Spontaneous and anodic oxidation of Tantalum and Zirconium in phosphate solutions

Spontaneous and anodic oxidation of Ta and Zr have been studied in 0.1 M solutions of H3PO4, NaH2PO4, Na2HPO4 and Na3PO4. Below 10 A/cm2 the dependence of the spontaneous‐oxidation rate on potential follows a Tafel relation. Te results indicate that the partial anodic reaction of film formation is balanced by the cathodic reduction of dissolved oxygen. The kinetics of anodic oxidation are discussed on the basis of a one phase oxide file liable to contamination by phosphate anions. Oxide growth follows the high field ionic conduction even below the oxygen evolution potential. The simple exponential relation of Güntherschulze and Betz is valid. This is supported by the linearity obtained between (1) reciprocal capacity and the logarithm of current density i and (2) the potential E and log i for oxide formation at constant charge. Approximate calculations of the effective activation distance show that the degree of contamination by anions is the same in the four equimolar solutions indepent of PH.