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

摘要

The corrosion of high alloy steels in N{sub}2-O{sub}2-HCl-SO{sub}2 and of Fe-Cr-alloys in N{sub}2-O{sub}2-HCl was investigated in exposures beneath different kinds of deposits from waste incineration plants at 600°C. In both kinds of deposits the formation of a Pb and Zn containing sulfate melt was observed. The main corrosion process of high alloy steels is 'hot corrosion' by a sulfate melt. In N{sub}2-O{sub}2 basic fluxing of the oxide scales occurs by formation of PbCrO{sub}4 on Cr{sub}2O{sub}3 forming alloys and zinc-rich spinel on FeCr{sub}2O{sub}4 forming materials. In N{sub}2-O{sub}2-SO{sub}2 acidic fluxing was observed, whereas Cr{sub}2O{sub}3-scales are resistent. In N{sub}2-O{sub}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)Cl reacts with the oxide scales to form Cl{sub}2, which initiates 'active oxidation' by metal-chloride formation at the metalscale interface. Addition of SO{sub}2 leads to retardation of the corrosion process, because the sulfates are stabilized by SO{sub}2 and acidic fluxing was observed. The Fe-Cr alloys show active oxidation due to the formation of ZnCl{sub}2 and PbCl{sub}2 in the deposits and also basic fluxing by ZnO.