Oxygen reduction on iron-Part VI. Processes in alkaline solutions

Abstract

The kinetics of oxygen reduction were studied on passivated iron in borate buffered and unbuffered solutions of pH ranging from 7.4 to 13.8. A rotating disk electrode was employed. The O2 reduction on iron under these conditions was found to occur on the oxidized surfaces. The reaction orders with respect to O2 and OH- are 1 and -1, respectively, in solutions of pH > 11 and 1 and 0 in solutions of pH < 11. The OH- reaction order was explained as being due to the acid-base properties of the passive film. The Tafel slope value is about -85mV. In solutions of pH < 11 there are two Tafel regions, whereas at higher pH values Tafelian behavior is observed only in the high current density region. Two reaction mechanisms are proposed having the same rate determining step which is the adsorption of O2 molecules on Fe(II)ox surface sites which act as the active centers.