DOI:10.14311/APP.2020.28.0008 Acta Polytechnica CTU Proceedings 28:8–14, 2020 © Czech Technical University in Prague, 2020 available online at http://ojs.cvut.cz/ojs/index.php/app HIGH-TEMPERATURE OXIDATION OF CHROME-NICKEL ALLOY Adéla Chalupováa,c,∗, Martin Steinbrückb, Mirco Grosseb, Jakub Krejčíc, Martin Ševečeka a Czech Technical University in Prague, Faculty of Nuclear Sciences and Physical Engineering, Department of Nuclear Reactors, V Holešovickách 2, 180 00 Prague 8, Czech Republic b Karlsruhe Institute of Technology, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany c UJP PRAHA a.s., Nad Kamínkou 1345, Praha-Zbraslav, 15610, Czech Republic ∗ corresponding author: chalupova@ujp.cz Abstract. The investigations in this paper deal with the Cr-Ni alloy. The material has been recently proposed as a potential ATF concept, primarily due to its behaviour under high-temperature oxidation. A set of experiments to determine the melting point and describe the oxidation kinetics of the Cr-Ni alloy were performed in Karlsruhe Institute of Technology. Presented results reveal its superb oxidation resistance comparing to zirconium alloys. Therefore, the alloy has a great potential for nuclear applications. Keywords: ATF, Cr-Ni, fuel cladding, high-temperature oxidation, LOCA, nuclear fuel. 1. Introduction Zr-based cladding has been used in light water re- actors for many decades. It has been chosen for nu- clear applications due to its low thermal neutron cap- ture cross-section together with good corrosion be- haviour under normal operation conditions. Yet, in high-temperature steam, such as might occur under severe accident conditions, a rapid increase in oxida- tion kinetics is observed. In response to the events in Three Mile Island (1979) and more recently in Fukushima Dai-ichi (2011), the development and de- ployment of advanced accident tolerant fuel (ATF) became of worldwide interest [1]. One of the possible approaches to achieve higher safety during severe accident scenarios is to replace the current zirconium alloys with new materials with higher oxidation resistance and lower heat and hy- drogen production. An alloy similar to 42XHM is considered as a promising material in the develop- ment of ATF. The 42XHM alloy is highly resistant to water vapour and it almost does not undergo to low-temperature embrittlement. The alloy has been already used as the structural material of absorbing rods in WWER-1000 reactors and as the fuel cladding in nuclear-powered ships and submarines. The previ- ous experiments on 42XHM alloy were presented in [2], [3]. Nevertheless, in comparison to the Zr-based cladding, the Cr-Ni has a large thermal neutron ab- sorption cross-section. Several solutions to counter- balance the absorption have been proposed. The strength of the alloy allows thinner cladding; hence the pellets can be slightly larger to give the same cold gap width in the rod. Still, even with a thinner cladding wall, it appears necessary to increase the fuel enrichment beyond the current 5% limit. An- other way to compensate for the high thermal neu- tron absorption is the implementation of high-density fuel instead of UO2 ceramics. To increase the capac- ity of uranium, it has been proposed to use heavily doped uranium-molybdenum fuel as well as modified uranium silicide fuel or a dispersion type fuel [4]. The melting point and high-temperature oxidation of the Cr-Ni alloy were studied and the results are presented. 2. Methods 2.1. Material The chemical composition of the Cr-Ni alloy used for these experiments is summarized in Table 1. Seg- ments were cut from non-irradiated longer tubes, de- burred, ground, and cleaned in acetone. The ultra- sonically cleaned specimen was measured using an an- alytical balance with a resolution of 0.0001 g and a calliper with a resolution of 0.01 mm. As the defor- mation is not isotropic, the length was measured at 3 locations of the specimen and the final length is the average of these three measurements. The geometry of the tested specimens was cylindrical with a height of 20 mm, an outer diameter of 8.3 mm, and an in- ner diameter of 7.1 mm. Before each experiment, the specimen was weighed in a balance with a resolution of 0.1 mg to determine the mass gain during the ex- position and to compare it with data obtained from TG. 8 http://dx.doi.org/10.14311/APP.2020.28.0008 http://ojs.cvut.cz/ojs/index.php/app vol. 28/2020 High-temperature oxidation of chrome-nickel alloy Element Cr Mo Ti Ni wt.% 42 1-1.5 0.25 Balance Table 1. Chemical composition of Cr-Ni alloy 2.2. Differential thermal analysis Differential thermal analysis (DTA) is a thermoana- lytic technique to analyse a change in properties of a sample, i.e., melting, decomposition, or changes in the crystal structure. In DTA, the temperature of a sample is compared with the reference material. The reactions are typically graphed from an arbitrary baseline as a function of temperature. Exothermic reactions per definition show upward curves and en- dothermic reactions downward curves from the base- line. The DTA analysis was performed to determine the accurate melting point of the Cr-Ni alloy. Thermal balance, particularly a simultaneous ther- mal analysis facility, STA 449 F3 Jupiter Eco from NETZCH company, allowing the measurement of mass changes and thermal effects, was used to per- form the differential thermal analysis. Contrary to most STAs, the balance does not use the external thermostat and the temperature of the chamber is regulated electronically. The sample of Cr-Ni alloy and the reference alu- mina powder (Al2O3) were weighted and placed sym- metrically in the STA facility. The furnace was pro- grammed to heat up to 1450°C (30 K/min) and subse- quently to cool down to 100°C (100 K/min). During the process, a differential thermocouple was set to detect the temperature difference between the sam- ple and the reference powder. The experiment was carried out in argon atmosphere to prevent oxidation reactions. 2.3. Thermo-gravimetric experiment The most common method of measuring oxidation rate is the observation of oxide accumulation in time. The gravimetric measurements in this study were performed continuously. The experimental facility consisted of a thermal balance STA 449 F3 Jupiter from NETZSCH company, the steam generator from Bronkhorst Company, and a quadrupole mass spec- trometer NETZSCH QMS 403 C. The facility STA 449 F3 Jupiter presents the older configuration com- pared to the STA mentioned before. The weighting chamber’s temperature is regulated by an external thermostat. The measured specimen was placed into the fur- nace. An alumina plate was used to support the specimen, as shown in Figure 1. First, the furnace was sealed and purged with argon for 10 minutes to remove the atmospheric gases. All experiments were performed with a flow rate of pure steam of 2 g/h. Additionally, 50 ml/min of argon was injected in the balance volume as a protective and carrier gas. Con- sidering the given flow rates and the molar volume Figure 1. Specimen before exposition under high temperature steam oxidation 0 200 400 600 800 1000 1200 1400 0 10000 20000 30000 40000 50000 60000 70000 80000 90000 Te m pe ra tu re [° C] Time [s] → 20 hours ↘ 30 K/min Ar: 50 ml/s H2O: 2 g/h Ar: 50 ml/s Ar: 50 ml/s ↗ 10 K/min Figure 2. Isothermal experiment of argon 22.4 dm3, the resulting steam molar con- centration in the fluid at the outlet was 45%. Due to the special construction of the furnace, the steam concentration of the sample was over 95%. Oxida- tion occurred on both sides. Generally, two varieties of the experiment were performed – isothermal tests with 20 hours annealing time and a transient test with a heating rate of 5 – 10 °C/min. To determine the oxidation kinetics of the Cr-Ni alloy, a set of isothermal experiments was performed. The course of an experiment is shown in Figure 2. The test was initiated at a temperature of 90 °C. The specimen was subsequently heated up to the desired temperature with a heating rate of 10 K/min. The 10-minute long isothermal phase followed to homog- enize the temperature and to adjust the vapour gen- eration. The subsequent temperature plateau with vapour supply took 20 hours. Finally, the specimen 9 A. Chalupová, M. Steinbrück, M. Grosse et al. Acta Polytechnica CTU Proceedings 0 200 400 600 800 1000 1200 1400 0 2000 4000 6000 8000 10000 12000 14000 16000 Te m pe ra tu re [° C] Time [s] ↗ 5 K/min ↘ 30 K/min ↗ 10 K/min → 10 min Ar: 50 ml/s Ar: 50 ml/s H2O: 2 g/hAr: 50 ml/s Figure 3. The transient experiment was cooled down to 100 °C with the cooling rate of 5 K/min. The isothermal experiments were performed at the temperatures of 700 °C, 800 °C, 900 °C, 1000 °C, 1100 °C and 1200 °C. To confirm the change in kinetics, the transient experiment was performed. The course of the ex- periment is summarized in Figure 3. The test was initiated at 90 °C. Subsequently, the specimen was heated with the heating rate of 10 K/min up to 500 °C. The isothermal period at the temperature of 500 °C followed for 10 minutes. Afterwards, the constant steam flow of 2 g/h was initiated, and the specimen continued to heat up to 1240 °C with the heating rate of 5 K/min. Finally, the specimen was cooled down to the temperature of 100 °C with the cooling rate of 30 K/min in argon. 2.4. Post-test examination The thermogravimetric analysis and mass spec- troscopy were measured online during the experi- ment. Eventually, the obtained data were analysed and the expected corrosion rate was calculated based on the specific oxidation behaviour, by means of parabolic or linear kinetics. The specimens were visually analysed to detect any relevant changes in appearance, i.e., the colour of the oxide, macroscopic dissolution, cracking, or spalling. To perform further analysis, it was necessary to cut the specimens into several rings. The metallo- graphic cross-sections were prepared using the stan- dard polishing procedure and metallographic evalua- tion followed. The thickness of the oxide scale and the wall were measured. Additionally, the specimen’s surface was treated with Au before scanning electron microscopy (SEM) analysis. A SEM is an electron microscopy technique that produces images of a sample by scanning the sur- face with a focused beam of electrons. The electrons interact with atoms in the sample, producing vari- ous signals that contain information about the surface topography and composition of the sample. The re- ceived signals are collected and examined. Specific X- ray energies or wavelengths are selected and counted Figure 4. The determined melting point and the solidification point of Cr-Ni alloy via energy-dispersive X-ray spectroscopy (EDS) for elemental analyses. The surface and under-surface morphologies of Cr- Ni alloy after oxidation at 1200 °C in steam for 20 hours were analysed using SEM/EDS. Chemical ele- ments present in the specimen were determined qual- itatively. 3. Results 3.1. The melting point determination During DTA, the transition temperatures were deter- mined. The DTA graph in Figure 4 visualizes DTA in relation to temperature. The nature of the re- action was derived from the peak orientation. The melting causes an endothermic reaction, the onset of the signal was determined at 1356.7 °C. The solidifi- cation causes an exothermic reaction. According to the graph, it was initiated at a lower temperature of 1355.8 °C. 3.2. High-temperature oxidation During oxidation, the water molecules are adsorbed on the surface of the metal, they dissociate, and the oxygen diffuses into the metal. After reaching the sol- ubility limit of oxygen in the metal, the oxide scale is formed and thickens at the expense of the metal. Generally, the rate of thickening determines the oxi- dation kinetics [5] [6]. The kinetics of oxidation can be described by equa- tion (1). WG S = km · tn (1) where WG is the weight gain of the specimen, S is the surface of the sample, km is the rate constant, and t is the oxidation time of the isothermal experiment. The weight gain per unit area in relation to time for isothermal experiments in steam at various tem- peratures is shown in Figure 5. There are two major trends. The weight gain of the alloy under oxidation 10 vol. 28/2020 High-temperature oxidation of chrome-nickel alloy Figure 5. Oxidation kinetics of Cr-Ni at 700 °C, 800 °C, 900 °C, 1000 °C, 1100 °C and 1200 °C in steam. The dot curves indicate a fitting relationship between weight gain over an area unit and oxidation time of 20 hours Temperature [°C] Time exponent n 700 1.03369 800 0.92483 900 0.82321 1000 0.47945 1100 0.33144 1200 0.41488 Table 2. Time exponents of the oxidation kinetics of Cr-Ni alloy at various temperatures temperatures between 700 °C and 900 °C is rather lin- ear. Above 1000 °C the curve is more likely parabolic, yet not precisely. The curves in Figure 5 show the fit- ting results of weight gain per unit area in relation to time. Time exponents from (1) are summarized in Table 2 for all experimental temperatures. Figure 6 shows the hydrogen concentration in the off-gas during the isothermal experiment at 1200 °C as a function of time. Considering that the steam molar concentration in the fluid is 45%, the hydrogen release rates measured by the mass spectrometer are proportional to WG/S and in good correspondence to the calculation. At the early stage of oxidation, the hydrogen release is increased due to the faster oxida- tion rate. The subsequent stabilization in oxidation rate can be attributed to the protective oxide scale formation. Figure 7 presents the general posttest appearance after isothermal oxidation in steam at 700 °C, 800 °C, 900 °C, 1000 °C, 1100 °C and 1200 °C for 20 hours and after oxidation at a temperature transient with the maximum temperature of, 1250 °C. All spec- imens showed a compact structure. They differ in colour and degree of degradation. Apparently, the oxide scale changes from green monotone grey to monochrome grey with a visible pattern. Despite the compact structure of the specimens, at temperatures Figure 6. Hydrogen content in time during the isothermal experiment at the temperature of 1200 °C Figure 7. Post-test appearance of Cr-Ni after oxi- dation at temperature transient for 150 minutes and after oxidation in steam at 6 different temperatures: 700 °C, 800 °C, 900 °C, 1000 °C, 1100 °C, 1200 °C for 20 hours of 1100 °C and 1200 °C, a fine powder falling from the specimens’ surface to the plate crucible was observed. The metallographic image in Figure 8 gives a closer view of the structure of the specimens. At a first glance, the specimens show discontinuous porous ox- ide scale. Thus, it appears unprotective. On the contrary, the oxidation kinetics remained in between parabolic and cubic (n=0.33-0.48) with no evidence of 11 A. Chalupová, M. Steinbrück, M. Grosse et al. Acta Polytechnica CTU Proceedings Figure 8. Microstructure of specimen cross sections after isothermal oxidation at 1100 °C in steam for 20 hours kinetics transition. A possible explanation could be the delamination of the oxide scale during the cool- down period and breaking of the hard oxide during the metallographic preparation of the samples. This fact is supported by the presence of fine powder in the TG balance volume after the experiment. Micrographs in Figure 9 to Figure 11 present SEM analysis of the surface and under-surface morpholo- gies of the Cr-Ni alloy after oxidation at 1200 °C in steam for 20 hours with corresponding EDS measure- ments. The SEM/EDS image in Figure 9 identified the discontinuous surface layer of Cr2O3. The carbon and Au impurities are detected due to the polishing procedure and surface treatment before SEM/EDS. EDS mapping in Figure 10 shows Ni-Cr morphology in the specimen’s wall. Isolated Ti-enriched islands were occasionally observed by EDS, as shown in Fig- ure 11. 4. Discussion The melting point of the Cr-Ni alloy was determined using the differential thermal analysis to 1356.6 °C. According to the Ni-Cr phase diagram [7], the melt- ing point of nickel and chromium at the pressure of 101325 Pa is 1390 °C, regarding the content of chromium 42%. The phase transformation of the real alloy is influenced by the heating rate and its compo- sition. In comparison, the melting point of zirconium alloy reaches 1850 °C. However, since the exothermic oxidation rate of Ni-Cr alloy is far more moderate than in the case of zirconium cladding, the melting temperature is expected to be reached later in a typ- ical accident scenario, hence the coping time is in- creased. Consequently, the lower melting point may not be a limiting factor for ATF requirements. Figure 9. SEM micrograph of oxide scale formed on the surface of Cr-Ni after oxidation at 1200 °C in steam for 20 hours accompanied with EDS measure- ment of the marked point The oxidation kinetics of the Cr-Ni alloy were stud- ied in the temperature range from 700 °C up to 1200 °C. The alloy exhibited rather linear kinetics after oxidation in steam at temperatures 700 °C and 800 °C. At temperatures from 900 °C to 1200 °C the ox- idation rate of the alloy starts to be controlled by diffusion and thus it resembles the parabolic kinet- ics. It was confirmed that the Cr-Ni has very low weight gain, i.e., WG/S from equation (1), under high-temperature oxidation. In view of this fact, a very high precision of the experiment is desirable. Figure 12 shows the Arrhenius plot for various pro- posed ATF designs (Cr-Ni. FeCrAl, Inconel 617) in comparison to standard Zr-based cladding mate- rials (Zr1%Nb, Zircaloy-4) and Cathcard-Pawel cor- relation. On the whole, all potential ATF materials outperform Zr-based alloys. It needs to be mentioned 12 vol. 28/2020 High-temperature oxidation of chrome-nickel alloy Temperature range [°C] A [mg.dm−2.s1/2] B [K] 700-900 0.2 -6951.9 1000-1200 3339.8 -15468.0 Table 3. Calculated parameters A and B from Arrhenius equation for oxidation kinetics of Cr-Ni alloy Figure 10. SEM micrograph of the wall beneath the oxide scale of Cr-Ni after oxidation at 1200 °C in steam for 20 hours accompanied with EDS measure- ment of the marked point Figure 11. SEM micrograph of the Ti grain formed beneath the oxide scale of Cr-Ni after oxidation at 1200 °C in steam for 20 hours accompanied with EDS measurement of the marked point that the correlation for Cr-Ni alloy is not highly ac- curate, given that its kinetics does not correspond to the parabolic law, see Table 2. To determine the activation energy from exper- iments at isothermal temperatures, the Arrhenius equation (2) has been used. k = A · e E R·T = A · e− B t (2) where k is the rate constant, A is the frequency factor or preexponential factor, E is the activation energy for the reaction, R is the gas constant, and T is the absolute temperature. Therefore, the oxida- tion rate is a function of reaction temperature and examined material. Figure 12. Arrhenius-plot of the estimated oxida- tion rate constants for various alloys, i.e. Zr1%Nb, Zircaloy-4, Cr-Ni, FeCrAl in steam [8] The parameters A and B were estimated consid- ering parabolic kinetics. Results are summarized for two temperature ranges in Table 3. 5. Conclusions The presented results show a better resistance of Cr- Ni alloy compared to zirconium alloys during high- temperature oxidation in steam. All visual evalua- tion, microstructure analysis, and weight gain, to- gether with the derived oxidation kinetics parame- ters confirmed its stability under severe accident con- ditions. These conclusions confirm that Cr-Ni is a promising ATF material candidate. The main disadvantage of the the alloy is the high cross-section of thermal neutrons. Accordingly, a few possible ideas on how to improve the alloy’s neu- tron performance were introduced. It has been de- clared that the Cr-Ni alloy (42XHM) allows a thinner wall of the cladding compared to zirconium. How- ever, sufficient compensation for the high thermal neutron absorption requires a substantial increase in fuel enrichment [4]. The alloy is often associated with fourth-generation reactors. An implementation of high-density fuel instead of uranium dioxide pellets increases the capacity of uranium, hence the usage of the oxidation-resistant Cr-Ni alloy as a cladding material could be a possible way to achieve a highly efficient and essentially safe fuel system. Acknowledgements The authors thank the whole group of High-temperature Materials Chemistry (namely: P. Severoloh, Ulrike Stegmaier, Jürgen Moch) for sample preparation and posttest evaluation. This research was held during the internship, finan- cially supported through the ENEN+ project, that has 13 A. Chalupová, M. Steinbrück, M. Grosse et al. Acta Polytechnica CTU Proceedings received funding from the Euratom research and training Work Programme 2016 – 2017 – 1 #755576. References [1] K. A. Terrani. Accident tolerant fuel cladding development: Promise, status, and challenges. Journal of Nuclear Materials 501, 2018. DOI:10.1016/j.jnucmat.2017.12.043. [2] A. V. Vatulin, V. P. Kondrat’ev, V. N. Rechitskii, M. I. 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