Negative plate lugs of flooded lead-acid battery were corroded during partial state-of-charge (PSoC) pattern cycle life tests simulated from stop and go vehicle driving. Potential step was applied to Pb–Ca–Sn alloy electrode at various potential and time regimes, and the electrode surface was observed by in situ electrochemical atomic force microscope (EC-AFM) to investigate the corrosion mechanisms during the potential step cycles. It was found out that the severe corrosion occurs when the oxidation of Pb to PbSO4 and partial reduction of passive layer of PbSO4 take turns many times. It was also found out that the periodic full charge, the optimization of the alloy composition, addition of the material that may make the reaction mechanism change to electrolyte were effective to suppress the corrosion rate.