The electro-oxidation/electro-reduction processes on the surface of a carbon steel electrode were studied in borate buffer solutions (pH=8.4) in the presence and absence of the corrosion inhibitor phosphor-polymaleic acid (PPMA)-ZnSO4 at room temperature by using the voltammetry-photocurrent response method. For the carbon steel electrode in solutions containing the corrosion inhibitor, its voltammograms displayed a smaller oxidation peak current and a larger photocurrent response compared to those in the reference solution. Further Auger electron spectroscopy (AES) analysis indicated that the passive films formed on the electrode consisted of ferric oxides and precipitation containing the corrosion inhibitor, which effect through blocking the metal surface, thereby, preventing the metal from further corrosion or dissolution.