On the pitting corrosion currents of zinc by chloride anions

Abstract The changes in the pitting corrosion current density with time on zinc electrode concerning the concentration of both the passivating borate and the aggressive chloride anions were followed using a simple electrolytic cell. The pitting corrosion currents started to flow after an induction period, τ . This period is found to be a function of the concentration of Cl − anion, according to the relation log τ = β − γ log C Cl − . The pitting corrosion currents finally reached a steady-state value, which depended on the concentration of both B 4 O 7 2− and Cl − anions. At a constant B 4 O 7 2− anion concentration, the pitting corrosion current varied with the concentration of Cl − anion according to the relation log i pit = a 1 + b 1 log C Cl − . It also varies at constant Cl − anion concentration and various B 4 O 7 2− anion concentration according to the relation log i pit = a 2 − b 2 log C B 4 O 7 2− . The susceptibility of the passivating zinc to pitting corrosion was found to be increasing as the temperature and pH of the solution increases. Results are discussed on the basis of adsorption of the aggressive anion on the passivating film, followed by penetration through the film and incorporation in it. This undermines the oxide film and causes pitting corrosion.