Potential step was applied to Pb–Ca–Sn alloy electrode at various potential and time regimes. No severe corrosion was observed during potential step cycle with cathodic potential under −140mV or over −40mV versus Pb/PbSO4 (3.39M H2SO4), or at constant potential without stepping. On the other hand, the Pb–Ca–Sn alloy was severely corroded during potential step with cathodic potential from −120mV to −60mV and with anodic potential of +40mV or more positive. The corrosion could not be decreased with periodical rest at 0mV, while it could be decreased with periodical reduction at high polarization of, e.g. −160mV. It was found out that the severe corrosion occurs when the oxidation of Pb to PbSO4 and partial reduction of passive film of PbSO4 take turns many times.