The Kinetics of Cathodic Oxygen Reduction on Thin Films on Ni-Cr-Mo (W) Alloys

Abstract

The kinetics of cathodic oxygen reduction is important to the evolution of crevice corrosion of Ni-Cr-Mo (W) alloys in high temperature brines. Various electrochemical and surface analytical techniques are being employed to investigate these kinetics on oxide-covered Alloy 22 surfaces and the film properties. Potential step experiments demonstrate that steady state currents depend on temperature and applied potential. The oxygen reduction currents were significantly suppressed by the growth of a passive film. Cyclic voltammetric experiments were conducted on surfaces pre-oxidized at different potentials throughout the passive region (-0.6 V to 0.6 V vs. Ag/AgCl) and temperatures (30˚C - 90˚C) in 5 mol L^-1 NaCl solution. The data demonstrate that the kinetics of oxygen reduction depend on both temperature and pre-oxidation potential. Oxygen reduction currents are strongly suppressed in the passive region, but revived as the potential approaches the transpassive region (> 0.4V). With increasing temperature, the passive current increases suggesting a decrease in film resistance. TOF-SIMS depth profiles show a two-layer structure for the oxide film, with an inner region enriched in Cr₂O₃, NiO, MoO₂, WO₂, and an outer region of Cr(OH)₃, Ni(OH)₂, MoO₃, and WO₃. The thickness of the film increases with applied potential. EIS measurements show the film resistance reaches a maximum value in the passive region.

This research is supported by the Science & Technology Program of the Office of the Chief Scientist, Office of Civilian Radioactive Waste Management, U.S.Department of Energy. The work was performed under the Corrosion and Materials Performance Cooperative. DOE Cooperative Agreement Number: DE-FC28-04RW12252.