Corrosion Types - Immunity
Corrosion Type Immunity.
The first form of corrosion described is the lack of attack, or immunity. This can result from the action of two basic mechanism.
Corrosion test measurements that are used to measure very low corrosion rates must be used to validate that corrosion activity is completely absent.
Definition.
Immunity is the lack of measurable attack on a metal when exposed to operational environments.
Mechanism.
Immunity can result from two basic mechanisms. In the first case, the energy content of the metal is lower (more stable) than any of the corrosion products that could possibly form. Such metals are commonly found in nature as metals that indicates the stability of the metallic state for these elements.
Corrosion of such metals where an increase of energy is required will not take place naturally just as a ball will not roll uphill unless pushed.
In the second case, there is an energy tendency for the metal to suffer corrosion, but corrosion activity is prevented by the presence of a highly stable passive film.
Not only must this film be stable in the environment, but it must be able to repair itself by reaction with the environment if it is damaged.
Examples.
Gold and platinum are examples of metals that are normally immune in a wide variety of environments due to the low energy content of their metallic state.
Titanium and some highly alloyed materials, such as the complex nickelchromium- molybdenum alloy Hastelloy C-276, have an extremely stable passive film that will repair itself when damaged.
Appearance.
A metal that is immune to corrosion in a given environment will not show any change due to corrosion after exposure.
Significant Measurements.
Metals that do not suffer corrosion are unchanged by exposure to their environment. Their strength, weight, size, shape, and surface finish are unaffected by exposure.
To measure very low rates of corrosion, weight loss and other material property measurements may not be sufficiently sensitive.
In these cases, the metal is exposed to a small quantity of the electrolyte and the electrolyte is analyzed for the presence of metal atoms from the corrosion process.
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