Limits determination of toleration of aggressive anions by a certain passivator on zinc surface

The open circuit potentials of Zn electrode were followed as a function of time in different concentrations Na₂B₄O₇ solutions until steady-state, E_st., values were attained. The potential shifts immediately towards positive values, indicating film thickening and repair. The effect of addition of NaCl, NaBr and NaI as aggressive agent on the steady-state potential of a Zn electrode previously equilibrated in a passivating borate solution was also established. For each Na₂B₄O₇ concentration, the variation in the potential with the quantity of aggressive anions follows an S-shaped relationship. The new potentials are established after an induction period which decreases with the increase in the concentration of the aggressive anion, C_agg., and/or decreases in that of the passivator anion, C_pass.. The concentration, C_agg., that can be tolerated by a certain concentration of the passivator anion, C_pass., is given by the relation: log C_pass. = k + n log C_agg., where k and n are constants. This is derived on the basis of competitive adsorption of both types of anion and the structure of the double layer at the metal/solution interface. The implications of the equations are briefly discussed.