GEL FORMATION DURING GROWTH OF BARRIER-TYPE ANODIC FILMS ON ALUMINUM

The formation of gels attached to the surfaces of barrier-type anodic films, grown on aluminium at high Faradaic efficiency, has been studied using a combination of scanning and transmission electron microscopy and Rutherford backscattering spectroscopy. It is shown for the first time that continuous gel layers develop during anodizing at constant current density in 0.1 M sodium tungstate electrolyte at 293 K. The gels are principally composed of hydrated tungsten oxide, namely WO3.nH2O, which is a product of the reaction between tungstate ions from the bulk electrolyte and hydrogen ions that are generated at the film surface as a consequence of film growth. However, gels of significant thickness are only found on specimens anodized at less-than-or-equal-to 10 A m-2, and the amount of gel increases as the current density decreases. Following anodizing at 1 A m-2 to 200 V, the upper limit on the concentration of tungsten in the gel is 1.9 X 10(27) tungsten atoms m-3. Further, the thickness of the gel layer, as observed following drying, is about 55% of the anodic film thickness. The gels do not significantly affect the growth of the anodic film which forms in the usual manner by migration of Al3+ ions outwards and migration of O2-/OH- ions inwards, with tungsten species being incorporated into the outer layer of the film.