Hot corrosion behaviour of 18Cr-8Ni Austenitic Steel in presence of Na2S04 and transition metal salts

Abstract

The high temperature oxidation behaviour of 18Cr-8Ni austenitic steel has been studied in presence of Na2S04 and transition metal salts, e.g. NiS04, CoS04, Cr2(S04)3, (NH4)2Mo04, NaV03 or Na2W04 in the temperature range of 923-1273 K in air. The steels coated with a mixture of Na2S04+NiS04 and Na2S04+CoS04 show higher corrosion rates than either the Na2S04 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 Cr2(S04)3 or CoS04 coated steel, the steel coated with all other salts, e.g. Na2W04, NaV03 or (NH4)2Mo04, have much higher corrosion rates than the Na2S04 coated steel. The decomposition of these salts into volatile oxides, e.g. W03, V2Os or Mo03 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.