Hot Corrosion Studies on Alloy 617 OCC in the Context of Its Use in Advanced Ultra-Supercritical (A-USC) Power Plants
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Degradation by hot corrosion is a major cause of concern in superheater and reheater tubing in boilers operating in advanced ultrasupercritical power plants. Alloy 617 with optimized chemical composition (617OCC) has been identified in Indian context as a promising material for the manufacture of this tubing. The present research deals with study of hot corrosion of this alloy. The coal ash environment occurring in A-USC power plants was simulated by coating the experimental material with a mixture of salts—namely 5% K2SO4, 5% Na2SO4, 30% Fe2O3, 30% SiO2 and 30% Al2O3. A flowing gas mixture comprising of 15% CO2, 3.5% O2, 0.25% SO2 and the rest N2 (by vol.%) was used to simulate the flue gas environment in the power plant. Hot corrosion tests were also carried out without flowing gas mixture for the sake of comparison. In both the cases, the exposure temperature was 700 °C and the exposure time extended up to 5000 h. The samples were cooled to room temperature after every 500 h of exposure to simulate the thermal cycling occurring in the power plants. Extent of hot corrosion was monitored using thermogravimetry. The corroded samples were characterized using Scanning Electron Microscopy coupled with Energy Dispersive X-ray Spectroscopy and X-Ray Diffraction. Degradation was very severe when samples coated with the salt mixture were tested in the flowing gas mixture. Degradation was comparatively less in extent in the salt environment without gas mixture. In the salt-without-gas experiments, oxides of Cr, Ni and Co and their spinel oxides—FeCr2O4, NiCr2O4 and CoCr2O4—were the corrosion products. CrS and Cr2(SO4)3 phases were observed additionally in salt + gas experiments, indicating that SO2 present in the synthetic flue gas environment was actively participating in the corrosion process. Corrosion in both salt-without-gas and salt + gas environments showed two distinct phases—initiation and propagation; the initiation phase was shorter in the (salt + gas) environment. The paper presents the findings and discusses the corrosion mechanisms coming into play.
KeywordsHot corrosion 617OCC Ultra-supercritical power plant Coal ash/flue gas environment
The authors are very much thankful to the Science and Engineering Research Board and to the Department of Science and Technology for the support and the funding received from them. We would like to extend our sincere thanks to the BHEL (Bharat Heavy Electricals) Hyderabad for providing the super alloy.
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