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

Abstract The open circuit potentials of Zn electrode were followed as a function of time in different concentrations Na 2 B 4 O 7 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 Na 2 B 4 O 7 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.