Inhibitor pretreatment synergies demonstrated using a scanning Kelvin probe technique

Abstract

A scanning Kelvin probe technique has been used to study the influence of pretreatment and pigment interaction on the kinetics and mechanism of corrosion driven delamination processes affecting model polyvinyl butyral coatings adherent to the intact zinc surface of hot dip galvanised steel. The effects of incorporating dispersions of strontium chromate (SrCrO₄), silica and bentonite based Cerium (III) cation exchange pigments within the polyvinyl butyral coating are investigated on galvanised steel specimens in the absence and presence of a mixed oxide/chromate rinse surface pretreatment. Placing aqueous sodium chloride electrolyte onto a penetrative coating defect establishes an electrochemical delamination cell in which cathodic O₂ reduction at the delamination front is coupled to anodic zinc dissolution at the coating defect by a thin layer of electrolyte which ingresses beneath the delaminated polyvinyl butyral film. The SrCrO₄ pigmented polyvinyl butyral films allow CrO₄^2- diffusion directly into the underfilm electrolyte layer and profoundly inhibit delamination both in the presence and absence of pretreatment. Without pretreatment delamination is progressively retarded by increasing SrCrO₄ pigment volume fraction. The addition of the pretreatment decreases the delamination rate in all cases by around between 20 and 50%, probably through improved adhesion to the metal substrate. In contrast, polyvinyl butyral films containing dispersed cerium exchanged bentonite clay (CEB) and cerium exchanged Shieldex (CES) pigments do not efficiently inhibit delamination in the absence of the mixed oxide pretreatment. In the presence of pigment volume fractions of up to 0.15, the delamination rate is only decreased to ∼20% of its original value. However, when used in conjunction with a pretreated surface, both CEB and CES pigments profoundly inhibit corrosion driven delamination, with observed rates reduced to levels below those measured for the SrCrO₄ pigmented system. Thus there appears to be a synergy between the CEB and CES pigments and the mixed oxide (chromate rinse) pretreatment. This synergy is believed to be related either to the ease with which a cerium hydr(oxide) inhibition film can form on the oxide covered surface of the pretreated material, or to the underfilm interaction of pigment derived cathodic (Ce³⁺) and pretreatment derived anodic (CrO₄^2-) inhibitors.