Fireside corrosion and ferritic/austenitic steels and high-alloy materials for superheaters,especially reheaters of coal-fired steam generators

In the past the performance of superheater and reheater tubes in coal-fired systems manufactured of ferritic and austenitic steels with respect to high-temperature corrosion has been the subject of various studies. After having adopted the steel X20 CrMo V 12 1 in the sixties the assumption was to have found a high-temperature corrosion resistant material. But already after 60,000 operating hours first tube failures occurred. As a result of the changes in fuel application at the beginning of the sixties interest in solving this problem faded, only to regain significance after the energy crisis. Already more than 20 years ago in England and the United States of America the development of tube materials having an upgraded corrosion resistance had started. Thus, e.g. compound tubes consisting of a creep-resistant tube core made of Incoloy 800 H and a cladding tube of Inconel 671 were introduced. They exhibited excellent characteristics in coal-fired units. Deutsche Babcock AG suggested to install, for test purposes, such a tube in Boiler 14, Unit 3 of Grosskraftwerk Mannheim. The test tube had safe ends of X 20 CrMO V 12 1, thus allowing a direct comparison of the two tube materials. After 22, 000 operating hours the tube was removed for examination. Wall reduction and corrosion damage was found to be extremely small. The paper will discuss this finding and the microstructure modifications in the material.