Doctor of Philosophy
David W. Shoesmith
Ni-Cr-Mo alloys exhibit exceptional corrosion resistance and are widely used in chemical processing industries. The reliable performance of these alloys under extreme industrial conditions, are generally attributed to the presence of a passive film on the alloy surface. O2 reduction is the most likely cathodic process able to sustain metal oxidation (corrosion) in industrial environments. However, the kinetics of O2 reduction on these oxide-covered alloys has hardly been studied, despite the possibility it may be the rate controlling process for corrosion, especially localized corrosion. The objectives of this research are to characterize the properties of the oxide film on Ni-Cr-Mo alloys, and to investigate whether O2 reduction occurs on oxide-covered surfaces, and, if it does, the conditions under which it occurs and the mechanism of the reaction.
Various surface analytical techniques such as angle-resolved X-ray photoelectron spectroscopy (XPS), synchrotron radiation XPS (SR-XPS), time-of-flight secondary ion mass spectrometry (ToF-SIMS), and SEM are employed to characterize the properties of oxide film on Ni-Cr-Mo alloys as a function of applied potential, temperature and pH. The presence of a layered structure in the passive film (< 5 nm) has been demonstrated on these alloys, with the outermost surface being enriched in Cu (or Cu oxide) and Mo oxide, the intermediate region dominated by Cr/Ni/Cu hydroxides, and the inner region comprising Cr/Ni oxide.
A number of electrochemical techniques are used to measure the reduction of O2 on a series of Ni-Cr-Mo alloys (Alloy C22, C2000, C276, C4 and 625) in 5M NaCl solution over the temperature range of 30-90°C. Both potentiostatic and cyclic voltammetric (CV) experiments demonstrate that O2 reduction currents on these alloys are either very small or completely suppressed by the presence of a film grown in the passive region, but revived to different degrees when the film-growth potential was extended into the transpassive region. The impedance properties obtained from EIS measurements show that the polarization resistance reached a maximum in the passive region, and decreased with the increase of film-growth potential in the transpassive region, due to the oxidative injection of cation defects.
Keywords: Ni-Cr-Mo alloys, passive film properties, kinetics, O2 reduction.
Zhang, Xiangrong (Sarah), "Kinetics of O2 Reduction on Oxide-covered Ni-Cr-Mo Alloys" (2012). Electronic Thesis and Dissertation Repository. 526.