Dissolution Current and Pitting Potential of Zinc in KOH Solutions in Relation to the Concentration of Aggressive Ions

Abstract

Potentiodynamic anodic polarisation curves for zinc were obtained in dilute (10⁻³ M) solutions of KOH that favoured metal dissolution with only a slight tendency to passivate the zinc, and in more concentrated (10⁻² M) solutions that favoured passivation of the zinc. Addition of aggressive halide ions (Cl⁻, Br⁻, I⁻) to the 10⁻³ M solution caused a shift of the dissolution potential to more negative values and a marked increase in corrosion rate, suggesting adsorption of the anions on to the bare metal surface. The difference between the dissolution current density, i_o, in 10⁻³ M KOH alone, and in the presence of halide ions, i_agg, was given by: log (i_agg−i_o) = a + b log C_agg, where a and b are constants.

In 10⁻² M KOH, small additions (2 × 10⁻⁴ M) of aggressive ions did not affect the dissolution kinetics. At higher concentrations the c.d. increased sharply at a well defined potential, or pitting potential, E_p, given by the relation: E_p = α − β log C_agg, α and β being constants. It is assumed that in 10⁻² M KOH the aggressive anion acts by penetrating the oxide, or by becoming incorporated in it as an impurity, altering its physical and electrical properties.