The Anodic Oxidation of Tungsten in Aqueous Base

Abstract

The kinetics of the anodic oxidation of tungsten metal in basic aqueous solution has been investigated using galvanostatic polarization. Anodic Tafel slopes were constant over all concentrations of hydroxide at 0.136 V/decade. Cathodic Tafel slopes ranged from 0.069 to 0.197 V/decade, increasing with hydroxide ion activity. Anodic exchange current density, obtained by extrapolation, averaged . Single ion activities, estimated by Harned's rule, in sodium hydroxide solutions buffered with sodium nitrate to constant ionic strength, yielded rate data indicating first‐order dependence on hydroxide ion activity over a concentration range of 0.02–6.00M OH−. A mechanism commensurate with the observed behavior is proposed involving oxidation to intermediate tungsten oxides, followed by nonelectrochemical dissolution to form the soluble tungstate anion. A brief discussion of the data for the hydrogen evolution reaction is also presented, in the light of recent studies on tungsten covered by oxide layers.