Formation of barrier-type amorphous anodic films on Ti–Mo alloys

Barrier-type amorphous anodic films with uniform thickness grow at high current efficiency on sputter-deposited Ti–Mo alloys containing 11.5–37.0 at.% molybdenum to high voltages of more than 100 V in 0.1 mol dm−3 ammonium pentaborate electrolyte, in contrast to an amorphous-to-crystalline transition of anodic film on titanium and subsequent oxygen formation at relatively low voltages of less than 20 V. During anodizing of the alloys at a constant voltage of 80 V, the currents decrease to the order of 10−2 A m−2 within 3 h, whereas the current for titanium decreases only to the order of 1 A m−2, due to the presence of flaws in the film, associated with oxygen evolution. The capacitance of the anodic films formed on the Ti–11.5 at.% Mo alloy is almost comparable to that formed on tantalum at the same voltage. The increase in molybdenum content in the alloy results in a slight decrease in the capacitance of the films. From these results, the structure of anodic films on titanium can be modified by incorporation of molybdenum species from the substrate so that uniform amorphous anodic films with a relatively high capacitance and a low defect density have been obtained.