Corrosion resistance of surface-modified T22 steel intended for use as superheater tubes in waste incinerators was examined by laboratory and field corrosion tests. An alloy of 40%Ni-30%Cr-20%Mo-10%Nb (wt.%, referred to alloy 4321) was used as a coating and applied to T22 steel by either HVOF metal spray or weld overlay. A thin film of alloy 4321 approximately 10gm thick, generated by unbalanced-magnetron (UBM) sputtering of target metals and subsequently ion-plated on either a metal
spray or weld overlay coating of alloy 625 was prepared and tested as well. According to a laboratory corrosion test conducted by coating the samples with a synthetic ash and exposure to gas mixture of
1000ppmHCl-50ppmSO2-10%O2-10%CO2- 20%H2O-bal.N2 at 550°C for 100h, alloy 4321 coatings showed considerably better corrosion resistance than alloy 625. However, field corrosion testing in a stoker type furnace exposed at 500-550°C for 3100 h showed the corrosion resistance of alloy 4321 coatings was similar or even worse than alloy 625. Thus, Nb additions between 3.5% to 10% and Mo additions between 9% to 20% to Ni-base alloys are considered not effective in combating corrosion in the present environments.
Keywords: high temperature corrosion, waste incinerator, fused salt, hot corrosion, fireside corrosion, superheater tube, surface modification, ion plating.