A critical review of the protection of metals by paints

The literature has been reviewed in order to categorize the reasons why paint films protect metals, and how they eventually degrade. The early work is examined which showed that the rate-controlling process in the corrosion degradation of some painted metals was the diffusion of ions through the paint film. The processes of water and oxygen permeation, being faster than on unpainted samples, were not rate controlling for these paint films. Later work is then discussed which indicates the importance of other factors, e.g. oxygen permeation may well be rate determining with some paint films; in some cases ionic diffusion rates are so low that, although they may be the slowest process, they cannot be rate controlling; and although water permeability in most cases cannot be the rate-controlling step because it is too fast, it may well be the rate-controlling factor in the loss of adhesion. It is demonstrated that the rate-controlling factor for one type of painted metal is not necessarily the same as for another and therefore it can be sometimes misleading to extrapolate the results of one system to that of another.