High temperature corrosion via chlorine is a key factor in the degradation of boiler tubes in waste-to-energy (WTE) plants. Corrosion rates are particularly high in the superheater where material temperatures may exceed 450°C and where carbon or low alloy steels may be used. Although increasing sulfur, in the form of SO2, in WTE flue gas has been shown in previous works to have potential for decreasing the corrosion of these materials, the inhibitive effect is not well understood.
This work investigated the corrosion of SA178A, a low carbon steel alloy (0.07 wt% C), and NSSER-4, a stainless steel (17.3Cr-13.1Ni-2.5 Si-Fe), via exposure under various well-defined environments, SO2:HCl ratios between 1:8 to 2:1 (HCl fixed at 800 ppm), 8% O2, 12% CO2, 0 and 15% H2O, N2 (balance) at 500°C in a horizontal tube furnace for 50 hours. Additional coupon testing was performed on NSSER-4 after application of 4 mg/cm2 ± 10% NaCl or Na2SO4 at 500 and 700°C for 24 hours to assess the impact of higher SO2 in the against both deposit and gaseous corrosion. Specimen preparation and corrosion assessment followed ASTM method G1-03.
Experiments demonstrated little to no trends in corrosion rates at SO2:HCl ratios between 1:8–2:1 under mixed gas environment. However corrosion reduction was observed when SO2:HCl was increased from a reference condition of 1:8 to greater than 1.4:1 in tests with NaCl present, which was also not observed under dry conditions. These results suggest that one possible explanation for the reduction of boiler materials corrosion rate with higher concentrations of SO2 may be largely attributed to the conversion of metal chlorides to sulfates.