On the pitting corrosion of tin in aqueous solutions

Four independent techniques were employed to prove that Sn undergoes pitting corrosion. The first was based on the measurement of the variation of the open circuit potential of the Sn electrode in aerated Cl^? solutions of various concentrations. Steady-state potentials were attained slowly and erratically from negative values, and were more positive the higher the dilution. Sn electrodes prepassivated in CrO₄^2? solutions responded readily to additions of Cl^? ion. The potentials developed were more noble than those measured in presence of the passivator alone, and changed to positive values with the increase of the concentration of the pitting corrosion agent. Attack was under cathodic control.Galvanostatic polarization of the Sri electrode was carried out in 0.005 to 0.1M NaOH, in the presence of various additions of Cl^?. Above a certain Cl^? content, contingent upon the alkali concentration, the aggressive anion prevented the evolution of Oa on the electrode, and oscillations in the E?t curves were recorded. Competitive adsorption of Cl^? and OH^? is assumed to occur, which affected both the quantity of electricity, Q_p, consumed along the oxide formation steps, and the rate of potential increase, dE/dt, following oxidation. Plots of the two variables as a function of the Cl^? ion concentration exhibited a definite break at the value characteristic for the initiation of pitting attack. Potentiodynamic polarization showed that the pitting corrosion potential progressively shifts towards negative values as the concentration of the aggressive agent in solution was increased. The pitting corrosion currents resulting from the addition of Cl^? to Sn electrodes prepassivated in CrO₄^2? solutions were measured. The dependence of the maximum currents on solution composition is explained on the basis of competitive adsorption.