Chromate Inhibition of Corrosion-Driven Organic Coating Delamination Studied Using a Scanning Kelvin Probe Technique

A scanning Kelvin probe technique is used to study the influence of chromate on the kinetics and mechanism of corrosion-driven delamination processes affecting polyvinyl butyral (PVB) coatings adherent to the intact zinc surface of hot dip galvanized steel. Placing aqueous sodium chloride electrolyte onto a penetrative coating defect establishes an electrochemical delamination cell in which cathodic reduction at the delamination front is coupled to anodic zinc dissolution at the coating defect by a thin (2.5-5 μm) layer of electrolyte which ingresses beneath the delaminated PVB film. Soluble chromates in the external electrolyte reduce delamination rates by less than 25% because anions are excluded from the underfilm electrolyte layer by the delamination cell electric field. In contrast, dispersions of particulate in the PVB coatings allow diffusion directly into the underfilm electrolyte layer and profoundly inhibit delamination. It is proposed that replacement of underfilm reduction by a self-limiting reduction process is the most significant factor in decreasing delamination rates.