Effects of chloride ion and applied current density on the stress generation during anodic oxidation of tungsten in 0.1 M H2SO4 solution

Surface stresses developed during the anodic oxidation of tungsten in 0.1 M H2SO4 solution have been investigated as a function of chloride ion concentration and applied current density. Before the oxide film breakdown, specimens moved increasingly in a compressive direction with increasing chloride ion concentration. In contrast, after the oxide film breakdown, the reverse movement in a tensile direction was encountered and the amount of tensile deflection increased with increasing chloride ion concentration. Ac impedance measurements substantiated the existence of relatively stable tungsten-oxychlorides formed due to the incorporation of chloride ion in the anodic oxide film on tungsten during the oxide growth. The movement in the compressive stress direction due to chloride ion incorporation before the oxide film breakdown increased with decreasing applied current density. The present experiment results are discussed with respect to oxide film growth-related electrochemical reactions occurring both at the metal/oxide film interface and at the oxide film/electrolyte interface. The changes in the sign of deflection are well accounted for in terms of the combined effects of the increasing compressive stress development due to the substitution of chloride ion for oxygen vacancy and of the increasing tensile stress development due to the cation vacancy accumulation at the metal/oxide film interface with increasing chloride ion concentration.