Cerium(III) Inhibition of Corrosion-Driven Organic Coating Delamination Studied Using a Scanning Kelvin Probe Technique

The scanning Kelvin probe is used to study the influence of trivalent cerium cations ( Ce 3 + ) 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 aerated aqueous sodium chloride onto a penetrative coating defect establishes an electrochemical delamination cell in which cathodic O 2 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 Ce(III) salts in the external electrolyte reduce coating delamination rates by <25% because Ce ( aq ) 3 + cations migrating beneath the coating immediately encounter a countercurrent migration of cathodically produced OH − anions. Consequently, Ce ( OH ) 3 precipitation occurs at the coating/defect boundary, with little or no Ce ( aq ) 3 + reaching the active sites of cathodic O 2 reduction at the delamination front. In contrast, dispersions of silica and bentonite-based Ce 3 + cation-exchange pigments in the PVB coatings enable Ce ( aq ) 3 + to exchange directly into the under-film electrolyte and significantly retard coating delamination. Delamination is not halted completely because under-film Ce ( aq ) 3 + exchange continually lags behind the advancing delamination front so that precipitated Ce ( OH ) 3 increases under-film resistivity by displacing electrolyte but does not form a coherent cathodic film capable of stifling O 2 reduction.