Sulfidation Behavior of an Iron-Nickel Alloy

An alloy containing 41 weight percent Ni was reacted at temperatures of 793, 888, and 938 K with f lowing H2/H2S/N2 gas mixtures corresponding to equi l ibr ium sulfur partial pressures in the range from 2 • 10-5 to 6 • 10-1 Pa. At 793 K the reaction product consisted of an inner compact layer of (Fe, Ni)I_=S plus pentlandite overgrown by layers of pentlandite and Ni-rich sulfide whiskers. Morphologies at the higher temperatures were similar except hat pentlandite was absent. At all temperatures increases in the H2S partial pressure led to a more compact scale morphology. The scale phases are shown to be consistent with local equil ibrium, although the scale-gas interface is not exactly at equi l ibr ium because H2S dissociation is slow. Reaction kinetics were irregular at 793 K, due to scale-alloy separation, but parabolic at higher tem-peratures. Growth kinetics of the compact (Fe, Ni)I-xS layer were parabolic and are shown to be consistent with DFe/ DNi = 0.4 and highly nonideal defect levels. Austenit ic stainless steels are used for service at high and intermediate temperatures in environments contain-ing sulfur. The effect of chromium on the sulfidation be-havior of these steels is well known (1-4) but that of nickel is not so clearly defined. At high temperatures (T> 973 K