Hot corrosion of Fe, Ni and Co based alloys

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  

According to Shi, a reduction of sulphate ions occurs at the scale/melt interface a negative gradient of concentration of and thereby of 2Fe02 could be established across the melt. He also proposed further fluxing of Fe203 with 

5 Phase stability diagram for Na-V-S-0 system at 900°C (Hwang and Rapp, 1989). 

The kinetics of corrosion and the morphology of the scales formed on pure iron, manganese and chromium with Na2S04 deposits have been studied in the temperature range 600-800°C under 1 atmosphere of a gas mixture containing O2 (3.6%), SO2 (0.25%) and N2 (balance) by Nanni et al. (1987). At all the temperatures, salt-coated iron has been observed to exhibit accelerated attack whereas the corrosion rate of chromium was not appreciably affected by the deposited salt. Naimi et al. have further suggested that the enhanced corrosion phenomena are due to low melting liquid sulphate formation. 

High temperature corrosion of Inconel-600 tube used as a furnace accessory has been reported by Krishna and Sidhu (2001). The corrosion of the tube was found to be due to severe oxidizing and sulphidizing atmospheres generated by interdiffusion of base metal constituents and sulphur through the microchannels. 

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