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Corrosion resistant alumina-forming alloys for lead-cooled reactors
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Generation IV nuclear power technologies provide attractive solutions to the common issues related to conventional nuclear power plants currently in operation worldwide. Through a significant reduction of the long-term radiotoxicity of nuclear waste, a more efficient use of nuclear fuel resources, and implementation of inherent safety features, Generation IV will make nuclear power sustainable and thus increase the public acceptance of nuclear power. Due to its attractive safety features, the lead-cooled fast reactor (LFR) is one of the most studied Generation IV reactor concepts currently. It is well known that liquid lead is corrosive to steels at elevated temperatures, thus limiting the operation temperature of the LFR. The use of alumina-forming FeCrAl alloys has been proposed to mitigate oxidation and corrosion issues. Commercial FeCrAl alloys have Cr-concentrations typically about 20 wt. % and are thus prone to α-α’ phase separation and embrittlement at temperatures up to about 500 °C. Reducing the Cr-concentration to levels around 10 wt. % would theoretically resolve the said issue. However, the oxidation and corrosion resistance may be impaired. In the scientific literature, compositional limits indicating the formation of protective alumina layers at various temperatures have been presented. Long-term corrosion studies are however scarce. Moreover, in-depth studies on the compositional limits regarding α-α’ phase separation are lacking. In this thesis, the long-term (up to 10,000 h) corrosion resistance and phase stability of alumina-forming alloys are studied at temperatures up to 550 °C. In addition, the influence of reactive elements (RE), e.g. Ti, Zr, and Y, on the liquid lead corrosion resistance of Fe10CrAl alloys is evaluated. By balancing the reactive element and the carbon content, with respect to carbide formation, it is demonstrated in this thesis that it is possible to form protective alumina layers on Fe10Cr4Al alloys from 450 °C, despite the low Al and Cr concentrations. It was found that the RE/carbon ratio needed to form protective alumina layers on Fe10Cr4Al alloys must be larger than unity to mitigate the detrimental effect of Cr-carbide formation.  The underlying phenomena are discussed, and a mechanism is suggested based on the outcome of the long-term oxidation studies. The phase stability of Fe10CrAl alloys was studied through thermal aging experiments in the temperature interval of 450 to 550 °C. In addition, the results were well reproducible using a developed Kinetic Monte Carlo (KMC) model of the FeCrAl system. Furthermore, the model indicated that the Cr-concentration should be limited to about 11 wt. % in a FeCr4Al alloy to mitigate α-α’ phase separation at all temperatures of interest for an LFR. The liquid lead corrosion resistance of alumina-forming austenitic stainless steels was shown to be superior compared to regular stainless steels, albeit the effect of ferrite stabilizing elements needs to be further addressed. The results included in this thesis provide a valuable input on the key issues related to the development of corrosion resistant alumina-forming alloys of interest for liquid lead applications. Moreover, the superior oxidation properties of the studied alumina-forming alloys make them of interest for use in other energy applications, where corrosion issues limits the operation temperature and thus the efficiency.

 

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2015. , xviii, 51 p.
Series
TRITA-CHE-Report, ISSN 1654-1081 ; 2015:1
National Category
Corrosion Engineering
Research subject
Chemistry
Identifiers
URN: urn:nbn:se:kth:diva-157861ISBN: 978-91-7595-345-8 (print)OAI: oai:DiVA.org:kth-157861DiVA: diva2:772477
Public defence
2015-01-12, F3, Lindstedtsvägen 26, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20141217

Available from: 2014-12-17 Created: 2014-12-17 Last updated: 2014-12-18Bibliographically approved
List of papers
1. Oxidation studies of Fe10CrAl-RE alloys exposed to Pb at 550 C for 10,000 h
Open this publication in new window or tab >>Oxidation studies of Fe10CrAl-RE alloys exposed to Pb at 550 C for 10,000 h
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2013 (English)In: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 443, no 1-3, 161-170 p.Article in journal (Refereed) Published
Abstract [en]

Five experimental FeCrAl-RE alloys have been exposed up to 10,000 h in stagnant liquid Pb at 550 C. The test matrix consisted of three 10 wt.% Cr alloys, with an Al content ranging from 4 to 8 wt.% (10Cr-4Al, 10Cr-6Al and 10Cr-8Al), one alloy without additions of reactive elements (RE) (10Cr-6Al), and one reference alloy with 21 wt.% Cr and 5 wt.% Al (21Cr-5Al). The evaluation showed a clear difference in oxidation properties, and it was possible to divide the alloys into two distinct groups. A critical Al concentration in the interval of 4-6 wt.% at the given RE content was required to form a thin protective oxide. However, the absence of RE addition in one of the two 10Cr-6Al alloys resulted in a significant reduction in oxidation resistance, comparable with 10Cr-4Al. None of the alloys were severely corroded, however Pb penetrated to a relatively large extent into the porous oxide of the low performing alloys. A 100 nm thick oxide scale, partly consisting of alumina (Al2O 3), was observed for the high performing 10Cr-6Al alloy. The Fe10CrAl-RE alloys showed overall very good corrosion resistance and are hence a promising new alloy category for liquid Pb applications.

Keyword
Al-concentration, Oxidation properties, Porous oxides, Protective oxides, RE additions, Reactive elements, Test matrix, Thick oxides
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-133892 (URN)10.1016/j.jnucmat.2013.07.023 (DOI)000327905800022 ()2-s2.0-84881143124 (Scopus ID)
Funder
Swedish Research Council
Note

QC 20131111

Available from: 2013-11-11 Created: 2013-11-11 Last updated: 2017-12-06Bibliographically approved
2. Microstructural stability of Fe–Cr–Al alloys at 450–550 °C
Open this publication in new window or tab >>Microstructural stability of Fe–Cr–Al alloys at 450–550 °C
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2015 (English)In: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 457, 291-297 p.Article in journal (Refereed) Published
Abstract [en]

Iron–Chromium–Aluminium (Fe–Cr–Al) alloys have been widely investigated as candidate materials for various nuclear applications. Albeit the excellent corrosion resistance, conventional Fe–Cr–Al alloys suffer from α–α′ phase separation and embrittlement when subjected to temperatures up to 500 °C, due to their high Cr-content. Low-Cr Fe–Cr–Al alloys are anticipated to be embrittlement resistant and provide adequate oxidation properties, yet long-term aging experiments and simulations are lacking in literature. In this study, Fe–10Cr–(4–8)Al alloys and a Fe–21Cr–5Al were thermally aged in the temperature interval of 450–550 °C for times up to 10,000 h, and the microstructures were evaluated mainly using atom probe tomography. In addition, a Kinetic Monte Carlo (KMC) model of the Fe–Cr–Al system was developed. No phase separation was observed in the Fe–10Cr–(4–8)Al alloys, and the developed KMC model yielded results in good agreement with the experimental data.

Keyword
Aluminum, Chromium, Corrosion resistance, Embrittlement, Iron alloys, Phase separation, Atom probe tomography, Candidate materials, Excellent corrosion resistances, Kinetic Monte Carlo modeling, Microstructural stability, Nuclear application, Oxidation properties, Temperature intervals
National Category
Corrosion Engineering Other Materials Engineering
Identifiers
urn:nbn:se:kth:diva-157834 (URN)10.1016/j.jnucmat.2014.11.101 (DOI)000349169100039 ()2-s2.0-84918521359 (Scopus ID)
Funder
Swedish Research Council
Note

QC 20141217

Available from: 2014-12-17 Created: 2014-12-17 Last updated: 2017-12-05Bibliographically approved
3. Long term corrosion resistance of alumina forming austenitic stainless steels in liquid lead
Open this publication in new window or tab >>Long term corrosion resistance of alumina forming austenitic stainless steels in liquid lead
2015 (English)In: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 461, 164-170 p.Article in journal (Refereed) Published
Abstract [en]

Alumina forming austenitic steels (AFA) and commercial stainless steels have been exposed in liquid lead with 10-7 wt.% oxygen at 550 °C for up to one year. It is known that chromia forming austenitic stainless steels, such as 316L and 15-15 Ti, have difficulties forming protective oxides in liquid lead at temperatures above 500°C, which is confirmed in this study. By adding Al to austenitic steels, it is in general terms possible to increase the corrosion resistance. However this study shows that the high Ni containing AFA alloys are attacked by the liquid lead, i.e. dissolution attack occurs. By lowering the Ni content in AFA alloys, it is possible to achieve excellent oxidation properties in liquid lead. Following further optimization of the microstructural properties, low Ni AFA alloys may represent a promising future structural steel for lead cooled reactors.

Keyword
Alloy steel, Alloys, Alumina, Austenite, Austenitic stainless steel, Austenitic steel, Building materials, Corrosion, Corrosion resistance, Nickel, Titanium oxides, Chromia, Lead-cooled reactor, Liquid lead, Micro-structural properties, Ni content, Oxidation properties, Protective oxides, Structural steels
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-157859 (URN)10.1016/j.jnucmat.2015.03.011 (DOI)000355023900021 ()2-s2.0-84925355830 (Scopus ID)
Note

QC 20150522. Updated from manuscript to article in journal.

Available from: 2014-12-17 Created: 2014-12-17 Last updated: 2017-12-05Bibliographically approved
4. Optimizing the oxidation properties of FeCrAl alloys at low temperatures
Open this publication in new window or tab >>Optimizing the oxidation properties of FeCrAl alloys at low temperatures
(English)Manuscript (preprint) (Other academic)
Identifiers
urn:nbn:se:kth:diva-157860 (URN)
Note

QS 2014

Available from: 2014-12-17 Created: 2014-12-17 Last updated: 2014-12-17Bibliographically approved

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