Chemical dissolution resistance of anodic oxide layers formed on aluminum

Chemically resistant anodic oxide layers were formed on pure aluminum substrates in oxalic acid-sulphuric acid bath. Acid dissolution tests of the obtained anodic layers were achieved in accordance with the ASTM B 680-80 specifications: 35 mL/L 85% H3PO4+20 g/L CrO3 at 38 °C. Influence of oxalic acid concentration, bath temperature and anodic current density on dissolution rate and coating ratio was examined, when the sulphuric acid concentration was maintained at 160 g/L. It was found that chemically resistant and compact oxide layers were produced under low operational temperature (5 °C) and high current densities (3 A/dm2). A beneficial effect was observed concerning the addition of oxalic acid (18 g/L). The morphology and the composition of the anodic oxide layer were examined by scanning electron microscopy (SEM), atomic force microscopy (AFM) and glow-discharge optical emission spectroscopy (GDOES).