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.

Subject
Acids, Films, Water, Corrosion resistant alloys, Metal surfaces, Anode surfaces, Pretreatment, Film formation, Salt spray corrosion testing, Corrosion resistance, Aluminum alloys, Alloys, Anodizing
Keywords:
Al 2024, Al 7075, alkaline etching, boric acid, chromic acid, deionized water, electropolishing, film, salt spray tests, sulfuric acid
NACE International
1999
You do not currently have access to this content.