The kinetics of the oxygen reduction reaction on steel in alkaline solution

Oxygen electroreduction on steel is considered the major electrochemical process on steel under cathodic protection (CP) in air. The kinetics of O ₂ reduction on steel in aqueous alkaline (pH = 14) electrolyte was examined to probe the electrochemistry of steel during CP. The cyclic potential ring measurement (CPRM) rotating ring disk electrode (RRDE) method was used to study the O ₂ reduction on steel, because the CPRM RRDE method permitted quantitative detection of the peroxide formed during O ₂ electroreduction. Two regimes of O ₂ reduction on steel were observed. The first regime, between -0.2 to -0.9 V versus SCE (saturated calomel electrode), was accompanied by virtually 100% electrochemical conversion of O ₂ to peroxide. The second regime, between -0.9 to -1.3 volt versus SCE, corresponded mainly to hydroxide formation. O ₂ reduction and peroxide oxidation currents were well behaved - that is, increased linearly with the inverse square root of all rotations rates (400 to 3600 rpm) - so a mechanistic analysis of the ring and disk data could be done to determine the potential dependent rate constants for O ₂ reduction on steel. A series mechanism was found operative. Oxygen electroreduction was found to proceed by two sequential 2e ^- steps, first yielding peroxide and then hydroxide. This study shows peroxide generation is expected on steel in air at potentials as highly negative as -1 V versus SCE at pH = 14, so coating disbonding models should consider the effects of peroxide on coated pipelines under cathodic polarization.