High Temperature Hard Anodizing with Tartaric Acid–Oxalic Acid–Triethanolamine Bath

Tartaric acid does not produce uniform films on A6063 alloy (Al–Mg–Si). Uniform films are formed when oxalic acid is added to the tartaric acid bath, but hardness above Hv 400 cannot be obtained at a voltage below 100 V. When 1 mol dm−3 tartaric acid–0.4–0.6 mol dm−3 oxalic acid baths were adjusted with triethanolamine (0.26–0.54 mol dm−3) to pH 1.00 and 1.25, crack-free films with hardness above Hv 430 were formed at 40 °C in the range 65–85 V. The hardness of the film is independent of the anion content of the film, increasing with increase in the thickness of cell wall and lowering in porosity.