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Comparative Studies On Corrosion Of Alloy 617 OCC At 700°C In Air And Simulated Coal Ash Environments
Hari P.R, , Rao M.N, Pavan A.H.V.
Published in Elsevier BV
Volume: 5
Issue: 5
Pages: 11452 - 11459
Hot corrosion is one of the major problems observed with the boiler tubing operating in ultra-supercritical (USC) power plants. It is due to the combined coal ash and flue gas environments encountered in USC power plants. Alloy 617OCC, a variant of the nickel based super alloy 617 with an optimized chemical composition, has been identified as one of the promising materials for manufacturing such boiler tubing. Hot corrosion behavior of alloy 617 OCC at 700°C in a simulated coal ash environment has been studied in the present work. Simulation was carried out by coating the alloy samples with a synthetic salt containing 5% Na2SO4, 5% K2SO4, 30% Fe2O3, 30% Al2O3 and 30% SiO2. In addition, oxidation tests were conducted at 700°C in laboratory air environment for reference purpose. In both cases the exposure time extended up to 5000 hours. The thermal cycling taking place in USC power plants was simulated by cooling down the alloy samples to room temperature after every 500 hours of exposure. Weight change of the samples was monitored as a function of exposure time to study the kinetics of hot corrosion. Characterization of the corrosion products was carried out using Scanning Electron Microscopy coupled with Energy Dispersive X-ray Spectroscopy (SEM/EDS) and X-Ray Diffraction (XRD). Oxidation in the air environment resulted in the corrosion product made up entirely of Cr2O3 over the entire range of exposure times. On the other hand, corrosion products observed on the samples tested in the salt environment contained Cr2O3, Fe2O3 and spinel oxide-NiCr2O4. There was no perceptible weight gain during the oxidation tests. There was significant weight gain, in contrast, in the salt environment tests. Weight gain plot for the salt testing indicated the presence of two distinct stages-initiation and propagation; the former extended to ∼ 1000 h with essentially no weight gain. The adherent, impervious Cr2O3 layer forming during testing in air environment is highly protective and prevents further ingression of oxygen into the metal. In the presence of salt mixture, however, Cr2O3 reacts with nickel oxide leading to the formation of the spinel phase NiCr2O4. The paper presents the findings and discusses the corrosion mechanisms coming into play. © 2017 Elsevier Ltd.
About the journal
JournalData powered by TypesetMaterials Today: Proceedings
PublisherData powered by TypesetElsevier BV
Open AccessNo