INFLUENCE OF TUNGSTEN SPECIES ON THE STRUCTURE OF ANODIC TITANIA

A sputter-deposited Ti-20 at.% W alloy has been anodized at high effic iency in order to examine the influence of alloy species on the struct ure of anodic films on titanium. Interestingly, amorphous oxide films develop on the alloy to voltages of about 200V, in contrast with cryst alline film formation on relatively pure titanium at voltages less tha n 20V. The amorphous films consist of an outer TiO2 layer and an inner layer containing units of TiO2 and WO3. The layered films form as a c onsequence of the migration of Ti4+ ions about 2.4 times faster than t hat of W6+ ions, probably owing to the relative strengths of the W6+-O and Ti4+-O bonds. Film growth proceeds by migration of both the O2- a nd/or OH- ions and the metal cations, typical of amorphous anodic film s, with the transport number of cations, determined using a silicon sp ecies marker, of 0.39. During anodizing above about 200V, local region s of thicker film material develop, possibly owing to nucleation and g rowth of crystalline material. The voltage eventually reaches a maximu m value of about 340V associated with copious oxygen evolution from th e specimen and an extensively flawed film surface.