Alloy 617 OCC, a variant of INCONEL 617 with optimised chemical composition, has been produced in India for manufacture of superheater and reheater tubing in boilers operating in advanced ultrasupercritical (A-USC) power plants. The tubing encounters intense hot corrosion conditions during service. The present study deals with hot corrosion behaviour of 617 OCC in A-USC environment. The environment occurring in A-USC plants was simulated in the laboratory by exposing the material coated with a mixture of salts at 700°C in a flowing gas mixture. For use in A-USC boiler technology, the metal loss due to fireside corrosion of the material should be less than 2 mm in 200,000 hours. The loss obtained in the present study was nearly 5 times this value. The corrosion processes were studied using SEM/EDAX, XRD and thermogravimetry. The degradation mechanisms coming into play, disqualifying the material for use in A-USC plants, would be discussed.

Arivazhagan N

Department of Manufacturing Engineering

School of Mechanical Engineering

Vellore Campus

Nageswara Rao M

P.R. Hari

A.H.V. Pavan

Vellore Institute of Technology (VIT) is a private university located in&nbsp;Tamil Nadu, India. Founded in 1984, as Vellore Engineering College, the institution offers 20 undergraduate, 34 postgraduate, four integrated and four research programs. It has campuses in Vellore, Amravati, Bhopal and Chennai.

VIT is one of the top ranked private universities in India according to NIRF, THE and QS Rankings.&nbsp;Govt. of India has recognized&nbsp;VIT, Vellore as an&nbsp;Institution of Eminence. This has allowed VIT to take independent quality initiatives and move up in world ranking.

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VIT University

Materials Science Forum

High-temperature corrosion of the nickel based superalloy 617 OCC (Optimised Chemical Composition) was studied in the context of proposed application in ultra-supercritical (USC) power plants. Studies were carried out in USC power plant environment simulated in the laboratory. Simulation was performed by coating the alloy with a salt mixture consisting of K2SO4, Na2SO4, Fe2O3, SiO2 and Al2O3 and exposing it to a flowing gas mixture comprising of 15% CO2, 3.5% O2, 0.25% SO2 and the rest N2 (Vol%) at 700 °C. For the sake of comparison, corrosion studies were also carried out under the same conditions, but without the gas mixture. Progress of corrosion was monitored by thermogravimetry, XRD and cross sectional studies on corroded specimens using SEM/EDAX analysis. There was evidence for both external and internal sulfidation with the flowing gas mixture, suggesting that the SO2 present in the gas mixture plays a dominant role. Chromium present in the alloy reacted with the sulfur-containing species in the environment forming chromium sulfide. No internal oxidation of aluminum and titanium was observed in the presence of flowing gas mixture. Internal oxidation was observed, in contrast, without flowing gas mixture at longer exposure times. The paper discusses the findings and possible corrosion mechanisms, coming into play. © 2019 IOP Publishing Ltd.

Materials Research Express

High temperature corrosion of alloy 617 OCC at 700 °C in simulated USC power plant environment

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. © 2017, The Indian Institute of Metals - IIM.

Transactions of the Indian Institute of Metals

Hot Corrosion Studies on Alloy 617 OCC in the Context of Its Use in Advanced Ultra-Supercritical (A-USC) Power Plants

Acta Metallurgica Sinica (English Letters)

Corrosion Behavior of the Ni–Cr–Fe Base Superalloy GH984G in a Synthetic Coal Ash and Flue Gas Environment

Nuclear Engineering and Design

Corrosion of Alloy 617 in high-temperature gas environments

Corrosion Science

Effect of temperature on coal ash hot corrosion resistance of Inconel 740 superalloy

Energy

High-efficiency coal-fired power plants development and perspectives

Hot Corrosion of Alloy 617 OCC in Simulated USC Power Plant Environment

Journal	Materials Science Forum
Publisher	Trans Tech Publications, Ltd.
ISSN	02555476
Open Access	No