Metal-Pigmented Paints for the Protection of Steel

Summary

Under certain geometrical conditions, rapid breakdown of a paint coat can occur in service. Maintenance costs could be reduced if paints were devised to resist such conditions. Consequently, the scientific causes of the breakdown have been studied, paint compositions modified to resist them and simple tests applied to ascertain how far success has been achieved. Under certain marine conditions, alkali, the cathodic corrosion product, accumulates, and attacks, softens or loosens either vehicle or pigment; conventional zinc-rich paints can be improved by replacing part of the zinc by alkali-resisting substances, including metallic cadmium, which continues to provide electrical connection with the zinc in the outer part of the coat after the zinc particles next to the steel have been destroyed. Nests of certain salts included in rust (notably ferrous sulphate and ammonium chloride) cause rapid bulging of paint coats applied over such rust; the incorporation of calcium and barium phosphates greatly diminishes this tendency. One of the outstanding features of zinc-rich paint is its power to protect at gaps. This may be lost if a conventional zinc-rich paint is applied over rust containing the salts mentioned; the new paints show promise for application over rust. Gap-protection may also be lost if an impervious outer coat is applied, but it is shown that gap-protection is obtained when conventional zinc-rich paint (or, to a less extent, one of the new paints) is covered with an outer coat selected to avoid interruption of the protective circuit—for instance, a coat that is pervious to water or one that is an electronic conductor.