The effects of the low antimony content and polarisation time on passivation of lead-antimony alloys under deep discharge conditions of the lead-acid batteries were investigated at a potential of +0.7 V versus Hg \ Hg2SO(4) \ K2SO4sat., in a 0.5 M H2SO4 solution. Electrochemical techniques and metallographic analyses revealed that the antimony level controls the electric passivation of Pb-Sb alloys, used as positive grid alloys. For low antimony alloys (Sb < 0.75 wt.%), this passivation phenomenon is due to the formation of α-PbO, acting as an electric barrier at the grid surface, and growing through a solid-state diffusion process of O2- anions in a local electric field. Thickness measurements and monitoring of PbO growth by electrochemical impedance spectroscopy have demonstrated, by computation of the diffusion coefficient and the diffusion resistance of O2- anions, that antimony incorporated into the oxide acts as a doping element for its growth. At higher antimony levels, the two-phase alloys (lead matrix+ Sb precipitates) promote the formation of a very thin layer of a Sb-rich oxide inhibiting the PbO growth. (C) 2003 Elsevier Science B.V. All rights reserved.