Boric/Sulfuric Acid Anodizing of Aluminum Alloys 2024 and 7075: Film Growth and Corrosion Resistance Available to Purchase

The influence of boric acid (H₃BO₃) additions to sulfuric acid (H₂SO₄) were examined for the anodizing of Al 2024-T3 (UNS A92024) and Al 7075-T6 (UNS A97075) alloys at constant voltage. Alloys were pretreated by electropolishing, by sodium dichromate (Na₂Cr₂O₇)/H₂SO₄ (CSA) etching, or by alkaline etching. Current-time responses revealed insignificant dependence on the concentration of H₃BO₃ to 50 g/L. Pretreatments affected the initial film development prior to the establishment of the steady-state morphology of the porous film, which was related to the different compositions and morphologies of pretreated surfaces. More detailed studies of the Al 7075-T6 alloy indicated negligible effects of H₃BO₃ on the coating weight, morphology of the anodic film, the thickening rate of the film, or corrosion resistance provided by the film. In salt spray tests, unsealed films formed in H₂SO₄ or mixed acid yielded similar poor corrosion resistances, which were inferior to that provided by anodizing in chromic acid (H₂CrO₄). Sealing of films in deionized water, or preferably in chromate solution, improved corrosion resistance, although not matching the far superior performance provided by H₂CrO₄ anodizing and sealing.