Anodic Behavior of Tungsten in H_{3}PO_{4}-K_{2}SO_{4}-H_{2}SO_{4}/KOH Solutions


Abstract: Potentiodynamic and potentiostatic polarizations were used to study the effect of pH on the anodic reaction characteristics of tungsten (W) in a very broad pH range (0.4-13). The open circuit potential was observed to decrease with a constant slope of 58 mV/pH, as pH increased. The oxide phase of W showed typical amphoteric surface behavior in the acidic regime. The oxide dissolution went through a minimum at pH 2.5, which was identified as the point of zero charge (pzc) of W--oxide. The oxide dissolution below the pzc was H^{+} -assisted and above the pzc was OH^{-}-assisted. At around the pzc the main dissolution pathway was H_{2}O-assisted dissolution. In basic solutions, in the pH range 7 to 10.5, the hydrated oxide phase dissolution was pH independent. In very basic solutions (above pH 10.5), the dissolution rate of W was observed to increase as pH increased.


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