High chromium grades of austenitic stainless steels (e.g., 309 and 310) are used in oxidizing environments and high temperatures. According to ASM Specialty Handbook on Stainless Steels, it is difficult to generalize corrosion rates in flue and process gas environments because gas composition and temperature may vary considerably within the same process unit. Combustion gases normally contain sulfur compounds; sulfur dioxide is present as an oxidizing gas along with carbon dioxide, nitrogen, carbon monoxide, and excess oxygen. The resistance of stainless steels to normal combustion gases increases as the chromium content increases. SS 309 has one of the highest chromium contents (22-24%) in the austenitic family and has been successfully used in combustion gas environments. Whether 309 will work in your case, depends on the specifics of the environment. As an aside, corrosion allowances are not generally useful for austenitic stainless steels since they tend to experience localized corrosion.