Anodizing of aluminium and AA 2024-T3 alloy in chromic acid: Effects of sulphate on film growth

Chromic acid anodizing is important for the corrosion protection of aerospace aluminium alloys. Previous study has demonstrated that SO42 − impurity in the chromic acid affects the film growth on aluminium at a voltage of 100 V. The present work further investigates aluminium and extends the study to industrial anodizing conditions (Bengough-Stuart (B-S) process) and to the AA 2024-T3 alloy. It is shown that SO42 − concentrations between ~ 38–300 ppm reduce the film growth rate for aluminium anodized at 100 V in comparison with an electrolyte than contains ≤ 1.5 ppm SO42 −, whereas ~ 1500–3000 ppm SO42 have an opposite effect and lead to an unstable pore diameter. Under the B-S process, the film growth depends on the substrate composition, the SO42 − content of the film, the film morphology and, for the alloy, oxygen generation. Corrosion tests of the alloy in 3.5% NaCl solution revealed better protection with films formed in chromic acid containing 38 ppm SO42 − compared with ≤ 1.5 ppm SO42 −, which are within the specified limits for sulphate impurity for chromic acid anodizing. The difference in corrosion protection is proposed to be related to the observed differences in the film morphologies; it is speculated that this may influence the retention of residues of chromate ions in the films.