Shortlisted evaluation of alternative corrosion inhibitors to chromate for use in organic coatings using scanning reference electrode technique

Corrosion is the major mode of failure associated with organically coated galvanised sheet steels used in construction applications. In order to optimise performance and maximise the service lifetime of these materials, corrosion inhibitors are incorporated into the organic coating systems. Salts based on chromate (Cr(VI)) are highly effective for this purpose and have been used extensively in this role, however there is continuing pressure to develop effective alternative inhibitor systems owing to the known toxicity and carcinogenic properties of chromate. The scanning reference electrode technique (SRET) has been used to study the influence of a range of anodic and cathodic corrosion inhibitors on patterns of localised corrosion occurring on the intact zinc surface of galvanised sheet steel. SRET can spatially resolve localised anodic and cathodic activity occurring across the zinc surface and has highlighted some promising novel inhibitor species, namely rare earth metal salts containing cerium(III), lanthanum(III), and yttrium(II). The trivalent cations of cerium and yttrium have been incorporated into a range of inorganic ion exchange matrices to produce anticorrosive pigments. Fully formulated polyester resin based primer systems containing these pigments have been prepared. The primers were applied to a galvanised steel substrate under an architectural polyester top coat and subjected to accelerated corrosion testing. Salt spray results indicate a corrosion performance equivalent to, and in some instances exceeding, that of chromate.