Abstract
Nickel-based industrial alloy samples were pre-oxidized at different temperatures (540/760/ 980 °C) in pure or diluted (0.5% O2 in Ar) oxygen, followed by exposure to a high carbon activity gaseous mixture (10% CO in Ar) at 550 °C, in order to reveal factors critical to the initial carbon-formation preceding metal dusting corrosion. Fresh and treated samples were studied via optical-microscopy, EPMA, SEM/EDS, and depth profile analysis by Auger electron spectroscopy under Ar-ion sputtering. The resulting oxide layers strongly differ in thickness, ranging from 1 μm for the highest to less than 30 nm for the lowest temperature. Systematic compositional variations were also found, with the high oxidation temperature being associated with chromium saturation and nickel/iron depletion of the oxide, as well as aluminium enrichment at the oxide-bulk metal transition. Increasing oxidation temperature was found to result in a surface oxide layer with better resistance to carbon formation, irrespective of the oxygen concentration of the pretreatment, although composition plays a role under otherwise equal conditions. The carbon formation appears to be associated with inclusion of nickel and iron species in the oxide layer, which subsequently reduce to promote the kinetics of the process.
