Corrosion of high alloy steels and Fe-Cr-alloys beneath deposits from waste incinerator plants

The corrosion of high alloy steels in N-2-O-2-HCl-SO2 and of Fe-Cr-alloys in N-2-O-2-HCl was investigated in exposures beneath different kinds of deposits from waste incineration plants at 600 degrees C. In both kinds of deposits the formation of a ph and Zn containing sulfate melt was observed. The main corrosion process of high alloy steels is 'hot corrosion' by a sulfate melt. In N-2-O-2 basic fluxing of the oxide scales occurs by formation of PbCrO4 on Cr2O3 forming alloys and zinc-rich spinel on FeCr2O4 forming materials. In N-2-O-2-SO2 acidic fluxing was observed, whereas Cr2O3-scales are resistent. In N-2-O-2-HCl severe corrosion. occurs due to the formation of a sulfate-chloride eutectic by reaction of HCl with sulfates in the deposits to (K,Na)Cl. The (K,Na)CI reacts with the oxide scales to form Cl-2, which initiates 'active oxidation' by metal-chloride formation at the metal\scale interface. Addition of SO2 leads to retardation of the corrosion process, because the sulfates are stabilized by SO2 and acidic fluxing was observed. The Fe-Cr alloys show active oxidation due to the formation of ZnCl2 and PbCl2 in the deposits and also basic fluxing by ZnO.