Oxygen reduction mechanism on corroded zinc

The mechanism of oxygen reduction on the as-polished and corroded zinc specimens has been studied using a rotating ring disc electrode (RRDE) system. On the as-polished surface, oxygen was reduced into two distinct steps. In the first step, about 44% of O-2 was reduced to H2O2 in a 2-electron reaction with the rest being reduced to OH- in a 4-electron reaction. On the other hand, in the second step, with the increase of overpotential O-2 was almost exclusively reduced to OH- in a 4-electron reaction. The first step reduction occurred on an air-formed oxide-covered surface at more positive potential than -1.2 V vs. Ag/AgCl and the second step reduction (E < -1.2 V) took place on a semi-uniformly active surface. On the corroded surface, the second step was not distinctly observed on the polarization curve, because reduction of the zinc corrosion products simultaneously took place around -1.2 V. The O-2 reduction in the first step was inhibited by deposition of the corrosion products, though the ratio of amount of O-2 reduced to OH- in a 4-electron reaction was larger than that on the as-polished surface. The mechanism of oxygen reduction is discussed on the basis of results obtained from the RRDE experiment. (c) 2005 Elsevier B.V. All rights reserved.