Kinetic Studies of Zinc Passivation in Borate Solutions Under Different Conditions

The anodic polarization of zinc in sodium borate (Na₂B₄O₇) solutions of different concentrations was studied under galvanostatic polarization conditions. Potential-vs-time curves obeyed the empirical relation (dE/dt)_i = a(i)^b. The oxide formation rate increased with decreasing concentrations of borate and increasing current density. However, increasing the solution temperature and pH decreased the oxide formation rate because of the partial dissolution of the oxide film. The activation energy (∆H^*) for anodic dissolution of the oxide was found to be 16 kJ/mol. Saturation of the solution with oxygen increased the oxide formation rate more than with hydrogen gas or when naturally aerated. This behavior was attributed to the fact that oxygen assisted in passivation and delayed dissolution processes.