Influence of ultraviolet illumination on the passive behavior of zinc

The passive behavior of zinc in alkaline pH 13.0, 10.3, and 9.2 solutions, under dark and illumination conditions, has been studied using dc polarization and ac impedance techniques. It was found that illumination with polychromatic light caused dissolution of anodically formed passive layers on zinc, with this photoinduced dissolution effect being more pronounced in the more alkaline pH 13.0 solution. Photoinduced dissolution in the pH 10.3 and 9.2 solutions is explained in terms of the photodecomposition of ZnO through reaction with photogenerated holes. The more intense dissolution in the pH 13.0 solution is explained in terms of the photodecomposition of ZnO and the additional electrochemical dissolution in this alkaline solution which occurs as a result of the photodecomposition of the passive layer. Although the electrodes repassivated once the light source was removed, it was found that prior illumination led to a decrease in the donor densities and thus a modification of the defect structure of the passive layers