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

Abstract

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 H 2O 2 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 − 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.