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 chromat e (CrO42-) on the kinetics and mechanism of corrosion-driven dclamination p rocesses affecting polyvinyl butyral (PVB) coatings adherent to the intact zinc surface of hot dip galvanized steel. Placing aqueous sodium chloride e lectrolyte onto a penetrative coating defect establishes an electrochemical delamination cell in which cathodic O-2 reduction at the delamination fron t 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 f ilm. Soluble chromates in the external electrolyte reduce delamination rate s by less than 25% because CrO42- anions are excluded from the underfilm el ectrolyte laver by the delamination cell electric field. In contrast, dispe rsions of particulate SrCrO4 in the PVB coatings allow CrO42- diffusion dir ectly into the underfilm electrolyte layer and profoundly inhibit delaminat ion. It is proposed that replacement of underfilm O-2 reduction by a self-l imiting CrO42- reduction process is the most significant factor in decreasi ng delamination rates. (C) 2001 The Electrochemical Society.