An Investigation of the Inhibition of the Oxygen Reduction Reaction at a Rotating Cylinder Electrode under Isothermal and Controlled Conditions of Heat and Mass Transfer

Abstract

The inhibition of the oxygen reduction reaction on a carbon steel rotating cylinder electrode in naturally aerated 600 ppm Cl-solution was studied using an optimum inhibitor blend, i.e., Sodium Nitrite (SN): Sodium Hexametaphosphate (SHMP) = 500:100 obtained via a weight loss technique. Potentiostatic technique, then, was applied at different bulk temperatures and various flow rates using un-inhibited and inhibited solutions under isothermal and controlled conditions of heat transfer. In an un-inhibited solution and under isothermal conditions, with limiting conditions of concentration polarization, the limiting current density of oxygen reduction reaction is flow and temperature dependent. The charge transfer of the oxygen reduction reaction is a 4 electron process in the range of bulk temperature employed from 303 to 323 K. Under heat transfer conditions, the charge transfer is still 4 electron process up to 336 K interfacial temperature, above which the contribution of the 2 electron process appeared. Moreover, the limiting current density values of the oxygen reduction reaction in inhibited solutions is much lower than those under identical conditions in un-inhibited solutions. This confirms the inhibition of the cathodic reaction, i.e., the oxygen reduction reaction under isothermal and heat transfer conditions, due to SHMP.