AN ELECTROCHEMICAL IMPEDANCE SPECTROSCOPY STUDY OF PASSIVE ZINC AND ELECTRODES IN ALKALINE AND NEUTRAL AQUEOUS-SOLUTIONS

The electrochemical behaviour of passive films on Zn and low alloyed Zn-Co coatings (Co < 1.0%) has been studied in alkaline and neutral aqueous solutions (8.4 less-than-or-equal-to pH less-than-or-equal-to 12.9) by means of electrochemical impedance spectroscopy (EIS). For comparison, measurements on ZnO single crystals were also performed. From Mott-Schottky plots the flat band potential E(FB) and the donor concentration N(D) were determined as a function of pH and Co content. A Nerstian dependence of E(FB) on pH and an increase of N(D) proportionally to the Co content were found. The passive current of Zn coatings decreases with decreasing pH value and increasing Co content. Photocurrent and photovoltage measurements are in satisfactory agreement with the electrochemical results. The decrease of the passive current density observed with increasing Co content can be explained in terms of a chemical stabilization of the passive film.