High protective, environmental friendly and short-time developed conversion coatings for aluminium alloys

There are a variety of procedures, described in the bibliography, for producing conversion coatings using salts of lanthanide elements, and the coatings obtained by means of some of these procedures show acceptable levels of protection. Nevertheless, the principal limitation usually presented by these procedures is the excessively prolonged treatment times required to achieve such levels of protection. This therefore limits the applicability of these treatments on an industrial scale. Coatings on the alloy AA5083 obtained by using Ce(III) are characterised by having a mixed or heterogeneous nature, being composed of a layer of alumina covering the matrix, together with islands of cerium formed over the cathodic intermetallics that are present on the surface of the alloy. The results obtained indicate that, once these precipitates have been covered, the level of protection provided is conditional upon the thickness of the layer of alumina. In this study, a procedure is proposed for obtaining conversion coatings on the alloy AA5083 based on immersion in solutions of Ce(III) at temperatures higher than ambient. By this means, coatings can be produced in only a few minutes, and of such quality that salt fog tests of 168 h duration are successfully passed. Furthermore, studies conducted employing electrochemical techniques of linear polarisation indicate that the degree of protection provided by these coatings is several orders of magnitude superior to that achieved with other