Intergranular Corrosion Mechanism of SCH 13 Heat Resistant Steel in Waste Incineration Environment

High temperature intergranular corrosion of a SCH 13 heat resistant steel in the waste incinerator, where plastics and used tires were burned under thermal cycling condition as at 1423K after 6.2ks and at 1150K for 17ks then cooled down to room temperature, was investigated by means of scanning electron microscope, electron probe micro-analysis and X-ray diffraction analysis. The network of intergranular corrosion within the alloy beneath the outer scale were identified. Si and Cr oxides as well as Cr and Mn sulfides were observed in the center of corroded area that was surrounded by Fe and Ni concentrated zone where Cr, Si, Mn were depleted. The sulfur partial pressure calculated from composition of combustion gases was smaller than the dissolution pressure of sulfides of alloy element in SCH 13, it was supposed that Cr and Mn oxides that were near the surface and Cr, Mn sulfides that were at the inside of alloy were produced by a reaction between Cr, Mn and SO2, O2 gases. At the initial stage of intergranular corrosion, Cr-Fe carbide channel that developed on grain boundary by repeated thermal cycle in waste incineration furnace was changed to oxide and then SiO2 and voids were formed. Cr, Mn oxide and sulfide were formed by a reaction between Cr, Mn and SO2, O2 gases that were introduced by means of voids from gas side. It is concluded that the Si, Cr, Mn depletion zone is caused by the formation of Si, Cr, Mn oxides and Cr, Mn sulfides, then Fe and Ni are relatively enriched on this depletion zone.