CORROSION-RESISTANCE OF ZN-CO ALLOY COATINGS

The corrosion behaviour of commercial zinc and Zn-Co alloy coatings, with and without chromate conversion treatment, was studied by means of salt spray tests and electrochemical tests in aerated 0.6 M NaCl solution at various pH values. At almost neutral pH values, the chromated Zn-Co coatings show a better corrosion resistance than that for conventional chromated zinc coatings, as a result of a particularly protective chromate conversion layer which increases both the cathodic and anodic overvoltages. At lower pH values, the corrosion resistance of both chromated zinc and chromated Zn-Co alloy decreases. This is attributed to the depolarization of the cathodic reaction (hydrogen reduction) and to the quick destruction of the chromate layer.