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 (Ce3+) 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 mum) 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 Ce3+ 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. (C) 2002 The Electrochemical Society.