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.
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1 November 1999
Research Article|
November 01 1999
Boric/Sulfuric Acid Anodizing of Aluminum Alloys 2024 and 7075: Film Growth and Corrosion Resistance
L. Zhang;
L. Zhang
*Corrosion and Protection Centre, University of Manchester Institute of Science and Technology, PO Box 88, Manchester M60 1QD,
U.K.
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C.J.E. Smith;
C.J.E. Smith
**Defence Evaluation Research Agency, Structural Materials Centre, Farnborough, GU14 6TD,
U.K.
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P. Skeldon
P. Skeldon
*Corrosion and Protection Centre, University of Manchester Institute of Science and Technology, PO Box 88, Manchester M60 1QD,
U.K.
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Online ISSN: 1938-159X
Print ISSN: 0010-9312
NACE International
1999
CORROSION (1999) 55 (11): 1052–1061.
Citation
G.E. Thompson, L. Zhang, C.J.E. Smith, P. Skeldon; Boric/Sulfuric Acid Anodizing of Aluminum Alloys 2024 and 7075: Film Growth and Corrosion Resistance. CORROSION 1 November 1999; 55 (11): 1052–1061. https://doi.org/10.5006/1.3283942
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