Oxygen reduction on iron .7. Temperature dependence of oxygen reduction on passivated iron in alkaline solutions

The temperature dependence of the kinetics of O-2 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 degrees C. The apparent enthalpy of activation was determined and was found to be pH dependent. The Tafel slope 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 O-2 reduction on passive Fe was attributed to the pH, temperature and potential dependence of the passive film properties.