Anodic Oxidation of Tungsten in Nonaqueous Electrolyte II . Electrochemical Coloring and Bleaching of the Oxide Film

Ellipsometric and electrochemical measurements are used to determine the mechanisms by which anodic oxide films on tungsten are reduced (colored) and reoxidized (bleached) in acetic acid electrolyte. In the reduction process, a layer of HWO 3 forms on the outer surface of the oxide, and, as reduction proceeds, a phase boundary between HWO 3 and WO 3 moves inward toward the substrate. Competition from hydrogen evolution limits the depth to which the boundary penetrates, but films up to 50 nm in thickness can be reduced completely to HWO 3 with an appropriate sequence of cycles