The mechanism of the anticorrosive action of calcium-exchanged silica

Abstract

The objectives of this work were to study the efficiency and the mechanism of the anticorrosive action of calcium-exchanged silica in paints. The anticorrosive properties of the pigment were evaluated by following the electrochemical behaviour of a steel electrode in the pigment suspension and analysing the protective layer formed on it. In a second stage, solvent-borne paints were formulated with 30% by volume (v/v) of the total pigment content. The selected PVC/CPVC (pigment volume concentration/critical pigment volume concentration) ratio was 0.8. Two resins were chosen as film-forming materials: an alkyd and an epoxy. The performance of the resulting anticorrosive paints was assessed by accelerated (salt-spray and humidity chambers) and electrochemical tests (corrosion potential, ionic and polarisation resistance). It was demonstrated that the anticorrosive performance of paints pigmented with calcium/silica is at least equal or better than that obtained with paints containing zinc phosphate. Good correlation was obtained between accelerated and electrochemical tests. Steel passivation by calcium-exchanged silica is a complex process accomplished by the high pH of the pigment suspension and the deposition of a siliceous film on the steel surface.