Oxidation-sulfidation behavior of nickel-base superalloys and MCrAl coatings

Abstract

The oxidation-sulfidation behavior of several nickel-base superalloys and a number of MCrAl coatings on an IN-738 substrate has been examined after exposure to ten different O 2S 2 mixed gas environments. Selected alloys have been tested in a thermogravimetric system in which the mass change of the specimens was continuously monitored. Extensive metallurgical analyses of the exposed specimens were performed and the results were used to develop generalized maps that depict the corrosion product morphologies as a function of the exposure environment. The results, in general, showed that nickel-base superalloys are susceptible to sulfidation attack and undergo breakaway corrosion in low O 2 environments that contained sulfur. Of several MCrAl coatings that were tested, CoCrAlY and NiCoCrAlY compositions showed superior sulfidation resistance over a wide range of S 2 partial pressures at the test temperatures of 1144 and 923 K. Other MCrAl coatings examined in this study showed promise at low to intermediate S 2 partial pressures.