Corrosion-resistant NiCrMo alloys in hot concentrated sulphuric acid with active carbon

Dewpoint corrosion by sulphuric acid has often been observed in the heat exchangers of coal-burning plants. If ashes contain active carbon, corrosion becomes particularly serious. Various nickel-base alloys with high corrosion resistance in such environments have been designed. The corrosion behaviour of these alloys was examined in hot concentrated sulphuric acid with and without active carbon. The corrosion resistance of ternary NiCiMo alloys in the acids without active carbon increases with increasing alloy molybdenum content. When active carbon is added to the acids, the corrosion rate increases significantly. This is due to acceleration of transpassive dissolution of molybdenum as a result of enhancement of cathodic activity with the addition of active carbon. The corrosion resistance in the acids with active carbon is enhanced by an increase in alloy chromium content, which improves the passivity in the acids.