Determination of the Initiation and Propagation Mechanism of Fireside Corrosion

A variety of deposit compositions were examined in short-term laboratory tests with the aim of determining the corrosion mechanisms of fireside corrosion for a range of chromia-forming alloys in various combustion systems. The deposits formed in boilers are complex, and despite decades of study, the propagation mechanism of fireside corrosion is not well understood. Alkali iron trisulfates, which are stabilized by SO3 in the gas atmosphere, have been believed to be the major corrosive species for many years. The propagation mechanism for fireside corrosion was investigated using T92 (a typical ferritic boiler steel) and a model austenitic Fe-Ni-Cr alloy and synthetic coal ash deposits. The metal loss, corrosion product morphologies, and compositions were carefully characterized in order to define a propagation mechanism. The corrosive species responsible for degradation was a (Na,K)2SO4-Fe2(SO4)3 solution and not alkali iron trisulfates, which is contrary to what has been believed for decades. The formation of the liquid deposit is similar to Type II hot corrosion of gas turbine engines. The mechanism is a synergistic dissolution process similar to that which can occur for hot corrosion as well. Simultaneous basic and acidic dissolution of protective Cr2O3 and Fe2O3 disrupts protective oxide formation and locally produces negative solubility gradients at the oxide/salt interface. The dissolved Fe2O3 and Cr2O3 reprecipitate where there is lower solubility, creating the observed corrosion products. The effect of the deposit composition, gas atmosphere composition, alloy composition, temperature, and deposit thickness were examined with respect to the proposed fireside corrosion mechanism. These measurements were found to be consistent with the proposed mechanism based on synergistic fluxing