Dislocation Etch Pitting Studies on Tungsten Single Crystals Under Potentiostatic Conditions

Dislocation etch pitting of the (100) surface occurs in chloride as well as nitrate and sulfate solutions during anodic polarization in a well-defined potential range (+0.3 to +0.7v vs H₂) where the current is relatively insensitive to changes in potential. In chloride solutions, increasing the chloride ion concentration or temperature results in an increase in the rate of pitting. This suggests that the chloride ion participates in the pitting process. A linear current-time law is obtained at constant potential. A pitting rate curve established for the (100) surface in 0.5M NaCl is Gaussian in shape with an apparent;maximum at a potential of +0.48v. The potential at which the maximum occurs appears invariant with composition or temperature, suggesting that the same chemical species adsorbs on the surface prior to pitting. The decrease in the pitting rate above this potential is explained by the desorption of the chemical species, considered to be WO₂⁻, as a result of further oxidation at more positive anode potentials to the species WO₄⁼. The inability to etch pit the (110) surface under the same conditions is attributed to the direct formation of the doubly charged monotungstate ion WO₄⁼ during anodic dissolution and its subsequent adsorption at all positive anode potentials up to as high at 10v. The specificity of etch pitting solutions is explained in terms of a “critical" potential range for pitting.