A study of the oxidation behaviour of FeCrAl-ODS in air and steam environments up to 1400 °C

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Abstract

As a candidate accident tolerant fuel cladding material, the high temperature oxidation behaviour of an Oxygen Dispersion Strengthened (ODS) Fe–12Cr–6Al alloy has been examined in isothermal air and steam environments. While the oxidation behaviour of FeCrAl has been extensively studied, very little has been reported in the literature on FeCrAl-ODS, which is the focus of this manuscript. By conducting Thermogravimetric Analysis (TGA), oxidation experiments were performed on specimens of Fe–12Cr–6Al-ODS at temperatures up to 1400 °C in air and up to 1215 °C in high purity steam. Oxidation kinetics were evaluated by calculating reaction rate constants and activation energies. The formation of an alumina scale was correlated to kinetics results from TGA by coupling Scanning Electron Microscopy (SEM) with Energy Dispersive X-ray Spectroscopy (EDS) line scans to investigate the chemical composition of the primary oxide layer, local oxidation effects, as well as the bulk alloy material. Electron Probe Micro-Analysis (EPMA) point scans were applied to the heavily oxidized specimen to achieve an accurate composition profiling of areas of interest. Finally, thermodynamic calculations were performed to interpret the formation of the complex oxide layer. These experiments support research efforts for the development of accident tolerant fuel technologies at Nippon Nuclear Fuel Development Co., Ltd.

Introduction

Due to the unfortunate events that occurred at the Fukushima Daiichi Nuclear Power Plant in 2011, the international nuclear community has put forth a concerted effort in developing Accident Tolerant Fuel (ATF) technologies to provide resistance to severe core damage in Beyond-Design-Basis (BDB) events. Conventional Zr-based alloys currently employed as cladding materials in commercial operation have demonstrated adequate corrosion resistance under normal operating conditions; however, aggressive oxidation and exothermic heat generation can occur in a high-temperature steam environment under BDB conditions. Various research and development programs in the international community are investigating ATF cladding technologies, as generalized by Pint et al. in Fig. 1 [1]. Three ATF candidate technologies have been exemplified: coated Zr-based alloys, FeCrAl alloys, and SiC/SiC ceramics [2].

As recognized by Pint et al. a central criterion for the design of ATF cladding has been established: exhibiting more than one hundred times slower oxidation kinetics than Zr-based materials [1]. Fig. 1 illustrates that the parabolic oxidation rate constant (kp) of FeCrAl is generally three to four orders of magnitude smaller than Zr-alloys [1]. The primary interest for the development of FeCrAl alloys is the improved oxidation kinetics as a result of the growth of a passive alumina (Al2O3) film. Exploiting the reported behaviour of Al2O3 to exhibit superior oxidation stability and kinetics at high temperatures, FeCrAl is currently under concerted investigation as a promising ATF candidate technology by the research community and industry [[3], [4], [5]]. For example, as a part of Global Nuclear Fuels (GNF) lead test assemblies, non-fuelled FeCrAl elements have been inserted in the Edwin I Hatch plant in Georgia, USA for testing [6]. As a result of lowering the oxidation rate with the substitution of conventional Zr-alloys by FeCrAl alloys, the rate of heat released and hydrogen generation in Light Water Reactor (LWR) cores during a severe accident can be reduced [2]; thus, safety margins can potentially be increased with the implementation of FeCrAl.1

As summarized by Israelsson, the oxidation behaviour of alloy materials is subject to the coupling of underlying thermodynamics, which drives oxide stability criteria of each alloyed element, and the availability of transport mechanisms for oxide elements and crystal defects, driving the kinetics of oxidation [5]. However, it is well understood that a number of material structure properties are thermally dependent, such as grain size coarsening, particle size ripening, grain boundary and dislocation transport, and atomic diffusion. Since crystal structure defects and transport phenomena play a significant role in oxidation kinetics, it is clear that the kinetics are innately related to the temperature and thermal history of the alloy.

With respect to FeCrAl, high temperature steady-state oxidation is achieved by the passive formation of (α-Al2O3), which exhibits a low defect density, and subsequently reduces its oxidation kinetics with a depleted transport path for diffusion of oxidation reaction constituents. Limited by diffusion, high temperature steady-state oxidation of FeCrAl is parabolic in nature according to Al2O3 scale formation [3,5,7]. Numerous compositions of FeCrAl with varying Cr and Al contents have previously been studied with the goal of improving cladding steam oxidation tolerance while maintaining reasonable mechanical and irradiation compliance [1,3,4]. Along with other FeCrAl alloy enhancements, the introduction of dispersed oxide particles within FeCrAl alloys is also being investigated as an improvement to its mechanical and/or oxidation properties [8]. By utilizing strengthening mechanisms, such as Orowan bowing or particle shearing through oxide dispersion, alloy strengthening can potentially be achieved. However, unlike similar techniques (e.g., precipitate hardening) that are solubility (temperature) dependent, dispersion strengthening is uniquely effective at high temperatures. While thermo-oxidation of FeCrAl has been extensively studied, there is limited information reported in the open literature on FeCrAl-ODS, which is the focus of this article. The dispersion strengthened FeCrAl alloy studied in this work included the addition of Y2O3 particles into the bulk FeCrAl material [8,9]. Furthermore, by improving the selective oxidation of Al2O3, improving grain growth resistance, and reducing the diffusion of ions (e.g., Al3+), the addition of reactive elements such as Nb, Ti, Y, and Zr has been shown to improve both mechanical and oxidation properties of the material [5,8,10].

The oxidation behaviour of an oxide dispersion-free FeCrAl alloy (Aluminium Powder Metallurgy (APM)) and a FeCrAl-ODS alloy (MA 956) has been compared by Merceron et al. between 1200 °C and 1300 °C in air [11]. As Merceron et al. demonstrate, both alloys exhibited alumina scale formation under short-term (40 h) oxidation; however, the morphology of the scale was different [11]. The oxide layer formed on the ODS alloy was reported to exhibit an Al2O3 layer that was compact, regular, and with no gap between the bulk material and the oxide layer [11]. On the contrary, the APM FeCrAl alloy appeared to be highly irregular and separated from the bulk material [11]. Oxide adherence plays an important role in oxidation kinetics, as the passivation layer at the metal-oxide interface acts as a diffusion barrier preventing the bulk material from being further exposed to the oxidation environment after the layer has been formed [11]. As noted by Merceron et al. oxide film adhesion is compromised by sulphur segregation at the metal-oxide interface [11]. As Merceron et al. and Israelsson suggested, the presence of reactive elements improves oxide layer adhesion and reduces the possibility of spallation at the metal-oxide interface by acting as a sulphur getter and reducing sulphur diffusion at the interface [5,11].

Efforts have also been made to understand the influence of water moisture on oxidation behaviour of FeCrAl [1,[12], [13], [14], [15]]. The general consensus is that only small differences in oxidation kinetics of FeCrAl alloys are found when subjecting the alloy to steam compared to dry air [15]. Unocic et al. reported that scales formed in steam environments have been observed to consist of outer regions higher in Cr and Fe concentrations than those formed in air [15].

This work studied high temperature oxidation kinetics of the Fe–12Cr–6Al-ODS alloy in high purity steam and dry air by employing Thermo-Gravimetric Analysis (TGA) to provide data for calculating the Arrhenius reaction rate constants and associated oxidation activation energy in each environment. The advantage of this approach is that the mass change over time is measured under precisely controlled experimental conditions, resulting in high confidence in the obtained results. Although, a similar kp determination of this alloy has been reported by one of the authors, K. Sakamoto, in a paper presented at the 2017 Water Reactor Fuel Performance Meeting, experiments were only performed in wet argon [16]. There is no reported information on the behaviour of this alloy in air at high temperature nor on composition of the oxide layer in either air or steam environments. That is one of the driving forces of this article. The interest of investigating the oxidation behaviour in both environments is justified by the different chemical composition of the outer layer, as reported by Unocic et al. [15]. Following TGA, the structure and integrity of the cross-section of oxidized specimens were inspected by Scanning Electron Microscopy (SEM) and characterized with Energy Dispersive X-ray Spectroscopy (EDS) for compositional analysis. Materials preparation and experimental design are described in §2, while experimental results are presented and discussed in §3.

Section snippets

Material sample and specimens preparation

Polished specimens of Fe–12Cr–6Al-ODS were provided by Nippon Nuclear Fuel Development Co. Ltd. in the form of 10 mm × 10 mm x 1.2 mm coupons. These specific dimensions were chosen to conform to the crucible dimensions of the TGA instrument. The Fe–12Cr–6Al-ODS alloy was fabricated through processes of mechanical alloying and hot extrusion. The Fe–12Cr–6Al-0.5Ti atomized powders and the Y2O3, Zr, and Fe2O3 powders were mechanically alloyed in Ar and packed into steel capsules. The capsules then

Thermogravimetric Analysis

Weight gain measurements of the Fe-12r-6Al-ODS specimens oxidized in air are shown in Fig. 2a, which indicate the different layer growth rates at 1200 °C, 1350 °C, and 1400 °C. Fig. 2b presents weight gain measurements of the Fe–12Cr–6Al-ODS specimens at 1115 °C and 1215 °C in steam.

The abrupt weight gain observed at the beginning of the oxidation process in both environments is likely due to the formation of the protective oxide layer on all specimens. The weight gain continued at a steady

Conclusions

High temperature oxidation kinetics experiments of Fe–12Cr–6Al-ODS specimens in air and steam were performed with a TGA, followed by characterization of selected specimens with SEM/EDS and EPMA. Mass gain of the specimens in both environments increasing at parabolic rate due to the formation of protective alumina layers at elevated temperatures. The moisture content was found to play a role at the early stage of oxidation, which led to earlier formation of the alumina scale at 1215 °C in steam.

Author’s contribution

K. Lipkina: TGA measurements, SEM/EDS and EPMA measurements, data curation, writing. D. Hallatt: Established TGA processes, writing. E. Geiger: SEM/EDS and EPMA measurements, thermodynamic modelling, writing. B.W.N. Fitzpatrick: Quality assurance. K. Sakamoto: Sample preparation, technical guidance. H. Shibata: Sample preparation. M.H.A. Piro: Conceptualization, writing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This research was undertaken, in part, thanks to funding from the Canada Research Chairs program of the Natural Sciences and Engineering Research Council of Canada, grant # 950-231328. Funding for equipment from the Canadian Foundation for Innovation, grant # 35712, the Ontario Ministry of Economic Development, Job Creation Trade, grant # 35712, and Ontario Power Generation, through the Ontario Tech University Research Infrastructure Fund are greatly acknowledged. Also, Emily C. Corcoran from

References (25)

  • GNF Lead Test Assemblies Ready for Loading - World Nuclear News

    (2019)
  • O. Albina Dionel

    Theory and Experience on Corrosion of Waterwall and Superheated Tubes of Waste-To-Energy Facilities. P.Eng. In Metallurgical Engineering

    (2005)
  • Cited by (0)

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