Conjugated poly(3,4-ethylenedioxythiophene) (PEDOT) film was electrochemically synthesized on stainless steel (SS). Redox interactions between the PEDOT film and the SS substrate were examined in 3.5 wt.% NaCl aqueous solution with the aid of electrochemical and spectroscopic analyses. The results show that the PEDOT film exhibited a barrier effect and mediated the oxygen reduction reaction, thus hindering ion diffusion to the steel substrate. Localized electrochemical impedance spectroscopy (LEIS) of the scratched area on the polymer film shows that PEDOT healed the defect by coupling with redox reactions on the steel surface to prevent charge localization and concentration. The electroactivity of the polymer film declined when PEDOT was polarized at potentials >−0.7 V. Prolonged exposure of the PEDOT film to dissolved oxygen in NaCl solution resulted in the polymer’s over-oxidation (degradation), evidenced by the formation of a carbonyl group in the spectroscopic result. The degradation of PEDOT was attributed to chain scissoring due to hydroxide ion attacks on the polymer chain.