedia Explains Immunity
The immunity of the metal is due to the material being thermodynamically stable in that particular environment. In this state, the corrosion reaction is unable to occur spontaneously. The immunity of a metal in a particular environment is influenced by various mechanisms, including:
- Low energy content - Metals with a lower energy content than that formed by the corrosion products are more immune. An energy increase is required to cause an effect on the metal, meaning that the metal remains stable under the normal conditions. Typical examples include naturally occurring noble metals such as gold, silver, palladium and platinum.
- Cold working - This destroys the continuous grain boundary paths and also generates imperfections that have a high affinity for carbon. This depletes the continuous paths, which carbon atoms tend to segregate.
- Cathodic protection - It is possible to bring the potential of the metal to the immunity region by imposing a cathodic shift.
The immunity of a metal also depends on the environment, and most metals exhibit immunity in noncorrosive environments. However, the immunity decreases as the corrosive factors increase. The presence of different corrosive environments means that it is possible to have a metal with immunity in one corrosive environment but less immunity in another environment.
Materials such as gold and silver have immunity in almost all environments. However, they are rarely used for engineering purposes due to their high cost and low mechanical strength. They are usually added to the other less-immune metals to produce stable and strong alloys.
The knowledge of a metal’s immunity is utilized in the development of corrosion-resistant alloys. The determination of the immunity is also important in determining the suitable operational environments or protective measures required to protect the metal.