Concentration and/or pH Effects of Oxalic Acid Bath on the Formation or Dissolution of Aluminum Oxide Film

Abstract

With the object of investigating a role of hydrogen and oxalate ion in galvanostatic oxidation of Al in oxalic acid-triethanolamine solutions, pH and/or concentration dependence of bath voltage, microstructure of anodic film and C₂O⁴ content of the film were examined, while the chemical dissolution rate of the film influenced by these factors were also examined. In the bath with constant (COOH), content, HC_2CO^2-- concentration decreased with decreasing pH. Then the total area of pores increased with decreasing pH so that electric field near the surface of barrier layer decreased. Nevertheless, C_2CO^2- content of the film increased as pH decreased. Such pH dependence of C_2CO^2- content of the film was in agreement with that of HC_2CO^2-- adsorption. In the bath of constant pH, the total area of pores decreased as oxalate concentration was increased. In this case, C_2CO^2- content of the film was increased since HC_2CO^2-- adsorption was accelerated by the high oxalate concentration and the increased electric field near the surface of barrier layer. Anodic film was more easily dissolved as CiO, content of the film was higher and/or pH at the pore-base was lower, and so the bath voltage decreased as pH of the solution was lower and/or oxalate concentration was higher. From the results obtained, it was considered that the initiation of pore formation was the dissolution of film accelerated by the oxalate ion involved in the film.