Characterization of Fireside Corroded Sections of Boiler Tubes

摘要

I1/4 Cr1/2 Mo3/4 Si (T1l) and 21/4 Cr1Mo (T22) grade steels are extensively used in critical parts of sub-critical thermal power plants like superheater, reheater tubes or headers. The tubes are designed to last for a very long time in service. These tubes are exposed to hot flue gas on the outer side and consequently, the inner side heats the steam passing through the tube. The outlet temperature of the steam ranges from 500-550℃. Therefore, not only the temperature on the outer side is much higher than the steam outlet but also the environment which the tube faces is extremely corrosive thereby leading to hot corrosion/ fireside corrosion. Some parts of the tubes are also exposed to fly ash coming from the combustion of coal leading to the formation of deposits on the tubes. In the present work, the tubes are cut and taken out during overhaul shutdowns and they are characterized by X-Ray Diffraction (XRD) for the likely phases that are formed on the surface of the fireside corroded layer. The deposits/scales are also qualitatively characterized. The cross-sections of the tubes are used for characterization of fireside corroded product layers using Optical Microscopy and Field Emission Scanning Electron Microscopy (FESEM) coupled with Energy Dispersive X-Ray Spectroscopy (EDS). This work will be extended further to develop rate laws and rate mechanisms of the corrosion product layers formed on the service exposed tubes.