Investigation of high and low temperature corrosion behaviour of nickel-base alloys

In this study, both low (T<1000oC) and high temperature (T>1000oC) oxidation behavior of two commercial Ni alloys containing more than 10% Mo, a refractory metal, is investigated. For this purpose, Hastelloy C-22 ve C-276 alloy samples were exposed to the stagnant air environment of a high temperature furnace for predetermined times. Extent of alloy oxidation was determined from sample mass change measurements while morphologies and chemical compositions of the product oxide scales were investigated with a scanning electron microscope (SEM) equipped with a backscattered electron (BE) detector and an energy dispersive spectroscopy (EDS) detector. A x-ray diffractometer (XRD) was also used to identify the crystalline phases in the scales. Important findings of this study can be summarized as follows: Significant mass changes was observed only at temperatures above 1000oC. Although oxide scales with similar microstructures and chemical compositions formed over both alloy samples, the C-276 alloy sample with the higher Mo content had larger amount of scale spallation. Furthermore, Mo-rich areas were found at scale locations where spallation took place. Detection of Mo over the inner surfaces of the ceramic crucible containing the C-276 sample indicated that Mo was preferentially removed, by evaporation, from the alloy sample during the high temperature tests. This observation suggested that Mo in the alloy affected scale spallation. These results clearly showed that presence of a large amount of refractory elements in an alloy can decrease its oxidation resistance at high temperatures.