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

Metallurgical examinations were conducted on a set of damaged high-alloy (Type 310H [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 fi reside 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 fluegas, namely, sodium chloride (NaCl), sulfur dioxide (SO₂), and water vapor (H₂O). Overall ranking in terms of increasing corrosion resistance to active oxidation follows the expected trend: T22 < SS310H < 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°C.