Gel formation and the efficiency of anodic film growth on aluminium

The development of gel layers during anodizing, and their influence on the efficiency of growth of anodic films, has been examined for formation of films at constant current density on aluminium in saturated potassium antimonate electrolyte and 0.1 M sodium molybdate, sodium silicate and sodium tungstate electrolytes. The gels are produced immediately above the growing anodic films by the reaction of H+ ions, generated at the film/gel interface, with the electrolyte anions to form uniform layers of hydrated oxide. The gels can enhance the efficiency of film growth by reducing, or eliminating, field-assisted ejection of Al3+ ions from the film to the electrolyte. The thicknesses of the gel layers increase at constant rates with thickening of the anodic films, although the efficiencies of gel formation per se are relatively low. The thickest gels are obtained following anodizing in antimonate electrolyte possibly reflecting a more favorable rate of gel formation, relative to its rate of dissolution, than that for other gel layers. The gel layers may shrink and crack on drying, most noticeably for the gels formed in antimonate and tungstate electrolytes. The gel layers formed in molybdate electrolyte appear, by comparison, highly resistant to cracking. (C) 1999 Elsevier Science Ltd. All rights reserved.