Breakdown of tin passivity in aqueous sodium stannate solution

Abstract

The electrochemical behaviour of tin anodes in 0·19 M Na₂Sn(OH)₆ solutions containing various concentrations of Cl⁻, Br⁻, or 1⁻ ions has been investigated. Galvanostatic potential-time curves at 1 mA cm ⁻² reveal that achievement of electrode passivity is inhibited by the presence of halide ions, but eventually attained; the extent of inhibition is directly proportional to halide ion concentration. At 5 mA cm⁻² and high Cl⁻ ion concentrations, a breakdown of anode passivity is observed. Potentiostatic current density-potential curves show that all the halides investigated inhibit anodic dissolution in the active region, but tend to break down passivity in the permanent passivation region, leading to pitting attack. The critical pitting potential shifts progressively to more negative values with increasing Cl⁻ ion concentration. In all the experiments, the aggressive action of the halides decreased in the order Cl⁻ > Br⁻ > l⁻. The breakdown is discussed in the light of earlier theories.