Formation of oxide films on the surface of tungsten disilicide during anodic oxidation

Abstract

Oxide-layer formation on the surface of tungsten disilicide is studied during its electrochemical polarization in a 3% NaCl solution at 20 °C. It is indicated that the total thickness of the oxide layer that forms over a period of 90 min does not exceed 10 nm. The following multistage mechanism of anodic oxidation is established: tungsten oxides (for example, W₃O and W_O3) in which the greater the polarization, the greater the extent to which the metal is oxidized, the lower tungsten silicide W₅Si₃, and silica are formed. It is demonstrated that a double electrolytic layer on the interphase boundary between the tungsten disilicide and 3% NaCl solution is formed owing to dissociation of the silanolic groups situated a priori on the surface of the silicide. Here, the surface of the electrode acquires a negative charge, while the positively charged diffusion layer contains hydroxone ions. As a silica film forms on the WSi₂, the material becomes more corrosion-resistant. On the whole, the scheme established for electrochemical oxidation of WSi₂ complies with the mechanism of its high-temperature oxidation.