An investigation of electrolytic coloring process of anodized aluminum coatings

In this paper specimens of pure aluminum, AA 5083 and AA 6111 anodized under standard conditions were electrolytically colored from acidic tin sulphate solutions at different rms voltages in order to further gain insight to the ac electrolytic coloring process. Electrochemical methods, SEM/EDS, RBS and XRF techniques were used in this study. The tin deposition efficiency increases with the increase of applied rms electrolytic coloring voltage and the purity of aluminum. The classical linear porosity of the film formed on high purity aluminum favors the tin deposition, while the heavily modified film morphology on AA 6111, impedes it. In the case of reanodizing the anodic–cathodic peak currents quickly reach maximum values and they are influenced by the applied rms voltage and the material types. Their ratio approaches unity indicating symmetry and cooperative movement of the ions through the oxide film. The corresponding ratio of faradaic charges is about 0.6, thus the anodic reactions occur in an extent of about 60% of the cathodic ones. However as the electrolytic coloring is going on and the currents decrease reaching a limited value, then these ratios change slightly not only with respect to the time of coloring but also to the material types. Regardless of the reanodizing or dissolution process being operative during the anodic half cycle, the tin deposition proceeds, when formation of new oxide and redistribution of pores take place. In the case of thinning of barrier layer more time is necessary in order to reach the peak currents their maximum values. The symmetry of these currents is considerably disturbed in the case of alloys but not of pure aluminum, while the ratio of faradaic charges is slightly modified. Thus the capacitance of the oxide films of alloys seems to be considerably altered during the barrier layer dissolution process.