The effects of the tin content and the polarisation time on the passivation of lead(calcium)-tin alloys in the deep discharge conditions of lead-acid batteries have been investigated at a potential of + 0.7 V versus Hg/Hg2SO4, K2SO4 saturated, in a 0.5M H2SO4 solution. Electrochemical techniques and metallographic analyses have revealed that the tin content in alloys must be controlled to avoid the formation of an electric barrier at the surface of the grid. For low tin levels in alloys (Sn < 1 wt.%), this passivation phenomenon is due to the formation of an insulating PbO layer, growing at a rate controlled by migration of O2- ions in a local electric field. When the solid solution contains more tin (1 < Sn < 1.5 wt.%), the PbO growth progressively decreases as a result of the precipitation of a conducting tin oxide in the PbO grain boundaries, then, the electronic conductivity of the oxide layer is enhanced. At higher tin levels, the alloy is two-phased (metallic tin precipitates in the Pb(Sn) matrix), which promotes the formation of a very thin and highly conducting oxide layer.