High temperature corrosion of martensitic steel of reheater pipes in a desalination power plant

X20CrMoV12-1 steel is one of the most common martensitic steel grades that is known for its excellent heat-resistant and superior performance against oxidation and creep. Consequently, it has been used as a replacement for the low-alloy ferritic 2.25Cr-1Mo steel reheater pipes in a desalination power plant. Fracture occurred unexpectedly in some X20 steel reheater pipes of the boiler. Visual inspection revealed clear signs of wall thinning and corrosion at the fireside of the failed tube. Assessment of the tube cross-section included metallurgical examination, indicating that the microstructure is tempered martensite with no clear indications of thermal degradation or voids formation by creep. This was consistent with the microhardness measurements showing a typical average Vickers microhardness of 240 Hv. In addition, evaluation of oxide layer was made using Energy-dispersive X-ray spectroscopy (EDS) showing formation of corrosive deposits on top of the protective oxide layer at the fireside. The deposits contain excessive amounts of sulfur and vanadium that were initially present in the firing fuel. Remedial actions should be necessarily taken to minimize high-temperature corrosion by including fuel additives such as magnesium oxide or alumina to increase the melting point of vanadium and sulfur compounds. Also, improving the combustion efficiency, e.g. use of improved atomizer, with less air should be considered to minimize corrosion attacks.