Lab-scale investigations of high-temperature corrosion phenomena in straw-fired boilers

The corrosion of superheater tube material in straw-fired boilers was inves tigated in the laboratory. Metal test elements of boiler tube steel (X20CRM V121 and AISI 347FG) were covered with synthetic (KCl and/or K2SO4) and rea l deposits, and exposed to a synthetic flue gas (6 vol % O-2, 12 vol % CO2, 400 ppmv HCl, 60 ppmv SO2, balance N-2) in 550 degrees C electrically heat ed ovens. Exposure times from 1 week to 5 months were used. The corrosion o f the metal test elements was, in general, quite uniform, and the corrosion products consisted mainly of oxides of iron and chromium. All test element s covered with KCl suffered from minor internal attack, and some elements h ad severe pits with chlorine found in the pit. A dense layer of potassium s ulfate and iron oxide was found adjacent to the metal oxide layers on all t he metal test elements covered with a deposit containing KCl. The layer had a characteristic structure, with iron oxide threads in a dense potassium s ulfate matrix. A mechanism for chlorine corrosion is suggested. The mechani sm is based on gaseous chlorine attack coupled with a fast sulfation of KCl to K2SO4 in a melt of KCl, K2SO4, and iron compounds formed adjacent to th e metal. Aspects of the sulfation of potassium chloride to potassium sulfat e are discussed in the paper.