The effect on a tarnishing reaction of an electric field across the growing product layer

Abstract

A study has been made of the effect of an applied electric field on the rate of oxidation of zinc and tungsten. An applied potential was established across the growing oxide layer by means of an inert porous platinum electrode at the gas/oxide surface. It was shown that for zinc at 350 °C and below, and for tungsten at 170 °C the direction of the effect was such that a reduction in the rate of oxidation resulted from a potential polarity with the porous platinum negative with respect to the metal substrate. For zinc at 390 °C and for tungsten at 260 °C the reverse dependence on polarity was observed. It was concluded that at 350 °C and below with zinc and at 170 °C with tungsten electron transport was rate-controlling, while with zinc at 390 °C and with tungsten at 260 °C mass transport was rate controlling. From a comparison with the known oxidation kinetics of these metals, it was suggested that these transitions in rate controlling species were accompanied by transitions from logarithmic to parabolic growth laws.