Electrical Breakdown and Pitting in Anodic Films on Tungsten in Halogen Ion-Containing Solutions.

Abstract The systematic investigation of the anodic behaviour of W in halogen ion-containing solutions reveals noticeable differences in the presence of different anions. Strong generalized dissolution is observed in fluoride solutions, the oxide growth being hindered at low anodizing current densities. Sparking phenomena occur in the presence of Br− and I− anions as in nitrate and sulphate solutions. Only in Cl− containing solutions is the growth of the anodic films limited by the occurrence of pitting phenomena at a critical thickness of the oxide. The laws of dependence of the phenomenon on the experimental parameters and the influence of ferrous ions on the voltage at which pitting occurs suggest an electrochemical mechanism with electron injection from the solution into the oxide conduction band, analogous to the case of electrical breakdown. The specific role of the Cl− anions is discussed in the frame of a simple model which extends the previous one regarding the electrical breakdown phenomenon in oxoacid solutions.