Materials at High Temperature Vol 17, Issue 2, 2000
Subscribers may view full papers here
Preface
The Fourth International Conference on the Microscopy of Oxidation was held at Trinity Hall, Cambridge from 20th to 22nd September 1999. Some 71 delegates contributed 64 oral and poster presentations. Fifty of these papers are published in this journal: 23 in part one and 27 in part two.
The conference followed the successful format of previous meetings with major sessions on oxidation studies of alumina formers (often oxide dispersion strengthened) and intermetallics, especially TiAl. This reflected the ongoing demand for materials which can withstand both oxidation and stress at high temperatures. The performance of these and other materials in mixed oxidant environments was also reviewed together with the use of coatings to enhance oxidation resistance. Papers on SiC containing composite materials as well as zirconium based alloys, used in the nuclear power industry, were also presented, while a session on Cr and Fe based alloys catered for the needs of the stainless steel community.
The conference was rounded off by talks on new techniques applicable to oxidation studies. Both the oral and poster sessions inspired lively debates, fuelled of course by the excellent food and drink laid on by Trinity Hall. The conference dinner and wine reception, sponsored by JEOL (UK) Ltd and FEI (UK) Ltd – suppliers of Philips Electron Microscopes – added to the convivial atmosphere, and delegates judged the whole meeting to have been very successful.
The next conference in the series has already been scheduled for 2002.
Gordon Tatlock
Simon Newcomb
The Effect of Surface Contamination on the Transient Oxidation of Ni-20Cr
Badrol Ahmad and Peter Fox
Materials Science and Engineering, The Department of Engineering, The University of Liverpool, Liverpool L69 3BX, UK
When a metal surface is heated in an oxidising environment for the first time, an oxide scale is formed. This scale is not the product of a process that occurs under steady state conditions, but is instead formed over a wide range of temperatures, as the sample heats. The oxides, often referred to as transient scales, that are formed under these conditions are frequently very different from those found after long oxidation times. The research presented here considers the oxides formed during the heating of a commercial Ni-20%Cr alloy and examines the effect of surface contamination on the oxidation process. Also examined is the role of surface pretreatment on the behaviour of the alloys during oxidation. The scales formed on these alloys are very thin (<100 nm) and the scales have thus been examined using transmission electron microscopy (TEM) and scanning transmission electron microscopy techniques (STEM). The samples produced were of cross-sections of the metal and oxide.
Keywords: surface contamination, transiet oxidation, Ni–20Cr
SEM and TEM studies of PtAl diffusion coatings under isothermal oxidation
J. Angenete and K. Stiller
Department of Experimental Physics, Chalmers University of Technology and Göteborg University, SE-412 96 Göteborg, Sweden
Three commercial Pt modified aluminide coatings (RT22, SS82A and MDC150L) and one conventional aluminide diffusion coating (PWA73), applied to the same substrate (CMSX4), were oxidised in still laboratory air at 1,050°C for times up to 2,000 h. The samples were investigated by gravimetry, scanning electron microscopy, (SEM), transmission electron microscopy (TEM) and energy dispersive x-ray spectroscopy (EDS). It was found that among the three Pt modified coatings, RT22 produced the fastest growing oxide and that at about 500 h, a decrease in growth rate occurred for all three coatings. The oxide on RT22 started to spall after 1,000 h, while the oxide on SS82A and MDC150L remained adherent. The conventional aluminide diffusion coating suffered from severe spalling after 1,000 h. Furthermore, results from microstructural and compositional measurements of the Pt modified aluminide coatings are presented.
Keywords: Pt modified aluminide diffusion coatings, Ni-based superalloys, high temperature oxidation, SEM, TEM
Adhesion of thermal barrier coatings – the development of metastable alumina
J. P. Banks, S. R. J. Saunders and C. J. Chunnilall
National Physical Laboratory, Queens Road Teddington, Middlesex TW11 0LW, UK
The adhesion of thermal barrier coatings (TBCs) is dependent upon the characteristics of the thermally grown oxide (TGO) that forms between the TBC and the corrosion resistant bond coat. Work has been carried out to investigate the properties of the TGO as a function of ageing treatments using piezospectroscopy. Residual stress maps were generated for an electron beam physical vapour deposited (EB-PVD) TBC which showed a large variation in residual stress over the surface of a coated sample. The two peaks generally associated with a alumina (R1 and R2) frequently appear as doublets with a high and low stress component. In addition, the presence of a metastable u alumina was detected in aged samples. It is believed that these observations can be related to incipient spallation of the TBC. The development of residual stress and the metastable oxide have been studied and correlated with the spallation behaviour of the TBC.
Keywords: thermal barrier coatings, metastable alumina
Isothermal and thermal cyclic oxidation behaviour of thermal barrier coatings: Pt aluminide bond coats
K. Bouhanek1, O.A.Adesanya1, F.H. Stott1, P. Skeldon1, D.G. Lees2 and G.C.Wood1
1Corrosion and Protection Centre, UMIST, PO Box 88, Manchester M60 1QD, UK
2Materials Science Centre, University of Manchester/UMIST, Grosevenor Street, Manchester M1 7HS, UK
_-phase and _–_′-phase Pt aluminide, two diffusion bond coats, were applied respectively to CMSX4
and RR3000, two single-crystal Ni-base superalloys, with and without a yttria partially-stabilised zirconia, EB-PVD top coat. The oxide scales developed on RR3000/_–_′-phase Pt aluminide systems without YPSZ top coat were essentially _-Al2O3 at 1,200°C and _-Al2O3 plus _-Al2O3 at 1,100°C. The oxide scales developed during thermal-cycling oxidation on RR3000/_–_′-phase, Pt aluminide/YPSZ were found to be more resistant to spallation than the scales grown on CMSX4/_-phase, Pt aluminide/YPSZ. Spallation of the oxide scale occurred mainly at the bond coat–oxide scale interface but also within the oxide and at the oxide–top coat interface.
Keywords: EB-PVD, thermal barrier coating, Pt aluminide
Correlation of alloy microstructure with oxidation behavior in chromia-forming intermetallic-reinforced Cr alloys
M.P. Brady, P.F.Tortorelli, L.R.Walker
Oak Ridge National Laboratory, Oak Ridge, TN 37831-6156, USA
E-mail: bradymp@ornl.gov
The oxidation behavior of the Cr(X) solid solution and intermetallic Cr2X Laves phases (X = Nb, Ta) was studied individually and in combination at 1,100°C in humid air. The two-phase Cr(Ta) + Cr2Ta alloys exhibited superior oxidation resistance to either the single-phase Cr(Ta) or the single-phase Cr2Ta alloys. However, the two-phase Cr(Nb) + Cr2Nb alloys exhibited lower oxidation resistance than either the single-phase Cr(Nb) or Cr2Nb alloys. The two-phase Cr(Ta) + Cr2Ta alloys were also found to exhibit superior oxidation resistance to the two-phase Cr(Nb) + Cr2Nb alloys. This was hypothesized to be related to the lower amount of the Cr2X phase in the Cr(Ta)–Cr2Ta eutectic and to the lower solubility of Ta in the Cr(X) phase, relative to the Cr–Nb system. Both of these factors resulted in significantly less oxidation and nitridation in the Cr(Ta) + Cr2Ta alloys.
Keywords: Cr, Laves phase, chromia, nitride, oxidation
Oxidation mechanism of Ni–20Cr thin strips and effect of a mechanical loading
G. Calvarin1, A.M. Huntz1 and R. Molins2
1LEMHE, UMR CNRS 8647, Université Paris XI, 91405 Orsay, France
2ENSMP, UMR CNRS 7633, Centre des Matériaux, 91003 Evry, France
The oxidation behaviour of Ni-20Cr foils of 200 _m thickness was studied in air at 600°C and 900°C. The oxide morphology and nature were determined by SEM and TEM. The scales formed at all temperatures were complex, with an outer NiO layer and an intermediate layer of equiaxed NiO, NiCr2O4 and Cr2O3 grains. At 600°C, internal oxidation of chromium occurred in the substrate while a continuous Cr2O3 layer was observed at 900°C. The effect of a tensile load on the oxidation kinetics and mechanism of Ni–20Cr alloy was also examined. The results indicated that applying a tensile load did not modify the oxide nature and morphology but induced an increase of the oxidation rate, promoting the formation of internal oxidation at 600°C, and of a NiCr2O4 layer at 900°C.
Keywords: Ni–20Cr alloy, NiCr2O4, internal oxidation, TEM, effect of a tensile load
The influence of NiAl3 on the high temperature oxidation of a plasma-sprayed overlay coating
W.Y. Chan1, H.E. Evans1, C.B. Ponton1, J.R. Nicholls2 and N.J. Simms2
1School of Metallurgy and Materials, University of Birmingham, Birmingham B15 2TT, UK
2School of Industrial and Manufacturing Science, Cranfield University, Cranfield, MK43 0AL, UK
MCrAlY overlay coatings have been successfully used as a means of improving the oxidation performance of gas turbine blades operating at elevated temperatures. However, depletion of aluminium can limit the ability of such coatings to form a protective oxide layer should spallation of the original _-Al2O3 oxide layer occur under thermal cycling conditions. It is the objective of the current research to evaluate the potential of NiAl3 as a reservoir phase for a NiCrAlY overlay coating on a IN738LC superalloy substrate at 1,100°C in air. The morphologies and microstructures of the conventional NiCrAlY and NiAl3-modified NiCrAlY overlay coatings in the as-sprayed and oxidised conditions were characterised using SEM, EDX and XRD techniques.
Keywords: nickel aluminide, overlay coating, _-Al2O3, reservoir phase, NiCrAlY.
Effects of a Reactive Element on isothermal and cyclic oxidation of chromia-forming alloys: SEM/EDX,TEM and SIMS investigations.
S. Chevalier1, G. Bonnet1, P. Fielitz2, G. Strehl2, S.Weber3, G. Borchardt2, J.C. Colson1 and J.P. Larpin1
1Laboratoire de Recherches sur la Réactivité des Solides, UMR 5613 CNRS, 9 avenue Alain Savary, BP 47870, 21078 Dijon cedex, France
2Institut für Allgemeine Metallurgie, Technische Universität Clausthal, Robert-Koch- Strasse 42, 38678 Clausthal-Zellerfeld, Germany
3Ecole des Mines, Parc de Saurupt, 54042 Nancy cedex, France.
Scanning electron microscopy (SEM), transmission electron microscopy (TEM), secondary ion mass spectrometry (SIMS), secondary neutral mass spectrometry (SNMS) and X-ray diffraction (XRD) were used to study the reactive element effect on chromia-forming alloys. The reactive element, neodymium, was introduced as an oxide film at the surface of the alloys. The analyses were performed during the early stages of oxidation at 1,273 K. Uncoated and Nd2O3-coated alloys have been oxidised for 1, 5, 30, 60, 120 minutes and 50 hours in air at atmospheric pressure. Chromia growth mechanisms were studied by two-stage 16O2/18O2 oxidation exposures followed by SIMS and SNMS analyses. Chromia grains quickly grew on uncoated samples, whereas they slowly developed on Nd2O3-coated specimens. A neodymium-containing phase rapidly evolved from Nd2O3, to NdCrO3 and then NdTi21O38. Indeed, the main phase evolution appeared during the first 60 minutes of the oxidation process. Chromia growth mechanism was not changed after 1 hour of oxidation because Nd was not yet incorporated into Cr2O3 scales. During the early stages of oxidation, Nd was mainly concentrated in the outer part of the scale composed of a spinel phase, Mn1.5Cr1.5O4. After two hours of oxidation, Nd was incorporated inside the chromia scale, leading to inward diffusion of oxygen. These results clearly demonstrated that the incorporation of the reactive element in the chromia scale as grain boundary segregant is the main explanation of the reactive element effect in the case of chromia-forming alloys.
Keywords: reactive element, chromia-forming alloys, chromia-growth mechanism
The development of aligned precipitates during internal carbonitridation of Fe–Ni–Cr alloys
S.I. Ford, P.R. Munroe and D.J.Young
School of Materials Science and Engineering, University of New South Wales, Sydney 2052, Australia
An austenitic Fe–25Cr–19Ni stainless steel alloy was carbonitrided at 1,000°C in an atmosphere with ac=1 and P(N2)=0.9atm to form two discrete precipitation zones. Local equilibrium between the precipitates and the austenite matrix carbon activity was achieved throughout the reaction zone. Small, globular Cr7C3 particles were formed immediately beneath the surface. High aspect ratio Cr23C6 lamellar plates were formed deeper in the precipitation zone and were found to have a cube–cube orientation relationship with the austenite matrix. The inward growth of these carbides was facilitated by the formation of an austenite/depleted austenite grain boundary at the precipitation front, which transformed the austenite to a more appropriate orientation and accelerated the segregation of chromium to the carbide tips.
Keywords: precipitate size, morphology, distribution
Direct observation and analysis of the oxide scale formed on Y-treated austenitic stainless steels at high temperature
Hisao Fujikawa1,Tadaoki Morimoto2 and Yoshitaka Nishiyama1
1Sumitomo Metal Industries Limited, 1–8 Fuso-cho Amagasaki, 660-0891 Japan
2Hitachi Limited, 1–1 Omika-cho 7-chome Hitachi, 319-1221 Japan
We have studied the oxidation behavior of conventional austenitic stainless steels using same small amounts of Y as is added for deoxygenation and desulphurisaton in steel making. The direct observation and analysis of the oxide scale formed on 19Cr–10Ni–l .5Si steels with and without small amounts of Y at high temperature have been carried out using several types of equipment.
The following results were found:
(1) Steel with 0.03Y showed good resistance to oxidation at l,000°C.
(2) Oxide scale was composed mainly of Cr oxide, and Si oxide was also detected at the oxide scale–metal interface and in the internal oxides. The Si oxide formed a network cell structure in the inner oxide scale with deeper internal penetrations. The steel with Y formed a uniform oxide scale in every oxide layer.
(3) Small amounts of Y and Si were detected at the grain boundaries of the inner oxide scale, but no Y was detected in the oxide grains.
The beneficial effect of Y addition was more notable in the Si containing austenitic stainless steels, as the existence of Y or Si prevents the diffusion of cations and anions through the oxide grain boundaries. As consequence, the steel treated with Y showed good resistance to oxidation.
Keywords: oxide scale, austenitic stainless steels
Isothermal transformation behavior of thermally-grown wüstite
B. Gleeson1,2, S.M.M. Hadavi1,3 and D.J.Young1
1School of Materials Science and Engineering, The University of New South Wales, Sydney, NSW, Australia
2Present address: Department of Materials Science and Engineering, Iowa State University, Ames, IA, USA
3GST Materials Research, Tehran, Iran
The transformation behavior of wüstite (FeO) below the FeO→Fe+Fe3O4 eutectoid temperature of about 570°C was studied using laser Raman spectroscopy in conjunction with optical microscopy, backscattered electron microscopy, and electron probe microanalysis. A thermally-grown wüstite layer was initially formed by oxidizing a mild steel in air at 900°C for 100 s. The thermally-grown layer was then isothermally annealed at 100–500°C for up to 6 hours. The transformation behavior was found to be very dependent on degree of undercooling below the eutectoid temperature. At temperatures down to about 350°C, transformation was initiated by Fe3O4 precipitation and then followed by lamellar Fe+Fe3O4 growth. However, at a temperature of 270°C, the FeO transformed directly to a fine, granular mixture of Fe+Fe3O4. No transformation was observed at 200°C and lower. The kinetic results were used to construct isothermal transformation diagrams.
Keywords: wüstite, oxidation, transformation, precipitation, laser Raman spectroscopy
Formation of chromium rich oxide scales for protection against metal dusting
H.J. Grabke, E.M. Müller-Lorenz, B. Eltester and M. Lucas
Max-Planck-Institut für Eisenforschung GmbH, Postfach 140 444, D-40074 Düsseldorf, Germany
Metal dusting is a disintegration of metals and alloys into graphite and metal particles, caused by strongly carburizing gas mixtures mainly in the temperature range 400–700°C. Protection of steels against metal dusting is possible through the formation of dense chromium rich oxide scales but it is not guaranteed that such scales are formed at low temperatures, even on high Cr-steels. Surface analytical studies have been conducted on the formation and composition of the oxide scales on 9–20%Cr steels. The growth of oxide films was followed by AES for 3 hours at 10–7 mbar O2 great differences were observed in dependence on surface finish. On ground samples, Mn and Si appeared early and Cr-rich oxide was formed, whereas on chemically etched samples Fe-rich oxides grew. After long term exposures (240 h) under metal dusting conditions, i.e. in CO–H2–H2O mixtures at 600°C, thin Cr-rich scales were observed on ground steels which were impermeable to carbon whereas on chemically etched steels thick Fe-rich scales had grown and carbon penetration was detectable. Accordingly, the oxide formation on Cr-steels at relatively low temperatures strongly depends on the surface treatment. Any surface working such as grinding and sand-blasting etc. introduces dislocations and causes a fine-grained microstructure near the surface, and the dislocations and grain boundaries act as rapid diffusion paths for supply of Cr to the surface in the first minutes of exposure, which leads to the formation of a protective oxide scale.
Keywords: metal dusting, chromium rich oxide scales
Study of the coating stability of a TBC system
Catherine Guerre, Régine Molins and Luc Remy
ENSMP, Centre des Matériaux, UMR CNRS 7633, BP 87, 91003 Evry Cedex, France
The degradation of thermal barrier coatings is closely linked to their spalling resistance which depends on the stability of the protective oxide scale produced by oxidation of the bond-coat. This study deals with a characterisation of the oxide layer, its interfaces with the bond-coat and the ceramic and their evolution during isothermal oxidation at 1,100°C. The microstructural evolution of the bond-coat, the outer part of the superalloy and the ceramic layer was also studied.
Keywords: alumina scale, TBC, nickel aluminide, TEM, scale adhesion
Characterization of Chromia Scales Grown on Pure Chromium in Different Oxidizing Atmospheres
S. Henry1, J. Mougin1,Y.Wouters2, J.-P. Petit2 and A. Galerie1
1Laboratoire de Thermodynamique et Physicochimie Métallurgiques, UMR 5614 – CNRS – INPG – UJF France
2Laboratoire d’Electrochimie et de Physicochimie des Matériaux et Interfaces, UMR 5631 – CNRS – INPG – UJF France
Ecole Nationale Supérieure d’Electrochimie et d’Electrométallurgie de Grenoble, BP 75 Domaine Universitaire, F – 38402 Saint Martin d’Hères Cedex France
Scanning electron microscopy, X-ray diffraction, photoelectrochemistry and microphotoelectrochemistry have been used to characterize thin chromia scales, in the micrometer range, grown on pure chromium at 900°C in oxygen and in water vapour. The duplex structure formed, more easily observable in water vapour grows by the opposite transport of chromium and of oxide/hydroxide ions. The external chromia subscale exhibits the usually reported 3.5 eV bandgap whereas the internal subscale presents a reduced gap possibly due to impurity contribution. Imaging the photocurrent generated in this subscale allows the observation of good metal-oxide interface properties of samples grown in H2O, whereas samples grown in O2, (liable to cracking during cooling), exhibit partially disrupted zones.
Keywords: Chromium oxidation, chromia scales, Cr2O3 forbidden gap, photoelectrochemistry
Microstructural evolution and degradation modes in cyclic and isothermal oxidation of an EB-PVD thermal barrier coating
F. Ibegazene-Ouali1, R. Mevrel2, C. Rio2 and Y. Renollet2
1SNECMA-Materials and Processes Department, 77550 Moissy-Cramayel, France
2ONERA-Metallic Materials and Processes Department, BP 72, 92320 Chatillon, France
Thermal barrier coatings constituted of a partially stabilised zirconia layer (~100 _m thick) deposited by EB-PVD on top of a platinum-modified aluminide bondcoat to protect a single crystal superalloy substrate, have been studied in cyclic and isothermal oxidation at 1,100°C in air. Close examination by high resolution SEM (with field emission gun) of the oxide scale and of the metallic surfaces after various durations, up to 600 hours at 1,100°C, show several morphological features which may affect the integrity of the systems: partial decohesion regions with large grain areas at the inner surface of the scale, subsequently replaced by a cellular morphology, cavities within the oxide scale as well as at the zirconia–alumina interface. The nature and distribution of these features depend on the type of test (isothermal or cyclic oxidation) and on the systems studied (standard superalloy or modified to lower the sulphur content). Scenarios are proposed to explain the observations reported.
Keywords: thermal barrier coating, EB-PVD, oxidation, microstructure
Mechanisms of simultaneous oxidationchloridation of austenitic stainless steels by NaCl in air
Jong-Won Kwon,Yun-Yong Lee and Yong-Deuk Lee
POSCO Technical Research Laboratories, P.O. Box36, Pohang, Kyungbuk, 790-785, Korea
Simultaneous oxidation-chloridation behavior of austenitic stainless steels by NaCl in air was investigated at 650°C. Surface oxide scales of the test alloys had double-layered structures that consisted of inner spinel layer and outer Fe2O3 layer. Condensed and gaseous chlorides were formed below the scale surface by NaCl vapor diffusing inward via pores or cracks of the scales. The rate of internal and uniform attack mainly depended on the porosity of the inner spinel layer. In order to predict the complex mixed oxidation behavior of alloys, new types of thermodynamic stability diagrams were utilized, which was constructed by computer-assisted calculations of phase equilibria of the multi-component reacting systems using the Thermo-CaIc program. The overall mixed oxidation process including the initial stage of reaction, the rapid formation and growth of the scale and the transport of gaseous species through the oxide scale was elucidated.
Keywords: chloridation, sodium chloride, stainless steel, stability diagram
Factors affecting the oxidation mode of stainless steels
C. Lille and R. F. A. Jargelius-Pettersson
Swedish Institute for Metals Research, Drottning Kristinas väg 48, S-114 28 Stockholm, Sweden
Studies have been made to investigate what effect different factors have on oxide formation on stainless steels at 600–1,000°C in air. The factors include the effect of microstructure, airflow rate, surface preparations, residual oxides and variations of the dew point of the air. Scanning electron microscope (SEM) equipped with energy dispersive X-ray spectroscopy (EDS), glow discharge optical emission spectroscopy (GDOES) and light optical microscope (LOM) were used to characterise the oxides.
Keywords: oxide formation, stainless steel
A study of the factors contributing to the metal dusting of Fe–Cr–Ni alloys in highly carburising atmospheres
M. Maier, J. F. Norton and P. Puschek
European Commission, Joint Research Centre, Institute for Advanced Materials, 1755 ZG Petten, The Netherlands
Metal dusting is a recurring problem within the petrochemical industry that has serious consequences for plant efficiency and reliability, as well as for operational safety. The underlying factors governing its occurrence are not fully understood and therefore studies under closely monitored conditions in atmospheres that simulate those encountered in service are essential. For this preliminary study, candidate reformer and pyrolysis tube materials have been selected and exposed at 560°C for periods of up to 200 hours in a highly reducing atmosphere. The intention has been to focus on some of the main variables that are thought to possibly contribute to metal dusting and evaluate their contribution through a series of controlled experiments. In addition to alloy composition, the principal parameters that have been addressed are surface condition and system pressure. In the series of experiments described in this paper, machined and ground as well as polished specimens have been exposed to an H2–24.4%CO–2.4%H2O gas mixture at pressures of nominally 1.5, 2 and 5 bar (absolute). As a consequence of the character of this gas (i.e. carbon activity, ac, >>1), extensive coke deposition occurs on the metallic specimens. Thus, a thermal cycle has been introduced into the test procedure, generally at 25 hour intervals, so that the samples can be inspected and coke removed. Cross-sectional optical and scanning electron microscope examinations of the morphology of the coke deposit and the nature and extent of attack have provided a deeper understanding of the importance of alloy composition and the contributions made by surface treatment and system pressure.
Keywords: metal dusting, reducing atmospheres, petrochemical industry, alloy composition, surface condition, pressure
Characterization of Al rich oxide layers on austenitic stainless steels by fluorescence spectroscopy
Yasushi Matsuda, Kiyoko Takeda and Shigeki Azuma
Corporate Research & Development Laboratories, Sumitomo Metal Industries, Ltd, 1–8 Fuso-cho, Amagasaki, 660 Japan.
An Al rich oxide passivation technique has been developed to improve the corrosion resistance of the stainless steel to ozone added ultra-pure water. Fluorescence spectroscopy was applied to the study on the selective oxidation of aluminum containing austenitic stainless steel in low oxygen atmosphere at 1,353K. It was found that in the thermal oxidation under low oxygen pressure, the minor alloying constitution of Al resulted in the formation of thin oxide layers. The frequency shifts of fluorescence spectra show that the compressive stresses exist in the oxide layers as a result of the difference of the thermal expansion coefficients between substrate steels and _-Al2O3, and depends on the thickness of the oxide layers. It is confirmed that pure _-Al2O3 protective layers grown on the stainless steels, which remain stable and attached to the stainless steels in ozone added ultra-pure water. These act as a diffusion barrier and protect the stainless steels from the metal dissolution.
Keywords: Raman spectroscopy, fluorescence spectroscopy, stress; _-Al2O3, piezospectroscopic analysis
The addition of Si to Fe-based alloys as a means of enhancing sulphidation resistance in complex mixed atmospheres
J. F. Norton1, M. Maier1 and W.T. Bakker2
1European Commission, Institute for Advanced Materials, JRC, 1755 ZG Petten, The Netherlands
2Electric Power Research Institute, 3412 Hillview Avenue, Palo Alto, CA 94303, USA
Of the three principal oxide-forming elements present in most high temperature engineering alloys (Cr, Al and Si), silicon has been the poor relation, largely as a result of its potentially adverse effect upon fabricability and weldability. However, several advanced heat resistant alloys containing between 2 and 3.5% Si have come onto the market in recent years with reported good resistance to corrosive atmospheres [1–8]. However, these alloys contain high levels of Cr, Ni and/or Co and are therefore relatively expensive. The present study explores the “Si-effect” using a cheaper 12%Cr ferritic, Type 410 class of steel. A commercial alloy, HCM 12, containing 0.2% Si has been selected as a base composition to which Si has been added in amounts ranging from nominally 1 to 4%. Samples of this commercial alloy have been exposed alongside similar specimens machined from specially cast higher-Si model alloys in highly reducing, non-equilibrated CO-based gas mixtures containing 4% CO2, 0–3% H2O and 0.1 and 0.8% H2S, balance H2. These atmospheres simulate those found in a dry-feed entrained slagging coal gasifier where corrosive degradation in the form of sulphidation attack limits operating temperature and therefore plant efficiency. Constructional costs remain an important issue, however, and comparisons are made with the performance of higher alloyed materials, HR 3C and Alloy 800H, exposed at the same time. Initially, experiments were carried out for up to 2,000 hours at 450°C in a 0.8%H2S containing atmosphere. Based upon the findings of these tests, a second series was carried out at a higher temperature, i.e. 550°C, for 1,000 hours in a 0.1%H2S gas mixture. Specimens were subjected to periodic interruptions for weight change measurements and visual inspection with selected samples also being discontinued for more in-depth surface and cross-sectional microstructural studies as well as metal loss measurements. The influence of Si upon corrosion kinetics and in modifying the nature and extent of attack has been established.
Keywords: Si-effect, 12%Cr ferritic steels, coal gasification, reducing gases, sulphidation attack.
Degradation of a SiC/SiC composite in the burner rig: investigation by fractography
Linus U.J.T. Ogbuji
Dynacs Engineering Co., NASA-Glenn Research Center at Lewis Field, Cleveland OH 44135, USA
Oxidation occurs at two levels in a fiber-reinforced composite, the more pernicious form being “internal oxidation”. In SiC/SiC composites a compliant interphase layer of carbon or boron nitride permeates the entire structure, and can serve as conduit for deep ingress of ambient oxidants. Because such insidious (“pest”) attack is not amenable to the usual analysis by measurement of weight change or oxide thickness, it thwarts efforts at routine prediction of service life. However, it leaves its signature in the microstructure. Hence, microscopy provides a useful tool for assessing pest degradation in SiC/SiC composites. This paper summarizes our studies on the Hi-Nicalon/BN/SiC composite, in which various microscopy tools were used to reveal features and disclose mechanisms behind the catastrophic degradation of this material in a burner rig.
Keywords: SiC/SiC composite, burner rig, fractography
Oxide scale damage and spallation in P92 martensitic steel
S. Osgerby
Centre for Materials Measurement and Technology, National Physical Laboratory, Teddington, Middlesex, TW11 0LW, UK
9 Cr martensitic steels are widely used in high temperature steam environments for their combination of creep strength and oxidation resistance. These materials are pushed to their limit in the quest for higher efficiencies in boilers and steam turbines and loss of the protective oxide scale may result in premature failure of critical components. The major cause of oxide scale spallation is a temperature drop. The mechanism behind the spallation process is the generation of internal stresses in the oxide scale due to the mismatch of thermal expansion coefficients between the substrate and the haematite, magnetite and spinel layers in the oxide scale. These scales can be partially relaxed due to creep processes if the cooling rate is sufficiently slow. Specimens of P92 have been oxidised in flowing steam for periods up to 2000 h and cooled at a linear rate of 100°C h–1. Acoustic emission (AE) of the samples was monitored during the oxidation and cooling periods. Specimens were sectioned and prepared metallographically and the damage in each layer of the oxide scale was examined. The spall from the specimens was also collected and characterised. The damage observed in the oxide scale and the AE signals have been explained by comparison with a simple stress analysis arising from thermal expansion mismatch. Future work will extend the modelling approach to incorporate creep relaxation effects.
Keywords: P92 martensitic steel, oxide scale damage
Influence of chromium diffusion and different surface finishes on the oxidation behaviour of chromium steels
C. Piehl, Zs.Toekei and H. J. Grabke
Max-Planck-Institut für Eisenforschung GmbH, Postfach 140 444, D-40074 Düsseldorf, Germany
Email: piehl@mpie-duesseldorf.mpg.de The present contribution is focused on the systematic investigation of the effects of different surface finishes (ground, polished, electropolished) on the oxidation behaviour of chromium steels. The specimens were oxidized in a H2–2.5%H2O atmosphere at 872 K for 1 h to 100 h. Depth profiles were recorded by secondary neutral mass spectrometry (SNMS) to determine the elemental composition of the oxide scale and the diffusion profiles below the scale. The surface finish was found to influence both the thickness of the oxide scale and the depletion of the selectively oxidized elements.
Keywords: chromium diffusion, surface finishes, oxidation behaviour
Phase reactions on surface of nickel-base alloys during tests and service
G.D. Pigrova
Central Boiler and Turbine Institute, 194021, St. Petersburg, Russia
X-ray diffraction analysis has been used to study the oxide phase composition of the surface of nickelbase alloys, which have different alloying levels under air conditions, and in tests with an ash film environment. Several stages of phase reaction and the directivity of phase reactions during oxidation have been revealed. The lattice parameter of the spinel oxide depends on the alloy content (mainly Cr/Al ratio). The surface structure of gas turbine blades after service in power plants and gas compressor stations has been examined. The different types of sulphate, oxide and sulphide compounds formed on the uncoated blade surface have been established. This suggested the development of sulphide-oxide corrosion process. The surface study of MCrAlY-coated blades revealed the formation of different types oxides during operation.
Oxidation behaviour in air of thin alloy 601 fibres
C.Salmon1, D.Tiberghien1, R. Molins2, C. Colin2 and F. Delannay1
1Université catholique de Louvain, Département de sciences des matériaux et des procédés, PCIM, Place Sainte Barbe 2, B-1348, Louvain-la-Neuve, Belgium
2Ecole Nationale Supérieure des Mines de Paris, Centre des Matériaux P-M Fourt, BP87, F-91003 Evry cedex, France
The oxidation behaviour in air of 12 _m diameter continuous alloy 601 fibres has been studied using thermo-gravimetric analysis (TGA) for kinetics identification and transmission electron microscopy (TEM) for determination of the nature of the oxide layers. The TGA allows two stages in the formation of the oxide layer to be distinguished: the first stage corresponds to the growth of a continuous layer of NiO above a discontinuous sub-layer of Cr2O3 whereas the second stage is attributed to the parabolic growth of the Cr2O3 sub-layer, from the time it becomes continuous. A third stage can be observed for high oxidation temperatures. The TEM observations of oxide layers formed after 30 min at 650, 750 and 900°C confirm these results. One common characteristic of these 3 oxidation conditions is the appearance of large cavities under the oxide layer. These cavities seem to be the consequence of the oxidation mechanism of Cr and of the particular morphology of the material (i.e. small diameter cylinders).
Keywords: thin alloy 601 fibres, oxidation kinetics, TEM
Development and application of a methodology for the measurement of corrosion and erosion damage in laboratory, burner rig and plant environments
N.J. Simms, J.E. Oakey and J.R. Nicholls
Power Generation Technology Centre, Cranfield University, Cranfield, Bedfordshire MK43 0AL, UK
Fuel gases derived from solid fuels such as coal, biomass and waste and their mixes have the potential to cause both erosion and corrosion damage to components in gas turbines and diesel engines. To allow the statistically valid assessment of materials performance in short term plant runs, burner rig tests and laboratory simulated environments a methodology has been developed to collect compatible quantitative data on materials degradation. Accurate measurement techniques based on pre-exposure contact metrology and post-exposure optical microscopy/image analysis have been developed. These take into account both the low level of damage required for practical systems and the localised nature of hot corrosion damage. The data produced have been used to derive and test quantitative models for the prediction of the performance of candidate materials in such power systems. For these models to be used with confidence, similar damage morphologies must be produced in both the real and simulated conditions, as well as similar damage rates.
Keywords: materials metrology, high-temperature corrosion, modelling, gas turbine corrosion, co-fired power plant
Observations of the spallation modes in an overlay coating and the corresponding thermal barrier coating system
M.P.Taylor, P. Niranatlumpong, H.E. Evans and C.B. Ponton.
School of Metallurgy and Materials, The University of Birmingham, Edgbaston, Birmingham B15 2TT, UK
The oxidation dynamics of an overlay coating and the corresponding thermal barrier coating system are presented. The particular systems examined are composed of a nickel-based superalloy with an air plasma-sprayed NiCrAlY bond coat and the thermal barrier coating system consists of air plasma sprayed yttria stabilized zirconia layer. Failure can occur in these systems by crack propagation within the ceramic outer layer at the interface with the bond coat. Defects, such as microcracks and pores, are common in plasma-sprayed coatings and within the thermally grown oxide scales. These can act as initiation sites for cracks. The subsequent growth of these cracks can lead to loss of the outer protective materials. Considerable information is available by microscopic examination of sections through test specimens that have been held at temperature for varying amounts of time. By careful sample preparation it is possible to monitor the development of the oxide scale formed during high temperature testing and the sites of failure. Identification of the initiation sites and growth of cracks is important in understanding the spallation process. In this study, scanning electron microscopy is used to provide evidence of the processes involved in the two systems. A comparison of the two coating systems reveals the effect the outer ceramic layer has on the oxide scale growth, and the spallation processes crucial to the understanding of the failure mechanisms of these coating systems.
Keywords: thermal barrier coatings, overlay coating, spallation, thermally grown oxides