Evaluation of alternative corrosion inhibitors to chromate tor use in organic coatings using scanning reference electrode technique

Corrosion is the major mode of failure associated with organically coated g alvanised sheet steels used in construction applications. In order to optim ise performance and maximise the service lifetime of these materials, corro sion inhibitors are incorporated into the organic coating systems. Salts ba sed on chromate (Cr(VI)) are highly effective for this purpose and have bee n used extensively in this role, however there is continuing pressure to de velop effective alternative inhibitor systems owing to the known toxicity a nd carcinogenic properties of chromate. The scanning reference electrode te chnique (SRET) has been used to study the influence of a range of anodic an d cathodic corrosion inhibitors on patterns of localised corrosion occurrin g on the intact zinc surface of galvanised sheet steel. SRET can spatially resolve localised anodic and cathodic activity occurring across the zinc su rface and has highlighted some promising novel inhibitor species, namely ra re earth metal salts containing cerium (III), lanthanum(III), and yttrium(I I). The trivalent cations of cerium and yttrium have been incorporated into a range of inorganic ion exchange matrices to produce anticorrosive pigmen ts. Fully formulated polyester resin based primer systems containing these pigments have been prepared. The primers were applied to a galvanised steel substrate under an architectual polyester top coat and subjected to accele rated corrosion testing. Salt spray results indicate a corrosion performanc e equivalent to, and in some instances exceeding, that of chromate.