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Turkish Journal of Chemistry

DOI

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Abstract

Potentiodynamic and potentiostatic polarizations, and a rotating disk electrode technique were used to study the influence of diffusion on the anodic reactions of tungsten (W) in a broad pH range (\sim 0.3--13.5) in H_3PO_4 buffered solutions. The anodic current was controlled totally by the kinetics of H^+ -assisted dissolution below pH 1 (Region A). At around pH 2.5 (Region B), in addition to the kinetics of H_2O-assisted dissolution, specimen rotation had an effect on the current. This observation was attributed mainly to the physical damage to an oxide phase by hydrodynamic effects and partly to the diffusion of surface H_2WO_4(aq) species. The anodic current was under the mixed control of the kinetics of OH^{-}-assisted dissolution and slow diffusion of surface WO_4^{2-} ions between pH 4 and 7 (Region C). The kinetics of the H_2WO_4(s) solubilization and slow diffusion of surface WO_4^{2-} ions were observed to influence the anodic current between pH 7.5 and 11.5 (Region D). In highly alkaline solutions (pH > 12; Region E), however, W anodic current was controlled totally by the slow diffusion of bulk OH^- ions to the electrode surface.

First Page

425

Last Page

440

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