Electrochemical roles of anti-corrosive pigments in sacrificial corrosion protection of painted galvanized steel and their relation to organic coating delamination

The effect of anti-corrosive pigments on the delamination of an organic film on a Zn coated steel was investigated using a cyclic wet-dry corrosion test (according to ISO 16539 Method A). The organic film was made from a mixture of polyvinyl butyral-co-vinyl acetate (PVB) ethanol, and a pigment. SrCrO₄ or Na₂HPO₄ was added as a pigment. The delaminated area around a deep scratch on the specimen with the SrCrO₄-containing film was smaller than those of the Na₂HPO₄-containing and pigment-free films. The corrosion rate and potential of the Zn layer coupled with the steel were estimated on the basis of the iR-drop in the films and the polarization curves of the Zn and steel electrodes. Also, the electrode potentials of the Zn under the PVB films were measured by scanning Kelvin probe force microscopy. From these results, it was confirmed that the delamination of the PVB films on galvanized steels was mainly caused by the anodic reaction of the Zn layer. The galvanic coupling of the delamination fronts at the interface of the Zn layer and the PVB film to the steel substrate at the scratches, inhibits the oxygen reduction reaction on the steel, in effect preventing Zn corrosion.