Carburization and Corrosion of Stainless Steel Tubes at Elevated Temperatures

Abstract The role of operational power changes on the corrosion of SA-213-TP-321H stainless steel tubes of a steam superheater was studied. The power unit where the tubes are installed is a 158MW fired with residual oil fuel and the plant is subjected to "peaking operation". The fuel oil consumed in the plant is high in sulfur, vanadium and asphaltens content. The study reveals that after 107,000 hours service the tubes are seriously corroded and develop cracking of the oxide layer and the cracking is associated with frequent temperature changes. Also associated with power changes are the bursts of carbon coke and soot particles that deposit on the tube surface and enhance a carburizarion process into the steel matrix. Carbon rich deposits are formed in the grain boundaries of the austenitic metallic matrix. A carburization-corrosion process is produced causing the intergranular wastage of the steel near the exposed surfaces. The evidence to characterize the corrosion-carburization mechanism is sustained by scanning electron microprobe EDS and WDS analysis, transmission electron microscopy, and microhardnes Vickers testing.