A PRETREATMENT AGAINST LOCALIZED AND FILIFORM CORROSION APPLIED TO ALUMINIUM ALLOYS: THE TRIVALENT CHROMIUM PRETREATMENT

摘要

To improve corrosion resistance and paint adhesion on aluminium alloys, a chemical conversion coating is frequently applied. The most effective pretreatment used before painting is chromate conversion coating. This conversion layer is obtained by immersion in solution containing toxic hexavalent chromium. Research on alternative methods not using any hexavalent chromium is developed for ecological and health reasons. The purpose of this study is to develop a pretreatment for aluminium alloys using relatively non-toxic chemicals. Baths containing trivalent chromium Cr_2(SO_4)_3 and NaF, adjusted with NaOH to pH 4.5 near the precipitation of basic compounds, are able to form films on aluminium alloys with significant corrosion resistance [1]. An oxidation post-treatment of Cr~(3+) to Cr~(6+), made by immersion of the pretreated aluminium alloy in a permanganate solution, enhances the corrosion resistance. Anodic oxidation, made under favorable conditions in a Na_2SO_4 solution, gives the same results. This alternative avoiding hexavalent chromium baths is now under study for aluminium alloys such as 1050, 5754-H22 and 6082-T6. With this pretreatment, the inhibition of the oxygen reduction reaction is effective and the pitting potential/corrosion potential interval is close to the one of chromate conversion coating. The results of corrosion resistance, studied with electrochemical impedance spectroscopy, show a good corrosion protection of pretreated aluminium alloys against pitting. The resistance to filiform corrosion is studied using epoxy electrophoretic paints. In conclusion, the pretreatment based on trivalent chromium baths with post-oxidation enhances pitting and filiform corrosion resistance of aluminium alloys.