Oxide film formation on zinc in borate solutions under open circuit

The open circuit potentials of Zn electrode were followed as a function of time in different concentration of Na2B4O7 solution until steady-state, Est., values were attained. The potential shifts immediately towards positive values, indicating film thickening and repair. The rate of oxide film thickening was determined from the linear relationship between the open circuit potential, E, of the Zn electrode and the logarithm of immersion time t as E = a1 + b1 log t. The liner plots consist of two segments indicating the duplex nature of the formed oxide film on the Zn surface. The final steady- state potential, Est., varied with the logarithm of molar concentration of Na2B4O7 solution according to: Est. = a2 − b2 log CNa2B4O7. The effect of rising pH and temperature was also studied. It was found that the rising of pH and temperature of the solution affect on the rate of oxide film thickening and the final steady- state potential.