Sulfidation Corrosion Failure of a Boiler Tube in a Fossil Fuel-Based Thermal Power Plant

Abstract

The present case study involves a thorough and methodical metallurgical analysis of a failed water wall tube to identify sulfidation corrosion as the cause of failure. The investigation into the failure of the water wall tube was conducted through various methods, including visual observation, chemical analysis, examinations using optical and scanning electron microscopy, fractography and surface analysis via SEM–EDS, x-ray diffraction analysis of deposits and reaction products, and evaluation of mechanical properties. The water wall tube exhibited failure in the form of an elliptical puncture, with no significant thinning at the location of the puncture. The chemical composition, mechanical properties, and microstructure of the failed water wall tube were in accordance with the specified standards for SA 210 grade C steel. The microstructure of the tube consisted of a ferrite–pearlite structure and showed no anomalies related to microstructure. The experimental results revealed that the tube failed due to sulfidation corrosion on its outer surface. This sulfidation reaction was initiated by sulfur from pyrite (FeS2) in the coal-ash deposits on the outer surface. Moreover, the buildup of ash deposits caused localized overheating, which accelerated the sulfidation reaction, followed by an oxidation reaction.