Mechanism:

Fireside corrosion remains a leading cause of failure in coal fired units. Fireside corrosion occurs via two modes. In the first mode of corrosion, tube wastage occurs by the formation of low melting species which dissolve the protective iron oxides on carbon or low alloy (Cr-Mo) steels. In the second mode, the tube wastage occurs when there are reducing conditions inside the furnace, and iron sulfide scales form instead of protective oxides. These iron sulfide scales are more porous and less protective than oxides. Reducing conditions may also promote carburization of Cr-Mo, ferritic and stainless steels, resulting in a loss of corrosion resistance. Unburnt carbon and carbon monoxide carry carbon to the steel surface under reducing conditions. Furnace wall tubes are also subject to fireside corrosion, but the low melting species differ from superheater/reheaters. Sodium and potassium pyrosulfate (Na₂S₂O₇ or K₂S₂O₇) have been responsible for furnace wall corrosion. Both of these species melt below 800oF, where the furnace wall tubes operate. The melting points of Na₂S₂O₇ and K₂S₂O₇ are 750⁰F and 570^oF, respectively. Mixtures of these two compounds could melt at even lower temperatures. Melting points as low as 635^oF-770^oF have been measured. 

Fireside corrosion produces wall thinning that eventually results in longitudinal, ductile rupture.  Wall thinning is uniform across several tubes in a particular location.  Circumferential grooving or cracking with deep finger-like penetrations into the tube wall has been found in super-critical boilers.  Hard, dark ash deposits are usually found on the external surface.