Sulphidation attack of the heat transfer surfaces in boilers: Materials performance and views on future prospects

Sulphidation attack of the heat transfer surfaces in boilers may be due to the presence of reduced sulphur compounds in gaseous phase, or due to the result of adverse deposit conditions. Under reducing conditions in the furnace, the safe upper temperature limit of heat transfer surfaces made of low alloyed steels may be no more than about 300-330oC. The use of austenitic stainless steels or high chromium - high nickel alloys may allow metal temperatures that are 50 to 80 degrees or more higher. However, deposition of alkali and heavy metal salts may create corrosion conditions where protective chromium oxide or chromium sulphide scales fail. Materials design for the mixed fuel or high-pressure boilers must be considered carefully. High wastage rates of heat exchangers made of highly alloyed metallic materials or claddings may not be feasible for economic and environmental reasons. New developments in the manufacturing of austenitic stainless steels, however, may provide economic solutions for the superheater steel tubes. Corrosion resistant thermal spray coatings may fail due to their porous or discontinuous structure, or due to the thermal mismatch with the substrate. Prospective technologies based on the application of nanosized raw materials aiming to manufacture of gas-tight, temperature cycling resistant coatings are in the development stage.