Logarithmic Oxidation Kinetics of Zinc

Oxidation rates of cold‐rolled zinc are reported at 0.2–0.8 mm O2 pressure in the temperature range 125°–206°C. Oxide thickness vs. time follows a two‐stage logarithmic equation similar in behavior to Cu, Ni, and Fe. Activation energies for first‐ and second‐stage oxidation are 0.34 and 0.35 ev, respectively. The data are interpreted in terms of a uniform density space charge oxide, corresponding to first‐stage oxidation, underlying a diffuse space charge oxide corresponding to second‐stage oxidation, with the rate being controlled in each instance by electron transfer from metal to oxide. The high density of trapped charge in explains in part the low initial oxidation rate of Zn compared to Cu and Ni. Second stage logarithmic oxidation kinetics is ascribed to thermal dissociation of trapped charge in the outer regions of oxide when the latter reaches a critical thickness, giving rise to a diffuse space charge.