Some Aspects of the Anodic Oxidation of V, Mo, and W

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Abstract
V, Mo, and W can be readily anodized in an electrolyte consisting of acetic acid, , and 1.0M additional water as proposed orginally by Keil and Salomon for V. The resulting films, when formed in the presence of air, are duplex, being crystalline and of lower stoichiometry near the metal‐film interface but having stoichiometries near , , and beyond the metal‐film interface. The films formed on V and Mo are subject to significant dissolution such that time‐independent dissolution currents of 0.11 and 0.04 mA/cm2 can be inferred; with W, by contrast, the dissolution current was only ∼0.001 mA/cm2. The electronic currents are estimated to be 0.09 mA/cm2 for V, 0.12 mA/cm2 for Mo, and 0.03 mA/cm2 for W, and the constant‐current efficiency follows as being given by the relation , where is the total applied current. The efficiencies for V and Mo thus lie in the interval 0.60–0.92 for , whereas W shows essentially unit current efficiency. Efficiencies are also deduced for conditions of mixed constant current and constant voltage. That of V is unusually low, as is to be expected for such conditions, whereas that of Mo is unusually high, suggesting that the average stoichiometry of the film is measurably less than . Again W shows essentially unit efficiency.

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