Effects of yttrium and zirconium additions on the high-temperature sulfidation behavior of an Fe?10Mo?20Al?8Mn alloy

The effects of zirconium and yttrium additions on the sulfidation behavior of an Fe–10Mo–20Al–8Mn(a/o, atom percent) alloy were examined in flowing H₂/H₂S gas of 4Pa sulfur partial pressure at 900°C. Good scale protection was obtained during the initial reaction stage of the base alloy. However, after 7–8 hr, the formation of internal (Mn,Fe) Al₂S₄ platelets triggered breakdown of the protective scale. The reaction products of the zirconium-containing alloy were nonprotective. Yttrium addition resulted in an Y(Fe_1–xAl_x)₁₂ network along the alloy ferrite grain boundaries. Preferential sulfidation of this phase led to almost complete manganese depletion from the engulfed ferrite, and consequently avoided the manganese-promoted scale breakdown.After an even slower initial stage, this alloy sulfidized at a parabolic rate two orders of magnitude slower than that of pure iron. The protection during the initial and following stages was believed to be provided by an Al₂O₃-containing layer and an Al_0.55Mo₂S₄+Fe_xMo₆S_8–z layer, respectively. The formation of Al₂O₃ is thought to be due to oxygen impurities in the H₂S gas, which cannot be removed by conventional means.