Deterioration of cathodic protection action of zinc-rich paint coatings in atmospheric exposure

Results obtained after 10 years' atmospheric exposure of steel specimens painted with six different zinc-rich paint (ZRP) compositions are presented and discussed. The paints were formulated with ethyl silicate and epoxy-polyamide binders and included different zinc dust and conductive extender contents. Experimental data refers mainly to electrochemical measurements of corrosion potential and polarization resistance. The authors propose an interpretation of the deterioration with time of the capability of the zinc particles in the paint to provide cathodic protection to the steel. In this respect, the role exercised by polarization of the ZRP coating acting as electrode and ratio of anodic to cathodic areas is discussed.