Effect of alloy types on the electrolytic coloring process of aluminum

The AC electrolytic coloring process at 15 Vrms in acidic tin sulfate solutions for specimens of AA 5083, AA 6111 unheat- and heat-treated was investigated in comparison with pure aluminum. For this purpose, electrochemical techniques, SEM/EDS and XRF were used. The specimens were anodized in sulfuric acid baths under different conditions, prior to electrolytic coloring. The alloy type affects the rate of tin deposition but certain qualitative characteristics and basic aspects of electrolytic coloring mechanisms are similar for alloys and pure aluminum. The temper of AA 6111 does not affect the electrolytic coloring process, although it influences the anodizing process. Under standard electrolytic coloring conditions, the current efficiency for tin deposition is low for all examined materials. However, pure aluminum has a much higher tin deposition efficiency than the alloys. A reduced amount of deposited metal with prolonged electrolytic coloring is observed for AA 6111, indicating deterioration of oxide structure during coloring process. For AA 5083, the increase of conductance of the oxide film results in an increase of the rms current and faradaic cathodic charge consumed during the electrolytic coloring process in comparison with the pure aluminum; this results in an increase of hydrogen evolution with no improvement in tin deposition efficiency. The anodizing voltage affects the electrolytic coloring process to a lesser degree for alloys than for pure aluminum, indicating the role of the morphology of the oxide film. The increases of anodizing temperature and porous layer thickness have marginal effect on electrolytic coloring process. Understanding the effect of alloy types on electrolytic coloring process will enable us to achieve color uniformity and to expand color and shade selections.