The corrosion of Ni3Al in a combustion gas with and without Na2SO4-NaCl deposits at 600?800�C

Abstract

The corrosion behavior of Ni3Al containing small additions of Ti, Zr, and B in combustion gases both with and without Na2SO4−NaCl deposits at 600–800°C has been studied for times up to four days. The corrosion of the saltfree Ni3Al leads to the formation of very thin alumina scales at 600°C but of mixed NiO−Al2O3 scales containing also some sulfur compounds at higher temperatures, while the rate increases with temperature up to 800°C. The presence of the salt deposits considerably accelerates the corrosion rate, especially at 600 and 800°C. The duplex scales formed at 600°C are composed mostly of a mixture of NiO and unreacted salt in the outer layer and of alumina and aluminum sulfide with some nickel compounds in the inner layer. The scales grown at 700°C contain only one layer of complex composition, while those grown at 800°C are similar but have an additional outer layer containing similar amounts of nickel and aluminum. At 600 and 700°C NiSO4 can be detected also in the salt layer. The samples corroded at 700°C and 800°C also show an Al-depleted zone containing titanium sulfide precipitates at the surface of the alloy. The hot corrosion of Ni3Al involves a combination of various mechanisms, including fluxing of the oxide scale as well as mixed oxidation-sulfidation attack. At all temperatures Ni3Al shows poor resistance to hotcorrosion attack as a result of the formation of large amounts of Ni compounds in the scales.