Passivation phenomenon of low antimony alloys in deep discharge conditions of lead–acid batteries

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 ∣ Hg2SO4 ∣ 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.