The influence of certain sulfonic triphenylmethane dyes, as additives, on anodizing of aluminum in phosphoric acid

Abstract

The anions of the triphenylmethane sulfonic dye Light Green and its isomer Brilliant Blue added in the bath during anodization of pure aluminum (99.96%) in 0.4 M phosphoric acid under constant voltage, are concentrated at the bottom of the pores of the anodic coating and influence the anodic process. This influence was investigated using electrochemical methods, X-Ray Fluorescence (XRF) and Scanning Electron Microscopy (SEM). The observed decrease of anodizing current density can be explained by the fact that the Light Green anions adsorbed at the oxide/electrolyte interface inhibit the transfer of Al3+ ions to the electrolyte bulk. This results in a reduction of the field-assisted dissolution and of the direct ejection of Al3+ ions as well as of the rate of formation of anodic oxide at the metal/oxide interface. These events can be correlated with the observed decrease of porosity and population density of the pores in the presence of Light Green, which indicates an increase of the cells diameter. In addition, from the above experimental methods no alteration of the diameter of the pores and the barrier layer thickness seems to occur. The dye Brilliant Blue has a similar but much smaller influence. This can be explained by its structural differences.