The influence of chlorides, sulphates and nitrates at the coating-steel interface on underfilm corrosion

Abstract

The influence of water-soluble salts at the coating-steel interface on underfilm corrosion was studied. Vinyl and polyurethane varnishes were applied on mild steel contaminated with eight water-soluble chloride, sulphate and nitrate salts with Na+, NH+ 4 and Ca2+ as cations. The coated samples were exposed to condensed humidity for 100, 300 and 600 h. Rusting and blistering were visually evaluated. The underfilm corrosion rate was determined using a gravimetric technique. The study was completed by water and oxygen permeability tests on free coating films and by conductivity tests in aqueous electrolyte solutions. The corrosion stimulating salts exert an important effect on underfilm steel corrosion. Water permeability governs the coating adhesion and oxygen permeability controls the underfilm corrosion. The effect of the cations on underfilm corrosion seems to be masked by the effect of the anions. © 1997 Elsevier Science S.A.