A comparision between the oxygen reduction rates on zinc in the active and passive state

Abstract

The oxygen consumption by zinc, due to the oxygen reduction reaction (ORR), was measured using a Pt microelectrode, operating in a redox completion mode. These experiments were performed on zinc anodically polarised, in solutions with pH 7 (active state) and pH 13 (passive state). In the electrode potential range -1.05 VSCE to -0.5 VSCE, the anodic currents in the pH 7 solution (range: 2.5-7.5 mA/cm2) are higher than that in a pH 13 solution (range: 0.01 to 0.5 mA/cm2). In the pH 7 solution, when anodically polarised, the oxygen concentration measured (using the Pt microelectrode) at a 50 micron distance from the zinc surface is 15-60% of the bulk oxygen concentration. Therefore at the same position, the oxygen consumption by zinc (due to ORR), is between 40-85% of the bulk oxygen concentration. In a pH 13 solution, as zinc becomes passive, the oxygen consumption by zinc (due to ORR) is around 70-80% of the bulk oxygen concentration. The rate of ORR on zinc oxides (ZnO/Zn(OH)2) is therefore significant when compared to actively corroding zinc.