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 O-2 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 O-2 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 (partial derivative E/partial derivative logi) of 0.058 V dec(-1) and 0.132 V dec(-1) are determined for 2e- and 4e- O-2 reduction on pure zinc, respectively. Aluminium is virtually inert and varying aluminium content between 0.1% and 55% exerts little influence on O-2 reduction kinetics. However, all the Zn-Al alloy surfaces give very much higher O-2 reduction currents at low polarization than does pure zinc and it is proposed that this arises through an electrocatalysis of 2e-O-2 reduction by traces of substrate derived iron. (c) 2005 Elsevier Ltd. All rights reserved.