Oxygen reduction on iron. Part VII. Temperature dependence of oxygen reduction on passivated iron in alkaline solutions

Abstract

The temperature dependence of the kinetics of O2 reduction was studied on passivated Fe in NaOH solutions having a concentration ranging from 0.010 to 1.0 mol dm-3 and in the temperature range from 2 to 42 °C. The apparent enthalpy of activation was determined and was found to be pH dependent. The Tafel slpe increased with temperature in the range -69 to -92 mV dec-1. The enthalpic and entropic components of the transfer coefficient were calculated and these values were also pH dependent. As a consequence of the pH dependence of the apparent enthalpy of activation and of the transfer coefficient components, the reaction order with respect to OH- could not be determined precisely. Its value depends on the potential and temperature and was estimated to be about -0.8. Such complex temperature and pH behavior of O2 reduction on passive Fe was attributed to the pH, temperature and potential dependence of the passive film properties.