Small concentration variations within a solution in contact with the metal may also affect the rate
and nature of corrosion reactions. Therefore, it is often impossible to predict the exact nature of
corrosion reactions. It is generally found, however, that for most metals exposed to an aqueous
environment the half-reactions involved in corrosion are the reduction reaction of Equation (2-4)
and an oxidation half-reaction of the type shown in Equations (2-1) through (2-3).
General corrosion is the process whereby the surface of a metal undergoes a slow, relatively
uniform; removal of material. This occurs on the surface of a single metal rather than dissimilar
metals. In general corrosion, a nearly infinite number of micro-cells are established on the metal
surface. Oxidation occurs at anodic areas and reduction at cathodic areas. The micro-cells are
uniformly distributed over the metallic surface, and as the reaction proceeds the cells may migrate,
or disappear and re-form. That is, any particular micro-region may be alternately anodic and
cathodic. The result is a uniform attack on the metal surface.
Under some conditions, relatively large regions become anodic or cathodic. Such regions have
less tendency to migrate and may remain operative for long periods of time. In this case, there
will be severe attack of the metal at the anodic (oxidation) region. The result may be a visible pit
in the metal surface.
Iron and steel are resistant to rapid corrosion in water despite the tendency of iron to oxidize as
indicated by its standard electrode potential listed in Table 1. The reasons for this resistance are
the passivating effect of the oxide film and cathodic polarization due to atomic hydrogen that
absorbs on the oxide surface, both of which are explained in the next section.
Passivity and Polarization of Metal
Metals that normally fall victim to corrosion will sometimes exhibit a passivity to corrosion.
Passivity is the characteristic of a metal exhibited when that metal does not become active in the
corrosion reaction. Passivity is caused by the buildup of a stable, tenacious layer of metal oxide
on the surface of the metal. This oxide layer is formed by corrosion on a clean metal surface,
where the corrosion products are insoluble in the particular environment to which the metal is
exposed. Once the layer, or film, is formed, it acts as a barrier separating the metal surface from
the environment. For further corrosion to occur, the reactants must diffuse through the oxide
film. Such diffusion is very slow or nonexistent, thus corrosion either decreases markedly or
Metals such as zirconium, chromium, aluminum, and the stainless steels form thin, tenacious oxide
films when exposed to the atmosphere or to pure water at room temperature. In some cases, the
film is extremely thin and may be invisible to the unaided eye, but it is still very effective in giving
these metals a marked passivity.
If there is a net conversion of reactants to products in a system, the system will be chemically
unstable, and the reaction will continue until a stable state is attained. This stable state is known