Metal—sulphide eutectics

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. 

Kassner used a rotating disc, for which the hydrodynamic conditions are well defined, to study the dissolution kinetics of Type 304 stainless steel in liquid Bi-Sn eutectic. He established a temperature and velocity dependence of the dissolution rate that was consistent with liquid diffusion control with a transition to reaction control at 860 C when the speed of the disc was increased. The rotating disc technique has also been used to investigate the corrosion stability of both alloy and stainless steels in molten iron sulphide and a copper/65% calcium melt at 1220 C . The dissolution rate of the steels tested was two orders of magnitude higher in the molten sulphide than in the metal melt. 

Many of the transition-metal-sulphur systems show low-temperature eutectics, for instance Ni-Ni3S2 (645 °C), Fe-FeS (985 °C) and C0-C04S3 (880 °C). Hancock exposed ifickel to hydrogen sulphide at 900 °C and found extremely rapid corrosion involving the formation of the liquid phase, as confirmed by the observation of dendritic features. 

The formation of a liquid scale would be expected to give rise to the rapid attack that can be associated with rapid diffusion through the liquid. If the liquid is also a eutectic between the sulphide and the metal, then the reaction at the scale-metal interface involves a dissolution step. Such dissolution is capable of occurring more rapidly from some surface orientations than from others. In addition the dissolution is likely to be very rapid at the disordered areas of the grain boundaries. It is to... 

To understand the accelerated oxidation of metals and alloys under sulphate salt coating, wide ranges of corrosion kinetics and morphologies have been observed by Shi (1993) under controlled laboratory conditions, and he has proposed several kinds of mechanisms. The Na2S04-Na20 eutectic melt was reported to be responsible for the formation of the abundant sulphide in the scale and was believed to play an important role in the present form of accelerated corrosion. Moreover Fe203 can dissolve into the eutectic melt according to the reaction ... 

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