Environmental Degradation of Thermowells in a Coastal Fluidized Bed Biomass Power Boiler

A metallographic examination was conducted on a set of degraded alloy thermowells, which were installed in the fluidized bed of a coastal fluidized bed biomass (waste-wood) power boiler. A better understanding of the thermowell environmental degradation mechanism was required to rationalize the observed relative materials performance, given the importance of the thermocouples in the fluidized bed temperature control strategy. Based on the set of observed features, a chlorine-associated corrosion involving a molten salt was implicated as the likely damage mechanism. The relative corrosion resistance ranking order of Alloy 600 > Alloy 625 > Type 310H stainless steel > Type 446 stainless steel is consistent with the reported influence of nickel and chromium alloying on the corrosion resistance to molten chloride-containing salts.