Composition and corrosion behaviour of galvanised steel treated with rare-earth salts: the effect of the cation

The corrosion performance and the composition of galvanised steel treated by immersion in nitrate solutions of rare-earth metals (cerium, yttrium and lanthanum) were evaluated by electrochemical techniques and surface analysis. The surface film consists of a mixture of rare-earth oxides/hydroxides, which hinders the corrosion reactions of the substrate by reducing the rate of both the cathodic and the anodic reactions. Although all the films provide corrosion protection, the effect seems to be more pronounced for lanthanum. The behaviour of the films formed on the surface is dependent on the rare-earth cation and on the treatment time. (C) 2002 Elsevier Science B.V. All rights reserved.