Corrosion of High-Alloy Superheater Tubes in a Coastal Biomass Power Boiler

Metallurgical examinations were conducted on a set of damaged high-alloy (Type 31 OH [UNS S31009] stainless steel and Alloy 625 [UNS N66250] weld overlay) superheater tubes removed from a coastal fluidized bed biomass power boiler, to better understand the degradation mode. Active oxidation (gas-solid reaction occurring underneath the chloride-containing fireside deposit) was found to be the mechanism responsible for the significant loss in tube wall thickness observed on damaged superheater tubes. There exists a complex, poorly - understood synergy between the critical factors that affect active oxidation, primarily tube temperature and the supply of reactants in the flue gas, namely, sodium chloride (NaCl). sulfur dioxide (SO2), and water vapor (H2O). Overall ranking terms of increasing corrosion resistance to active oxidationfollows the expected trend: 722 < SS31 OH < A625WO. Despite the favorable ranking, the Alloy 625 weld overlay can corrode by active oxidation at an appreciable rate (0. 45 mm/y [18 mpy]) under existing boiler operating conditions. A promising corrosion control strategy involves reducing the steam temperatures in the outlet tubes of the three superheater sections to below 500 degrees C.