Sulfidation behavior of Al-modified MP35N alloys

Abstract

A commercial superalloy, MP35N, which was modified by adding various amounts of Al up to 10 wt.%, has been studied over the temperature range of 600–900°C in 0.01 atm sulfur vapor. The Al-modified alloys contained a second phase whose amount increased with increasing Al. The sulfidation rate followed the parabolic rate law and decreased with increasing Al content. An activation energy of 46 kcal/mol was obtained for MP35N, MP35N-5Al, and MP35N-7Al, while a value of 56 kcal/mol was found for MP35N-10Al at 700–800°C. Triplex sulfide scales formed on MP35N, consisting of an outer layer of solid solution (Co, Ni) ₃S₄,an intermediate layer of Cr ₂S₃,and a complex, heterophasic inner layer of mostly MoS ₂.The Al-modified alloys also formed an outer scale of (Co, Ni) ₃S₄,and an inner layer containing a mixture of CoCr ₂S₄ and Al ₂S₃.An internal-sulfidation zone containing an Al-rich sulfide was formed in the substrate. Only a moderate beneficial effect of Al additions to MP35N was observed at 600°C. The observation of the location of platinum markers suggests that cobalt, nickel, chromium, and aluminum diffuse outward to form the major part of scale, and sulfur diffuses inward to produce an internal-sulfidation zone. Transport processes in the internal-sulfidation zone governed the sulfidation rate of the alloys.