Effect of ZnS/PbS deposits on high temperature corrosion of waterwall tubes in reducing atmosphere

Abstract

Zn- and Pb-rich deposits have been observed on the waterwall at fireside in three tangentially coal-fired boilers. This work aims to clarify the influence of ZnS or PbS deposits on sulfide corrosion behavior with the help of field and corrosion testing analyses. The characterization of field samples shows that the enrichment of various metal elements (Pb, Zn, Ga, Se, etc.) in the corroded layer is mainly attributed to vaporization-condensation of metal vapors, and gaseous migration of metal elements remained in unburnt char. In laboratory 12Cr1MoVG specimens were exposed to ZnS or PbS deposits in a mixed gas of 0.1%H2S-0.1%O2–5%CO-N2 at 400 or 500 °C for 120 h. The results show that pyrite is identified only at 400 °C while marcasite appeared at 500 °C needs a longer exposure time. Iron can be oxidized in the regions in which PbOx is stable, while in the regions where PbS is stable iron can only be sulfided. PbS and ZnS deposits could decrease the corrosion rate of specimens. But the capacity of PbS for hindering corrosion attacks is slightly weaker than ZnS, due to the generation of Pb(g) and more active S migrating into corrosion side.