OPTIMIZATION AND EVALUATION OF ALUMINUM SEALING

Preliminary studies showed that outstanding corrosion resistance to humid N2O4 and salt fog could be achieved on sulfuric acid anodized 7075-T6 aluminum alloys when sealed first with nickel acetate followed by sodium dichromate. In this study, the nickel acetate-sodium dichromate sealing procedure, termed duplex sealing, was investigated in detail for 2024-T3, 7075- T6, 7178-T6 and 7079-T6 aluminum alloys to identify the treatment combinations giving optimum corrosion resistance. The process variables studied were seal solution immersion times, temperatures and concentrations. Factorial designed experiments were used, followed by statistical analyses of the corrosion resistance of the treatment combinations. The nickel acetate seal temperature was found to be the predominant variable affecting corrosion resistance. The lowest temperature, 180 F, resulted in the best corrosion resistance. Based on these studies, maximum corrosion resistance was achieved by sealing first in 1. 25 wo nickel acetate for ten minutes at 180 F followed by a two minute seal in 4.15 wo sodium dichromate at 180 F. In duplex sealing, the chromium concentration in the sealed anodic film is not related to the chromate yellow color of the anodized alloy. High nickel acetate seal temperatures inhibit stable chemisorption or reaction of the chromate with the anodic film. Under such conditions, the chromium content is very low, even though the samples are intensely colored. Sulfuric acid anodizing times as short as five minutes, when duplex sealed, were resistant to 5 salt fog for times in excess of 1,000 hours.