The high temperature oxidation behaviour of 18Cr–8Ni austenitic steel has been studied in presence of Na
2SO
4 and transition metal salts, e.g. NiSO
4, CoSO
4, Cr
2(SO
4)
3, (NH
4)
2MoO
4, NaVO
3 or Na
2WO
4 in the temperature range of 923–1273 K in air.
The steels coated with a mixture of Na
2SO
4+NiSO
4 and Na
2SO
4+CoSO
4 show higher corrosion rates than either the Na
2SO
4 coated or transition metal sulphate coated steel at 923 K. This has been attributed to the formation of low temperature eutectics. At 1273 K, except the Cr
2(SO
4)
3 or CoSO
4 coated steel, the steel coated with all other salts, e.g. Na
2WO
4, NaVO
3 or (NH
4)
2MoO
4, have much higher corrosion rates than the Na
2SO
4 coated steel. The decomposition of these salts into volatile oxides, e.g. WO
3, V
2O
5 or MoO
3 seems to be the sole reason for catastrophic oxidation. A direct oxidation or sulphidation cum fluxing mechanism is adequate to explain hot corrosion. The scale morphology as predicted from mechanistic considerations is in fairly good agreement with the observed morphology.
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