Materials at High Temperature Vol 19, Issue 3, 2002
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Crevice corrosion of Cr2O3 and Al2O3 forming alloys in mixed nitrogen-oxygen environments
M. J. Bennett1, J. R. Nicholls1, G. Borchardt2 and G. Strehl2
1SIMS, Cranfield University, Cranfield, Bedfordshire, MK43 0AL, UK
2Institut für Metallurgie, Technische Universität Clausthal, Robert Koch Strasse 42, Clausthal- Zellerfeld , D-38678, Germany
When exposed freely to low potential bioxidant environments at elevated temperatures, either Cr2O3 or Al2O3 oxide scales can afford long-term protection of commercial alloys, even if such environments contain another potentially corrosive gaseous constituent. Circumstances arise, however, in many technological applications where specific geometry factors, such for example, as crevices/cracks or narrow holes, could result in the oxygen supply to internal regions becoming the dominant control parameter. This has been demonstrated previously with carbon bearing bioxidants, where carburisation of Cr2O3 formers then can ensue, often with disastrous consequences, controlling component integrity or life. To demonstrate that these crevice corrosion principles apply also to other mixed gas bioxidants, and for Al2O3 formers as well as Cr2O3 formers, the corrosion behaviours of two chromia formers, Nimonic 86 and Hastelloy X, at 950ºC and of the Al2O3 forming FeCrAlRE alloy, PM 2000, at 1200ºC have been examined in separate studies, using blended nitrogen–oxygen environments. The overall objectives were to confirm that protective scales are formed on all alloys in freely exposed conditions, but that with geometries restricting gas access, oxidant depletion led to internal nitridation, with the formation of CrN on Nimonic 86 and Hastelloy X and of TiN and AlN on PM 2000.
Keywords: crevice corrosion, Cr2O3, Al2O3, nitrogen–oxygen environments
Corrosion performance of the tubing austenitic alloy installed in the gasifier heat exchangers of the ELCOGAS IGCC power plant in Spain
A.M. Lancha1*, M. Alvarez de Lara1, D. Gómez-Briceño1 and P. Coca2
1CIEMAT. Avda. Complutense, 22. 28040-Madrid, Spain
2ELCOGAS, S.A., C.T. GICC Puertollano. Carretera Calzada de Calatrava a Puertollano, km. 27. 13500-Puertollano, Ciudad Real, Spain
The 335 MWe (ISO conditions) IGCC plant in Puertollano (Ciudad Real, Spain) has been constructed by the ELCOGAS consortiuma. The gas-side corrosion performance of the tubing alloy (Sanicro 28) in the high pressure heat exchanger is being evaluated by removing short lengths of sample tubing installed in membrane wall tube panels at different locations. Until now, test samples with 3000, 4100 and 7000 operating hours have been examined. The destructive examination performed has included mainly metallography, scanning electron microscopy (SEM) and analysis by energy dispersive spectroscopy (EDS). In addition, Auger electron spectroscopy (AES) analyses in depth have been carried out in order to characterise the different corrosion layers on the metal surface. The results obtained have shown good behaviour of the Sanicro 28 in the gasifier atmosphere.
Keywords: oxidation, sulfidation, Sanicro 28, gasifier atmosphere.
Waterwall corrosion mechanisms in coal combustion environments
J.C. Nava-Paza, A.L. Plumleya*, O.K. Chowb and W. Chenb
ABB Power Plant Laboratories, Materials and Chemical Technology, *Combustion Technology
When coal-fired boilers are operating with substoichiometric firing to limit NOx formation in order to comply with emission regulations, the reducing environments formed locally in the firing zone or under deposits can result in sulfidation attack and premature failure of components. Carbon and low-alloy steels are the preferred materials for waterwall applications due to their good thermal conductivity, creep strength (up to 550ºC) and relatively low cost. However, these materials, with at best low contents of Cr, are also the most susceptible to sulfidation attack. This paper presents the results of an extensive investigation to evaluate the effect of coal combustion characteristics, gas flow in the boiler, sulfidation potential of the environment, and the formation of deposits, on the kinetics of sulfidation in typical boiler materials. An interdisciplinary approach was adopted involving combustion analysis, corrosion studies and computational fluid dynamics along with information on operational boilers. The objective was to provide support for the low NOx retrofit program and to generate authoritative information for utility customers and also to develop a capability to predict potential combinations of coal and combustion conditions which could cause enhanced rates of attack.
Keywords: waterwall corrosion, coal combustion
Low-cycle fatigue life prediction for Waspaloy
J. T. Yeom1*, S.J. Williams2 and N.K. Park1
1Korea Institute of Machinery and Materials, 66 Sangnam-Dong, Changwon 641-010, South Korea
2Rolls-Royce plc, PO Box 31, Elton road, Derby, DE24 8BJ, UK
Methodologies of correlating and predicting life to failure in low cycle fatigue are investigated for Waspaloy at various temperatures. The viscoplastic material model proposed by Chaboche was initially adopted in order to predict the inelastic behaviour observed in low cycle fatigue. Life predictions under low-cycle fatigue loading conditions for the disc material Waspaloy were investigated with two models, the strain range partitioning method (SRP) and the linear combination of creep and fatigue damage laws. In the SRP rule, decomposition method using a computer modeling for determining the model parameters is proposed. The capabilities of the two models were assessed using strain-controlled LCF test results on Waspaloy.
Keywords: unified vicoplastic material model, life prediction, low cycle fatigue, Waspaloy
A numerical study of damage development and creep life in circular notched specimens during creep
Z.F. Yue* Z.Z. Lu and X.M. Wang
Department of Engineering Mechanics, Northwestern Polytechnical University, Xi’an, 710072, P.R. China.
In this study, numerical calculations of creep damage development and life behaviour of circular notched specimens have been performed with the Kachanov–Rabotnov damage law. The emphasis was placed on the role of specimen geometry and material ductility. The creep deformation will relax the elastic stress concentrations. The redistribution of the stress concentration depends on the material properties and specimen geometry. ‘Brittle’ materials will redistribute faster than ‘ductile’ materials. The creep crack initiation place can be the centre of the specimen (large radius of notches e.g. R = 3 mm in this study) or the tips of the notches (small radius of notches). The characteristics of ‘notch strengthening’ and ‘notch weakening’ depend on the notch radius and notch type as well as material properties. For the same notch type, the creep lives decrease with the decreasing of notch radius and the depth of cracks. Generally, the ‘V-type’ notched specimens have longer lives than ‘U-type’ notched specimens for the same notch radius. For all specimens studied, the ‘ductile’ material specimens have longer creep lives than ‘brittle’ material specimens.
Keywords: creep damage, circular notched specimens