Gel formation during growth of barrier-type anodic films on aluminium

Abstract

Abstract 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 0.1m 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, els of significant thickness are only found on specimens anodized at ≤ 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 × 1027 tungsten atoms m−3. Further, the thickness of the gel layer, as observed ...