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High Temperature Oxidation of Iron-Chromium Alloys

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High Temperature Oxidation of Iron-Chromium Alloys RIS0 High Temperature Oxidation of Iron- Chromium Alloys Lars Mikkelsen Ris0-PhD-2(EN) Ris0 National Laboratory Roskilde, Denmark June 2003 Author: Lars Mikkelsen Ris0~PhD~2(EN) June 2003 Title: High Temperature Oxidation of Iron-Cliromium Alloys Department: Fuel Cells and Solid State Chemistry Department This thesis is submitted in partial fulfilment of the requirements for the Ph D. degree at University of Southern Demnark Abstract (max. 2000 char.): ISBN 87-550-3353-9 See page 3 Contract no.: Group's ownreg. no.: Sponsorship: Cover : Pages: 170 Tables: References: Information Service Department Riso National Laboratory P.O.Box 49 DK-4000 Roskilde Denmark Telephone +45 46774004 biblfgirisoe.dk Fax +45 46774013 www.risoe.dk Preface This thesis is part of the fulfilment of the requirements for the Ph.D. degree at University of Southern Denmark. The majority of the work is based on studies performed at the SOFC group at the Materials Research Department, Riso National Laboratory. Part of the studies was carried out at the Physics Department, and at the Department of Chemistry, University of Southern Denmark. The work was carried out under the supervision of Eivind M. Skou, Department of Chemistry, University of Southern Denmark, and Soren Linderoth, Peter H. Larsen and Mogens Mogensen, Materials Research Department, Riso National Laboratory. The work was initiated in February 2000 and completed in June 2003 with the submission of the present thesis. Lars Mikkelsen Riso National Laboratory, June 2003 Acknowledgement During the work, I have received much help through fruitful discussions and help during the experimental stages. At the Physics Department, University of Southern Denmark, I would like to thank Per Morgen for providing equipment and for much help performing the Auger electron spectroscopy measurements. I would like to thank John Nicholls, Cranfield University, for good advise on preparing fractured specimens for the use in electron microscopy. Thanks to J0rgen Bilde-Serensen for much help with the electron microscopy, especially the TEM work, which could not have been done without his help. Thanks to Anette Hansson, Peter Friehling and to Charles Hatchwell for many good discussions during the present work and for many good hours. Also thanks to the many colleagues at the SOFC group for helping with many small things, and for making the stay at Ris0 a pleasant experience. Finally, thanks to my supervisors S0ren Linderoth, Eivind M. Skou, Mogens Mogensen and Peter H. Larsen for many good discussions during the work. 2 Abstract The high temperature oxidation of the ferritic alloy Fe78 Cr22 has been investigated in the present work. The effect of small alloying additions of cerium and/or silicon was also investigated. The alloys were oxidized at 973, 1173 and 1373 K in either air or a hydrogen/argon mixture. The various reaction atmospheres contained between 0.02 and 50% water vapour. The oxide scales formed on the various alloys at 973 K consisted of thin chromia layers. The oxide scales grown on the alloys at 1173 K also consisted of a chromia layer. The microstructure of the chromia scales was found to depend on the reaction atmosphere. The chromia scales grown in hydrogen/argon atmospheres formed oxide whiskers and oxide ridges at the surface of the scales, while the chromia scales grown in air formed larger oxide grains near the surface. This difference in oxide microstructure was due to the vaporization of chromium species from the chromia scales grown in air. Two different growth mechanisms are proposed for the growth of oxide whiskers. The growth rate of the chromia scales was independent of the oxygen activity. This is explained by a growth mechanism of the chromia scales, where the growth is governed by the diffusion of interstitial chromium. The addition of silicon to the iron-chromium alloy resulted in the formation of silica particles beneath the chromia scale. The presence of silicon in the alloy was found to decrease the growth rate of the chromia scale. This is explained by a blocking mechanism, where the silica particles beneath the chromia scale partly block the outwards diffusion of chromium from the alloy to the chromia scale. The addition of cerium to the iron-chromium alloy improved the adhesion of the chromia scale to the alloy and decreased the growth rate of chromia. It was observed that the minimum concentration of cerium in the alloy should be 0.3 at.% in order to observe an effect of the cerium addition. The effect of cerium is explained by the “reactive element effect”. The oxide scales grown on the various alloys at 1373 K exhibited the presence of thick iron rich oxides in the scales. The presence of iron rich scales is described as a result of failure of the chromia scale and subsequent breakaway oxidation of the alloy. The breakaway oxidation was initiated at the sample edges. Furthermore, the breakaway oxidation was affected by the oxygen activity in the reaction atmosphere, where a low oxygen activity protected the alloys from breakaway oxidation. A mechanism for the breakaway oxidation is put forward, which explains the observed oxidation behaviour of the alloys. The effect of water vapour on the chromia growth rate was investigated. It was found that the presence of water vapour in the reaction atmosphere increased the growth rate of chromia. It is proposed that hydrogen dissolves in the chromia scale and thereby increases the transport of reactants through the chromia scale. Furthermore, the growth rate of chromia was observed to decrease, when the alloys were oxidized in air containing large amounts of water vapour. This is a result of vaporization of chromium containing species from the chromia scale during the oxidation. The effect 3 of water vapour was studied through thermogravimetric measurements in various reaction atmospheres. The measurements were partly performed on an equipment developed during this Ph.D.-study. The development was performed through a collaboration with Netzsch. The development of the novel equipment is described in this thesis. 4 Resume Hej-temperatur oxidationen af en ferritisk legering Fe78 Cr22 er undersegt i dette arbejde. Oxidationsegenskaberne af legeringen er ogsa undersegt efter en lille legeringstils$tning af silicium og/eller cerium. Legeringerne blev oxideret ved 973, 1173 og 1373 K i luft eller i en hydrogen/argon blanding. De forskellige reaktions gasser indeholdt mellem 0.02 og 50% vand damp. Under oxidationen af de forskellige legeringer ved 973 K blev der dannet et oxidlag bestaende af krom(III)oxid. Under oxidation af legeringerne ved 1173 K blev der ogsa dannet et lag bestaende af krom(III)oxid. Mikrostrukturen af kromoxidlaget afhang af reaktionsgassen. Overfladen af kromoxidlaget bestod af oxidwhiskers og oxidhejderygge nar oxidet blev dannet i hydrogen/argon blandinger, mens overfladen af kromoxidlaget bestod af store oxidkorn nar oxidlaget blev dannet i luft. Denne forskel skyldes afdampning af kromholdige forbindelser fra oxidlaget nar det blev dannet i luft. To forskellige v$kstmekanismer freml$gges til at forklare dannelsen af oxidwhiskers. Dannelseshastigheden for kromoxidlagene er uafh$ngigt af oxygen partialtrykket. Dette er forklaret ved hj$lp af en v$kstmekanisme for oxidlaget, hvor v$ksten er bestemt af diffusionen af interstitielle krom. Tils$tningen af silicium til jern-krom legeringen resulterede i dannelsen af siliciumoxidpartikler under kromoxidlaget. Tilstedev$relsen af silicium i legeringen nedsatte v$ksthastigheden af kromoxidlaget. Dette forklares ved hj$lp af en blokerings mekanisme, hvor siliciumoxidpartiklerne under kromoxidlaget delvist blokerer den udadgaende diffusion af krom fra legeringen til kromoxidlaget. Tils$tningen af cerium til jern-krom legeringen forbedrede vedh$ftningen af kromoxidlaget til legeringen og nedsatte v$ksthastigheden af kromoxidlaget. Koncentration af cerium i legeringen skal dog v$re mindst 0.3 at.% for at iagttage en fordelagtig effekt af cerium. Effekten af cerium forklares ved hj$lp af ’’reactive element ” effekten. Under oxidation af legeringerne ved 1373 K blev der dannet et tykt oxidlag bestaende af jernholdige oxider. Tilstedev$relsen af jernholdige oxider forklares ved brud af kromoxidlaget. Dette resulterer i ’breakaway ” oxidation. Breakaway oxidationen startede omkring kanterne af praverne. Breakaway oxidationen afhang af oxygen partialtrykket i reaktionsgassen, hvor et lavt oxygen partialtryk beskyttede mod breakaway oxidation. En mekanisme for breakaway oxidationen freml$gges, som forklarer de observerede oxidationsresultater. Effekten af vandindholdet pa oxidationshastigheden af kromoxidlaget er undersegt. Tilstedev$relsen af vanddamp i reaktionsgassen foragede oxidationshastigheden af kromoxidlaget. Det foreslas at hydrogen opleses i kromoxidlaget og derved foreger transporten af reaktanter gennem kromoxidlaget. Oxidationshastigheden af kromoxidlaget aftog nar legeringerne blev oxideret i luft, som indeholdt store m$ngder vanddamp. Dette skyldes afdampning af kromholdige forbindelser fra kromoxidlaget under oxidationen. Effekten af vanddamp blev studeret ved hj$lp af termogravimetriske malinger i forskellige reaktionsgasser. En del af malingerne blev 5 udf0rt pa et eksperimentelt udstyr, som blev udviklet gennem dette Ph.D.-studie. Udviklingen af udstyret blev gennemf0rt i et samarbejde med Netzsch. Udviklingen af det nye udstyr er beskrevet i denne afhandling. 6 List of common symbols to Chemical potential ni Electrochemical potential K Boltzmann constant a; Activity
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