The furnace design is every bit as important as the material that it?s made from. Stainless steel may last longer than mild steel in an outdoor furnace, but not all stainless is corrosion resistant there are many grades and some of them are subject to rusting and corrosion. Automobile exhaust systems are made from one of the lower grades; they resist high temperatures but totally corrode. If you do purchase one of the lower grade stainless steel furnaces, be sure that the same model made in steel has a proven track record of corrosion resistance.