Corrosion of superheater materials in a waste-to-energy plant

A major drawback when generating electricity from waste-fired boilers is the rapid corrosion of critical components such as superheater tubes. In this work a number of commonly-used superheater materials have been exposed on internally cooled probes in a waste-fired grate boiler. The investigated materials are the ferritic steel 13CrMo44, the ferritic–martensitic steel HCM12A, the austenitic steels Super 304, 317L and Sanicro 28, and the nickel-base alloys Hastelloy C-2000 and Inconel 625. Short-term exposures (3 h) for analysis of deposit composition and initial corrosion, as well as long-term exposures (1550 h) to investigate corrosion rates and corrosion characteristics have been made.Analysis revealed a deposit dominated by CaSO4, KCl and NaCl, but also appreciable amounts of low melting salt mixtures such as ZnCl2–KCl, PbCl2–KCl, FeCl2–KCl and NaCl–NiCl2. Metal loss measurements showed unacceptably high corrosion rates for 13CrMo44, HCM12A and Super 304. The corrosion attack for these alloys was manifested by the formation of mixed metal chloride/metal oxide scales. A different type of behaviour was seen for the higher alloyed austenitic steels and nickel-base alloys, which were able to form a chromium-enriched oxide next to the metal. However, these alloys suffered from some localised pitting attack. The behaviour is explained by oxide dissolution in the molten salts that are present in the deposit.

► Superheater corrosion limits electric and economic efficiency of waste-fired power plants.
► Alloying level changes corrosion rates and type of corrosion attack.
► Oxide fluxing limits the use of high alloyed austenitic steels and nickelbase alloys.