SEALING OF ANODIZED ALUMINUM-ALLOYS WITH RARE-EARTH-METAL SALT-SOLUTIONS

Boric-sulfuric acid anodized (BSAA) aluminum alloys have been sealed i n hot solutions of cerium or yttrium salts. For comparison, sealing ha s also been performed in the presently used dilute chromate solution, bailing water, and a cold nickel fluoride solution. The corrosion resi stance 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 p rocesses. From the impedance spectra the time dependence of the pore r esistance, R-po, and the specific admittance, A(s), has been determine d. At the end of the exposure the pitted area, A(pit), was calculated. The relationship between A(s) and R-po has been evaluated. Two differ ent sealing mechanisms were detected. For sealing in dilute chromate t he pores in the outer oxide laver stayed open, while for hot water sea ling or sealing in cold nickel fluoride the pores were closed by an ox ide/hydroxide. Sealing of BSAA Al alloys in cerium or yttrium salt sol utions occurred according to one of these two mechanisms depending on alloy type and solution composition. Based on the experimental values of R-po, A(s), and A(pit) it was concluded that sealing in cerium nitr ate and yttrium sulfate solutions provided corrosion resistance simila r to that of chromate-sealed BSAA Al alloys.