The Role of Oxygen Reduction Reaction in Determining Pit Stability of FeCr Alloys

Abstract

Pitting corrosion is one of the most dangerous forms of corrosion, leading to sudden catastrophic failures in engineering system. Once initiated, pit propagation rates are largely dependent on the magnitude of the supporting cathodic current available from the oxygen reduction reaction (ORR) occurring on the external passive film. Here the ORR kinetics on conventional and high Cr content FeCr alloys made by arc-melter were investigated by experimental and computational methods. DFT calculations suggested that the overpotentials for the ORR on FeCr oxides are in the order Cr2O3 > Fe3O4 > Fe2O3 > FeCr2O4, which was experimentally confirmed on the FeCr alloys by rotating disc electrode experiments in both O2 saturated 0.1 M NaOH and 3.5 wt% NaCl. Koutecký-Levich analysis demonstrated that ORR follows a 4-electron pathway on all surfaces investigated. Moreover, the investigation of the Pourbaix diagrams of FeCr revealed that at potentials where pitting corrosion initiates the main constituents of the passive films should be a mixture of Fe2O3 and Cr2O3, rather than FeCr2O4, which is proved to be a good ORR catalyst. The results support the postulation that one role Cr additions play in the prevention of pitting corrosion is to suppress the ORR reaction.