Anodic behavior of tungsten in $H_3PO_4-K_2SO_4-H_2SO_4$/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 $H_2O$-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.