POTENTIOSTATIC PASSIVATION OF ZINC IN ALKALINE-SOLUTIONS

The potentiostatic passivation of upward-facing horizontal zinc electr odes in quiescent 0.4-4 m KOH solutions has been studied according to different models and by SEM, coulometry and ICP. An initial flat maxim um was found for all the potentiostatic transients except in 0.4 M KOH . The greatest part of the transients consisted in the initial maximum followed by a current plateau and a sigmoidal sharp decrease. The rem aining curves were gradual modifications of the latter. The first part of all the transients could be interpreted according to a diffusion-c ontrolled growth of a porous film coupled to metal dissolution from th e electrode surface. In this model, the metal is dissolved as its ions and part of them react with the anions already present in the solutio n to form the anodic film, the rate of electrocrystallization being de termined by the instantaneous concentration of the metal ions at the e lectrode surface. However, the second phenomenon appearing in practica lly all the potentiostatic transients, i.e., the current plateau and f urther sigmoidal sharp decrease must be explained by a model of 2D ohm ic-controlled film growth.