Nickel-based alloys sulphidation

Steels and stainless steels show preferential nucleation of pits at inclusions, most notably sulphide inclusions ". Other sulphur-rich regions in ferrous and nickel-based alloys may also lead to premature failure. It has been shown that accumulation of sulphur on the surface of these materials retards passivity and enhances dissolution of the metal. These effects occur in any solution in which the metal shows an active region and they are also preferential pitting sites in the presence of chloride. A recent notion for... 

Although iron sulphide also forms a eutectic with the metal this melts at 988°C, and at temperatures in the region of 700 to 800°C alloys with substantial proportions of nickel replaced by iron and a chromium level maintained at about 20% show advantages over nickel-chromium-base alloys in resistance to sulphur attack. 

Of course, if the protective scale of chromia or alumina is not penetrated by SO2, sulphide cannot form at the scale-metal interface. This was found for Ni-20 wt% Cr, Co-35 wt% Cr and Fe-35 wt% Cr alloys exposed to pure SO2 at 900 °C and emphasizes the resistance of a chromia scale to permeation. On the other hand, alloys in the Fe-Cr-Al, Ni-Cr-Al and Co-Cr-Al systems were exposed to atmospheres in the H2-H2S-H2O system. These atmospheres had compositions that supported the formation of chromia or alumina together with the sulphides of Fe, Ni and Co at the scale-metal interface. In these cases, a protective layer of chromia or alumina that formed initially was penetrated by sulphur to form iron, nickel, and cobalt sulphides at the scale-metal interface. Furthermore, iron, nickel, and cobalt ions apparently diffused through the oxide layer to form their sulphides on the outside of the protective scale. Thus the original protective scale was sandwiched between base-metal sulphides. 

Oxidation and hot corrosion in sulphate, chloride and vanadate environments of a cast nickel-based superalloy have been reported by Deb et al. (1996). Weight gain studies were carried out in air for uncoated samples and for samples coated with 100% Na2S04, 75% Na2S04 + 25% NaCl, and 60% Na2S04 -I- 30% NaV03 -i-10% NaCl. The presence of sulphur in the form of sulphates was reported to cause internal sulphidation of the alloy beneath the external oxide layer. Deb etal. observed the formation of volatile species by chlorides, which further led to formation of voids and pits at grain boundaries that reportedly provide an easy path for flow of corrodents. The presence of vanadate in conjunction with sulphate and chloride is proposed to provide... 

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