The kinetics and mechanism of cathodic oxygen reduction on zinc and zinc–aluminium alloy galvanized coatings

A rotating disk electrode technique is used to investigate the kinetics and mechanism of O2 reduction as it occurs at the surface of various hot-dip Al–Zn alloy coatings (on steel) immersed in weakly alkaline (pH 9.6) aqueous sodium chloride. The zinc component of coatings behaves electrochemically as though it were free zinc and the O2 reduction pathway is determined by the potential dependent state of zinc. A 2e− reduction to H2O2 predominates at potentials near the free corrosion potential, where zinc is (hydr)oxide covered. A 4e− reduction to OH− predominates at potentials where zinc is bare. Tafel slopes (∂E/∂log i) of 0.058 V dec−1 and 0.132 V dec−1 are determined for 2e− and 4e− O2 reduction on pure zinc, respectively. Aluminium is virtually inert and varying aluminium content between 0.1% and 55% exerts little influence on O2 reduction kinetics. However, all the Zn–Al alloy surfaces give very much higher O2 reduction currents at low polarization than does pure zinc and it is proposed that this arises through an electrocatalysis of 2e− O2 reduction by traces of substrate derived iron.