The effect of Sn concentration on the corrosion resistance of Pb-Sn alloy in H2SO4 electrolyte was investigated by potentiodynamic polarization. A new approach to calculate the exchange current density, i corr, was proposed and proved to be in accordance with the experimental results. This study shows that with the increase of alloying Sn, the corrosion rate of the alloy increased and then decreased, with its minimum appearing at 2.60wt.% Sn. It was also found that the cathodic reaction does not have to be a single-step process, i.e., there are multiple sub-steps involved and complex cations, including H4 3+ and H2 +, existing in the systems.