Corrosion of Fe-9Cr-(Mn, Si) alloys in CO2-H2O-SO2 gases

Model alloys, Fe-9Cr, Fe-9Cr-2Mn and Fe-9Cr-0.5Si (wt%) were exposed to Ar-20%CO2-20%H2O-(0, 0.1, 0.5 and 1.0%)SO2 gases at 650 °C. In the absence of SO2, all alloys grew thick iron-rich oxide scales. Adding SO2 led to partial passivation by chromia formation on Fe-9Cr and Fe-9Cr-0.5Si, but not the Fe-9Cr-2Mn. Alloy carburisation was extensive in sulphur-free gas, but substantially suppressed by SO2. Internal sulphidation and oxidation were also observed. A silica layer formed beneath the chromia scale on Fe-9Cr-0.5Si in SO2-bearing gas, but not in sulphur-free gas. Reaction morphologies are discussed in terms of oxide grain boundary diffusion, as affected by sulphur uptake.