Sealing of Anodized Aluminum Alloys with Rare Earth Metal Salt Solutions

Abstract

Boric-sulfuric acid anodized (BSAA) aluminum alloys have been sealed in hot solutions of cerium or yttrium salts. For comparison, sealing has also been performed in the presently used dilute chromate solution, boiling wateL and a cold nickel fluoride solution. The corrosion resistance of the sealed BSAA Al alloys Al 2024, Al 6061, and Al 7075 has been evaluated by recording impedance spectra during exposure in 0.5 N NaCl for 7 days. Shorter or longer exposure times have also been used depending on the corrosion resistance obtained by different sealing processes. From the impedance spectra the time dependence of the pore resistance, R,, and the specific admittance, A,, has been determined. At the end of the exposure the pitted area, was calculated. T1'he relationship between A, and R, has been evaluated. Two differ-ent sealing mechanisms were detected. For sealing in dilute chromate the pores in the outer oxide layer stayed open, while for hot water sealing or sealing in cold nickel fluoride the pores were closed by an oxide/hydroxide. Sealing of BSAA Al alloys in cerium or yttrium salt solutions occurred according to one of these two mechanisms depending on alloy type and solution composition. Based on the experimental values of R80, A,, and A61, it was concluded that sealing in cerium nitrate and yttrium sulfate solutions provided corrosion resistance similar to that of chromate-sealed BSAA Al alloys. Infroduction Oxide layers on aluminum alloys produced by anodizing