Influence of grain orientation on oxygen generation in anodic titania

Abstract The influence of grain orientation on oxygen generation during anodizing of chemically polished titanium at 20 mA cm − 2 in 0.026 M Na 3 PO 4 and 0.02 M Na 4 P 2 O 7 is investigated in the pre-sparking region using scanning electron, atomic force and transmission electron microscopies. The findings reveal preferential oxygen generation in the anodic film on titanium grains that tend to a basal-like orientation. The resultant oxygen-filled voids that develop within the film increase in size and coalesce with progress of anodizing. Their presence leads to formation of blisters and the eventual film rupture due to release of the high-pressure gas. In contrast, relatively few voids, and hence smoother film, develop on grains of prism-like orientation. The differing rates of oxygen generation are attributed to the grain orientation dependence of the amorphous-to-crystalline transition of anodic titania, with rutile being identified at sites of oxygen-filled voids in films formed in 0.02 M Na 4 P 2 O 7 . There may also be small contributions to oxygen generation from influences of substrate impurities incorporated into the film. The grain orientation also affected the rate of chemical polishing of the titanium in HF/HNO 3 , which was faster on basal-like planes.