Effect of pH on the Anodic Behavior of Tungsten

Potentiodynamic and potentiostatic polarization, and the rotating disk electrode technique, were used to study the anodic behavior of tungsten (W) in a broad pH range (0.5-13.5) in buffered solution. Surface oxides were found to play a prominent role in the anodic oxidation and dissolution of tungsten. Five distinct pH regimes and the corresponding reaction mechanisms were identified. Below pH 1 (region A) -assisted dissolution was the main dissolution pathway. As the pH increased, the role of diminished and at pH 2.6 (region B), which was identified as the point of zero charge (pzc) of the surface tungsten oxide, dissolution was mainly -assisted. The dissolution was observed to be -assisted above the pzc. The reaction order for was one between pH 4.5 and 6.5 (region C). The reaction order became zero at pH 8 (region D). This observation was attributed to the pH-independent dissolution of the hydrated oxide phase. Above pH 10, dependence of the anodic current commenced and at around pH 12.5 (region E) the reaction order for became one. © 2002 The Electrochemical Society. All rights reserved.