Possible scenarios for the causes of accelerated fireside corrosion of superheater tubes in coal-fired boilers

Recent experience of rapid corrosion of superheater tubes in coal-fired boilers has led to the application of weld-overlay coatings, which have proved effective but are expensive and can degrade mechanical tube life. Lack of understanding of how such corrosion differs from the established mechanism involving low-melting, complex, alkali-iron trisulphates is a major obstacle to devising alternative protective measures. Suggested scenarios for such accelerated corrosion are considered, including (1) increased tube surface temperatures allowing other sulphate mixtures to melt, analogous to Hot Corrosion mechanisms in gas turbines; (2) changes in the corrosivity of tube deposits from carryover of reduced species; and (3) increased release of corrosive species from the coal under substoichiometric combustion conditions. The potential for such scenarios to change the corrosive environment is assessed on the basis of current mechanistic understanding, though the general lack of phase diagrams for relevant ternary and quaternary sulphate systems is a major drawback to specifying the processes involved.