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Article

Accident-Tolerant Barriers for Fuel Road Cladding of CANDU Nuclear Reactor

by
Diana Diniasi
1,2,
Manuela Fulger
1,
Bogdan Butoi
3,
Paul Pavel Dinca
3 and
Florentina Golgovici
2,*
1
Institute for Nuclear Research Pitesti, Campului Street, No. 1, P.O. Box 78, 115400 Mioveni, Romania
2
Department of General Chemistry, National University of Science and Technology Polytechnic Bucharest, Splaiul Independentei Street, No. 313, 060042 Bucharest, Romania
3
National Institute for Laser, Plasma and Radiation Physics, Atomistilor Street, no. 409, 077125 Magurele, Romania
*
Author to whom correspondence should be addressed.
Coatings 2023, 13(10), 1739; https://doi.org/10.3390/coatings13101739
Submission received: 11 September 2023 / Revised: 26 September 2023 / Accepted: 29 September 2023 / Published: 7 October 2023
(This article belongs to the Special Issue New Strategies and Recent Advances in Investigations of Micro- and Nano-Coatings for Technological and Biomedical Applications)
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Abstract

:
The nuclear industry is focusing some efforts on increasing the operational safety of current nuclear reactors and improving the safety of future types of reactors. In this context, the paper is focused on testing and evaluating the corrosion behavior of a thin chromium coating, deposited by Electron Beam Physical Vapor Deposition on Zy-4. After autoclaving under primary circuit conditions, the Cr-coated Zy-4 samples were characterized by gravimetric analysis, optical microscopy, SEM with EDX, and XRD. The investigation of the corrosion behavior was carried out by applying three electrochemical methods: potentiodynamic measurements, EIS, and OCP variation. A plateau appears on the weight gain evolution, and the oxidation kinetics generate a cubic oxidation law, both of which indicate a stabilization of the corrosion. By optical microscopy, it was observed a relatively uniform distribution of hydrides along the samples, in the horizontal direction. By SEM investigations it was observed that after the autoclaving period, the coatings with thickness from 2 to 3 µm are still adherent and maintain integrity. The XRD diffractograms showed a high degree of crystallinity with the intensity of chromium peaks higher than the intensity of zirconium peaks. Electrochemical results indicate better corrosion behavior after 3024 h of autoclaving.

1. Introduction

Energy is essential for sustainable economic growth and improving the living conditions of the population. Nuclear power provides access to clean and reliable energy, eliminating the negative impacts of climate change. It is a significant part of the possible types of clean energy and the use of nuclear energy is expected to increase in the coming decades [1].
At the global level, countries are focusing on the direction of decarbonization and energy security, with nuclear energy being considered a critical factor for achieving these goals. The year 2022 was successful for nuclear power, with progress being made in increasing awareness of the value of nuclear power, among climate groups, investors, and other energy communities [2,3]. In this context of sustainable clean energy, the research in the field of materials development performance for nuclear energy is growing revealing the challenges related to safety and efficiency [4] and many alloys were investigated [5,6].
Environmental pollution is a central issue in the new vision of the global economy. Nuclear fission currently provides 10% of the world’s energy needs, compared to coal’s 63.3% [7,8]. The use of nuclear energy has reduced greenhouse gas (GHG) emissions by an average of two billion tons compared to the use of fossil fuels [9]. Such emissions have a major impact on the environment. Thus, the increase in global temperature and atmospheric pollution determines the orientation towards clean energy. One of the types of clean energy sources is nuclear energy. It is the least harmful energy source and has a minimal total environmental impact [10]. Even spent nuclear fuel, if it is properly stored, has no impact on the environment [9]. Coal is one of the most polluting fuels used. Solid particles of ash and soot resulting from coal combustion naturally contain uranium and other radioactive isotopes, as well as As, S, Hg, and Se [11]. The mortality rates per billion kWh according to the WHO (World Health Organization) are 100 for coal, 36 for petroleum products, 24 for biomass, 4 for natural gas, 1.4 for hydropower, 0.44 for solar power, 0.15 for wind power, and 0.04 for nuclear power [11]. More than 2 billion tons of CO2 are currently prevented from being emitted annually by operating nuclear power plants [12]. Additionally, nuclear fuel is a concentrated supply of energy; in a breeder reactor, 1 g of uranium or thorium provides the same amount of thermal energy as 3 tons of coal burning [13]. Considering these advantages, a transition from conventional energy to nuclear energy presents many benefits.
Zirconium-based alloys are currently used as fuel cladding materials in nuclear reactors due to their low neutron absorption cross-section, good corrosion resistance, and mechanical strength. The nuclear fuel cladding is a safety barrier to contain the fission products generated by nuclear fission reactions and prevent their release into the reactor pressure vessel. Under normal operating conditions (310 °C and 10 MPa), the zircaloy cladding undergoes an oxidation reaction on the outside in contact with the coolant. During the process of oxidation, a substantial amount of the hydrogen produced is absorbed by the cladding and precipitates as zirconium hydrides, which may weaken the cladding and affect the mechanical integrity of the fuel rods.
Through a Loss of Coolant Accident (LOCA)-type accident, the temperature of the cladding can exceed 1200 °C, so oxidation reactions and hydrogen generation are significantly accelerated. The consequences of this reaction were seen during the Fukushima-Daichi accident after the 2011 earthquake and subsequent tsunami [14,15], which resulted in explosions in the reactor building and the surroundings being exposed to radioactive material.
For current reactors, there is a demand to extend their design life and to increase reactor burnup from the current 35–45 GWd/Tu (gigawatt days per ton of uranium) to more than 50 GWd/tU to meet the increased demand for energy from industry and consumers, as well as to increase the efficiency of electricity production, which also implies increased profitability for reactor operators [16]. Nevertheless, higher burnups will cause the corrosion rate to increase [17]. These issues have driven the nuclear industry to focus some of its efforts on increasing the operational safety of current nuclear reactors and improving the safety of future types of reactors. A promising approach that has generated considerable research interest is the development of protective coatings or the application of surface treatments to materials that would improve the oxidation resistance of the zirconium alloy cladding to be more tolerant to degradation under LOCA conditions [18]. Thus, if the degradation of the cladding is delayed it will increase the response time in the event of an accident. Furthermore, this direction could have the advantage of reducing the oxidation rate and hydrogen uptake during normal operation, leading to increased overall reactor safety.
Operating nuclear reactors in a safe, reliable, and economical manner have always been a priority for the nuclear industry. The continued improvement of technology, including advanced materials and nuclear fuels, remains a priority to ensure the success of this industry. Decades of research and reactor operating experience have driven solid technology development and provided an extensive database and information on nuclear reactor fuel performance under both normal and accident conditions [19].
The current nuclear power industry is based on mature technology with an excellent safety and operational record. The current fuel, UO2 meets all performance and safety requirements, maintaining nuclear power as a clean and economically competitive energy option. Developing alternative fuel technologies to further improve the safety and competitiveness of existing nuclear power plants is the aim of accident-tolerant fuel (ATF) development [19,20].
The present paper is focused on testing and characterizing chromium coating deposited on Zy-4 substrate by the Electron Beam Physical Vapor Deposition (EBPVD) method. The corrosion behavior and morphological and structural properties of the coating were investigated. The results showed good protective properties of the chromium coating tested in primary circuit conditions of a CANDU reactor (lithiated water, 310 °C, pH = 10.5, 310 °C, 100 atm).
There are studies in the specialized literature about various procedures of coating for Zr-4 alloys including PVD technology [21]. The original character of the present investigation is based not only on the coating deposition, but also consists of a complex flow of performance coating characterization in time. The fact that surface and mechanical properties support electrochemical data is a confirmation of a good selection coating procedure. The purpose of the manuscript is to complete in the frame of nowadays energy strategy the data about nuclear materials improvement in aggressive conditions and for a longer time.

2. Materials and Methods

2.1. Materials

The alloy used as a substrate was a Zy-4 tube with an outer diameter of 13 mm and a wall thickness of 0.45 mm, the chemical composition [22] expressed in wt.% is 1.32 Sn, 0.29 Fe, 0.14 Cr, 0.12 O, and Zr balance. The samples were cut to a length of 20 mm, then the samples were sectioned longitudinally in half, and a hole with a diameter of 3.5 mm was made at one of the ends to fix them on the autoclave support. The preparation process of the Zy-4 samples was completed with their ultrasonication in isopropyl alcohol with a concentration (%) of min. 99.9 for 15 min and dried with hot air. The samples were allowed to cool. The heating cycle includes heating from room temperature to 310 °C, maintaining for 504 h, and cooling down. Then, 3 samples were selected to be weighed. At every 504 h up to 3024 h, 3 samples were weighed, and at 504 h, 1512 h, and 3024 h, some samples were removed for further analysis. There is a support with hooks for hanging the samples.
The material chosen for coating the Zy-4 alloy was metallic Cr due to its properties, such as low neutron abs cross-section, good thermal conductivity, high melting point, and high-temperature oxidation resistance [19,23,24,25,26]. Among the various deposition methods [27,28,29,30,31], PVD methods were identified to be the most suitable for developing coatings on materials used in the nuclear field [32,33,34,35,36]. In the present study, a 2 µm chrome coating was applied by the Electron Beam Physical Vapor Deposition (EBPVD) method [37,38]. The main deposition parameters used for applying the chromium coating are starting vacuum (4 × 10−5 Torr), working vacuum (2 × 10−5 Torr), deposition rate (2 A/s), total deposition time (4.5 h), E-beam power (1–2 kW), the distance between the crucible, and Zy-4 substrate (1.2 m).

2.2. Morphological and Structural Surface Analysis

Corrosion testing of Zy-4 samples deposited with metallic Cr by the EBPVD method has been realized in a 1 l static autoclave with a stainless steel support with hooks for hanging the samples. It used a pH = 10.5 lithium solution, and the test was conducted at 310 °C and 100 atm, simulating the environment in the primary circuit of the CANDU reactor.
Following the testing of Zy-4 samples covered with metallic Cr obtained by the EBPVD method, in a static autoclave, in lithium solution to simulate the environment in the primary circuit of the CANDU reactor, the samples were characterized by gravimetric analysis, optical microscope, electron microscope, and electrochemically evaluated.
At every 504 h of autoclaving, up to 3024 h, 3 samples were extracted for gravimetric analysis and only at 504 h, 1512 h, and 3024 h several samples were removed for further analysis.
The identification and highlighting of zirconium hydrides were carried out by metallographic analysis using the metallographic microscope Olympus GX 71 (Olympus Corporation, Tokyo, Japan). The highlighting of the hydrides was done by chemical etching in a solution composed of 45 mL HNO3 (67%), 45 mL H2O2 (30%), and 7 mL HF (30%), for 30 s. Vickers microhardness (MHV0.1) was determined by an OPL tester in an automatic cycle with an indentation load of 0.1 kgf [39]. The equation for Vickers microhardness takes the following form:
H V = 1854.4 P d 2 k g f   m m − 2
where P—load indentation, kgf; and d—diagonal length, mm2.
An FEI Inspect S scanning electron microscope (FEI Company, Hillsboro, OR, USA) in high vacuum mode, equipped with an energy dispersive spectra detector (EDS), was used for morphological research and elemental analysis.
The investigation of the crystalline structure of the coating after the autoclaving process was carried out with a table-top Bruker D8 Advanced. The diffraction system was provided with a Cu-Kα X-ray source with a specific wavelength of 0.154 nm. The samples were measured in a classical θ–2θ Bragg–Brentano geometry between 10–90° with a measurement step of 0.01° with a dwell time per step of 4 s.

2.3. Electrochemical Tests

Three electrochemical techniques have been employed to investigate the corrosion behavior: potentiodynamic tests, electrochemical impedance spectroscopy, and open circuit variation. A three-electrode electrochemical cell made up of a working electrode (Cr-coated Zy-4), a saturated calomel reference electrode (SCE), and two auxiliary electrodes (graphite rods) were used in the electrochemical tests. This system was called the PARSTAT 2273 computer-controlled electrochemical measurement system and was manufactured by Princeton Applied Research and AMETEK in Oak Ridge, TN, USA. Each test included three repeats and was conducted at ambient temperature (22 ± 2 °C).
Chromium-EBPVD-coated Zy-4 samples were evaluated for generalized corrosion behavior using the open circuit potential method in LiOH solution at pH 10.5.
The EIS measurements were conducted at open circuit potential (OCP) with an amplitude of 10 mV in the frequency range of 10−2 to 105 Hz following OCP stabilization in a chemically inert solution made up of 0.05 M boric acid and 0.001 M borax solution. The obtained spectra were represented in the form of Nyquist and Bode diagrams. The EIS experimental data were fitted using the Zview 2.90c software (Scribner Associates Inc., Southern Pines, NC, USA) and an appropriate electrical equivalent circuit.
Potentiodynamic measurements consisted of sweeping the potential over a field, starting from the cathodic zone (−250 mV), with a 5 mV/s scanning rate, to allow the studied corrosion process to take place, up to 1000 mV in a specific primary circuit solution (LiOH solution, pH 10.5).
A research diagram that describes the adopted algorithm and research stages is presented in Figure 1.

3. Results and Discussion

3.1. Oxidation Kinetics for Cr-EBPVD-Coated Zy-4, Autoclaved under Primary Circuit Conditions

Weight gain measurements were done for some of the samples tested at generalized corrosion by autoclaving at 310 °C and 10 MPa in LiOH solution. The samples were weighed at every 504 h, up to 3024 h, and as no exfoliation or weight loss was observed this analysis can be applied for oxidation kinetics calculations. Based on weight gains recorded for the chromium-EBPVD-coated Zy-4 corrosion kinetics was performed, represented by the mass gain as a function of exposure time, presented in Figure 2.
Comparing these data with the results obtained on uncoated Zy-4 [40], it can be seen a much lower weight gain for the Cr-EBPVD-coated Zy-4. As can be seen from the above figure, the corrosion kinetics follows a typical power law, in accordance with data presented by other researchers [40,41,42,43,44,45,46,47] described by:
Δ W = k p   ∗   t n
where ΔW is the oxide weight gain (mg × dm−2), kp is the rate constant, t is the exposure time (h), and n is the exponent.
Table 1 contains the oxidation constants that were established by fitting the data with Equation (2). The value of the correlation coefficient (R2 = 0.996) indicates that the experimental Cr oxidation data fit well.
Based on the scientific literature [36,41,44,45,46,47,48], if the value of exponent n is 0.3 indicates that oxidation kinetics follow a cubic law. Therefore, as for this case, the value of exponent n is close to 0.3, and it is considered that the oxidation kinetics of chromium coating follows a cubic law.
It was observed that after about 2000 h of autoclaving, the weight gain showed a plateau, which indicates a stabilization of the corrosion process. Compared with data obtained for uncoated Zy-4 [2], which does not show a stabilization of the weight gain, we can appreciate that the chromium coating obtained by Electron Beam Physical Vapor Deposition on Zy-4 induced a stabilization of the corrosion process.

3.2. Morphological and Structural Characterization

3.2.1. Metallographic Analysis

By optical microscopy, it was evaluated the behavior of the coating related to hydrogen absorption. Generally, many researchers have noticed a significant increase in the absorption of hydrogen as test time and temperature increase [49,50]. When the solubility limit of absorbed hydrogen is exceeded, excess hydrogen precipitates, causing the appearance of zirconium hydrides (ZrHx) [51,52,53]. These hydrides can cause the embrittlement and fracture of fuel cladding [50,54].
Figure 3 presents representative cross-section hydrides for chromium-EBPVD-coated Zy-4 autoclaved for various periods. It can be observed that a relatively uniform distribution of hydrides along the sample is in the horizontal direction. This is a qualitative evaluation, and we can only appreciate the evolution of hydrides with time. After 504 h of autoclaving, it was observed the appearance of zirconium hydrides. A higher density of hydrides was noticed after 1512 h of autoclaving, but after 3024 h of autoclaving, no increase in density was observed. Compared to the results obtained for uncoated Zy-4 alloy in a previous paper [39], the chromium coating does not decrease the hydrogen uptake.
The surface morphology of Cr-EBPVD-coated Zy-4 alloy autoclaved for various periods of time was also performed by optical microscopy and is presented in Figure 4.
Based on the above micrographs, changes were identified in the coloration of the surface of the samples as the autoclaving time increased. The golden color suggests a thin chromium oxide layer, while the shift to purple/bluish is given by the increase in chromium oxide layer thickness. According to the literature [54], the thickness of 100 nm is suggested by the micrograph’s golden color, while for the purple/bluish color, a thickness of several hundreds of nanometers corresponds.
For the Vickers microhardness test (MHV0.1) was used a 0.1 Kgf load was [39,55,56]. Before the test, the samples were cut into cross-sections and embedded in a cupric resin. Since small differences were observed between the microhardness results at the substrate/coating interface and the middle of the samples, 10 indenter prints were made for each sample, 5 for each of the mentioned areas. Figure 5 presents the average microhardness data for the tested samples.
In the graph above the coated samples show a slight increase in microhardness values as autoclaving time increases. This behavior may indicate a tendency of the material to harden over time due to working conditions (high pressure and temperature, chemistry of testing solution) or because of the formation of zirconium hydrides, which may induce a material hardening effect [57,58].

3.2.2. Scanning Electron Microscopy (SEM) Measurements

By SEM investigations, more information about the surface morphology of the autoclaved coated samples is presented in Figure 6.
It was observed that after every autoclaving period, the coating is still adherent, having a surface morphology like the as-received coating [59].
By EDS analysis has been obtained the elemental distribution on the surface of the sample and the concentrations of the elements are summarized in Table 2.
It can be observed that after 504 h of autoclaving, the higher variations of the concentration of the elements of interest are recorded: the concentration of oxygen increased by ~3 times compared to the initial value, that of chromium decreased by 3 percent, and zirconium reached a value of 0.35% Zr. However, during the autoclaving process, minor variations in the concentration of the elements were recorded. Thus, it can be considered that this film showed good stability under the respective test conditions.
The variation of the concentrations of the elements of interest correlates with the data obtained from the application of the XRD technique, with zirconium oxides and chromium oxide being identified. However, the existence of a chromium oxide layer is also considered, but the diffraction method is sensitive to the volume of the sample, it is considered that it has thicknesses maxima of the order of hundreds of nanometers, as was also considered from the metallographic analysis, based on the evaluation of the coloration of the oxides developed after different periods of autoclaving.
Integrity assessment and outer layer thickness measurement were performed for Zy-4 samples coated with Cr obtained by the EBPVD method and autoclaved for different periods of time. The cross-section SEM images, representative of the tested samples, are shown in Figure 7.
As can be seen in these micrographs, all coatings maintained their integrity until the final of the autoclaving process. The results of SEM measurements are in good correlation with the results of gravimetric measurements, both showing an increase in layer thickness with autoclaving time.
Figure 8 presents the results from the EDS line scan in cross-section.
It is observed that there are not considerable variations of the identified elements, in the area corresponding to the coating. Also, during the autoclaving process, the same ratio between the elements is maintained, and chromium is presented as the highest concentration. The oxygen concentration shows a slightly decreasing trend, and Zr is found in a very low concentration in the middle zone of the layer, increasing slightly towards the interface with the substrate, respectively, towards the outside of the coating. Following the analysis of the results presented in Figure 8, we can say that on the surface of all the autoclaved samples zirconium, and, respectively, chromium oxide appear in accordance with the results obtained by EDS analysis of the sample’s surface. If we consider the Ellingham diagram, we can say that Gibbs free energy formation for ZrO2 is significantly lower than Cr2O3, and the needed oxygen concentration to form ZrO2 is also much lower, which is why the zirconium oxide layer is beneath chromium oxide.

3.2.3. XRD Measurements

The XRD diffractograms are presented in Figure 9. The evolution of the crystalline structure was evaluated as a function of the duration of the autoclaving process.
The measurement range was selected to put into evidence the multiple Zr diffraction peaks and the ones associated with the Cr coating. Due to the half-cylindrical shape of the sample, only the ridge area was exposed to the X-ray radiation and consequently, the information regarding the crystalline structure was collected only from this area.
The polycrystalline nature of the investigated samples is highlighted by the textured diffraction patterns observed below (Figure 9). The prevalent crystalline phase corresponds to the substrate material, namely a Zr, P63/mmc (194) space group with a hexagonal arrangement of atoms in the elemental lattice. The main hkl planes associated with the Zr phase are (101), (102), (110), (103), (004), (112), and (104) with the main preferential crystalline growth on (002) plane (peak centered at 35°). To avoid the overloading of the graphs presented in Figure 9 only the three most intense reflection planes for Zr were presented. The EBPVD Cr deposited layer shows a high degree of crystalline order evidenced by a peak centered at 44.5°. This peak is attributed to the (110) reflection plane of a Cr metallic phase, Im-3m space group with a body-centered cubic structure. The Cr2O3 pattern shows diffraction peaks, at 35°, 36.3°, and 63.8° 2 theta, corresponding to the (104), (110), and (214). Some of the Cr2O3 peaks were superimposed on the Zr peaks [60].
It is also observed that the intensity of chromium peaks is higher than the intensity of zirconium peaks, even after 3024 h of autoclaving. Most likely this is because the Zr signal corresponding to the substrate is mitigated by the thick Cr coating. It is also shown that the intensity of the chromium peak remains like the as-received sample (0 h AC) both after 504 h and after 1512 h of autoclaving. Therefore, it is suggested good stability of the coating with autoclaving time. The XRD diffractograms shown in Figure 9 indicate an increasing trend of ZrO2 as a function of autoclaving time followed by a decreasing Cr content more pronounced after 3024 h oxidation time. This is an expected behavior since the autoclaving process induces oxidation, and, on the other hand, segregation of Zr substrate atoms due to high-temperature sample exposure can occur. In addition, the EDS line profile of the Cr element (Figure 8) along the cross-section shows a deeper presence of chromium after 3024 h autoclaving time.
This indicates a decrease in the X-ray scattering centers, with oxidation as a potential underlying cause. This is a hypothesis that is sustained only by the weight gain measurements.
In addition to the peaks specific to different reflection planes for Zr and Cr, another peak can be observed at ~28°. The correlation of this peak with a crystalline phase was very difficult due to the lack of additional specific orientations and the high intensity of Zr and Cr-associated peaks. However, by comparing the XRD measurements on the Zy-4 uncoated samples (not shown here), at similar autoclaved time, we identify this peak as the corresponding (−111) reflection plane of a monoclinic ZrO2 phase. This crystalline phase is formed in conditions like the autoclaving process. The absence of additional peaks can be attributed to the fact that the majority overlapped with the Zr-specific peaks. It can be clearly seen in Figure 9 that the (−111) intensity increases with autoclaving time which might suggest that a thicker layer of ZrO2 is formed. On the other hand, compared to the uncoated samples of the Cr coating deposited by EBPVD on Zy-4, we can observe the appearance of two oxides, ZrO2 and Cr2O3, respectively. These results are in good correlation with the results obtained by SEM/EDS, where good stability of the coating was also observed during the autoclaving process.

3.3. Electrochemical Characterization

3.3.1. Open Circuit Potential Evaluation

The variations of the corrosion potentials for the Zy-4 samples coated with Cr by the EBPVD method and autoclaved for different periods of time are shown in Figure 10.
As can be seen from the figure above, small fluctuations in the evolution of the corrosion potential were recorded during the test time. The non-autoclaved Cr-EBPVD-coated Zy-4 sample (0 h) showed a high rate of increase in Ecorr, after 1400 s increasing from −80 mV to −39 mV, followed by a plateau for about 6500 s, after which a slight decrease and leveling off was observed. The samples autoclaved 504 h and 1512 h, respectively, showed very close evolutions of the corrosion potentials for the first 2500 s. After this point, a shift in the curves was observed, the sample autoclaved 1512 h, moved towards more electropositive values of Ecorr, compared to the sample autoclaved 504 h. The sample autoclaved at 3024 h showed a different behavior, the corrosion potential starting from the highest values (89 mV) compared to the other samples (which had Ecorr in the cathodic range), but after approximately 2700 s, the potential of corrosion registered a continuous downward trend.

3.3.2. Electrochemical Impedance Spectroscopy (EIS)

EIS was the other electrochemical technique utilized to assess the protective properties of the Cr layer obtained by the EBPVD method on the Zy-4 alloy. The spectra of the Cr-EBPVD-coated Zy-4 alloy recorded after 10 min of immersion in LiOH solution, are shown as Nyquist and Bode diagrams from Figure 11a,b.
The Nyquist diagrams (Figure 11a) reveal that a single open capacitive arose for non-autoclaved samples (0 h), but for all autoclaved, Cr-EBPVD-coated Zy-4 alloy samples, two capacitive semicircles arise in accordance with the interfaces generated. In comparison to the non-autoclaved Cr-EBPVD-coated Zy-4 alloy, larger values of the capacitive semicircle diameter were obtained for all autoclaved samples. We can see from the Bode plots (Figure 11b) that greater impedance values were observed for all Cr-EBPVD-coated Zy-4 samples that were subjected to autoclaving for different periods. Since impedance, or |Z| values, are closely correlated with oxide resistance, we can conclude that these coatings have good corrosion resistance. The Bode diagram (Figure 11b) shows that two maxima of the phase angle are obtained, one at high frequencies and the other at low frequencies. The electrical equivalent circuit model shown in Figure 12 was used to fit all experimental data collected from electrochemical impedance spectroscopy studies.
Rs is the solution resistance between the electrode and the electrolyte in this equivalent electrical circuit; CPEdl is the constant phase element for the double layer and Rct is resistance to charge transfer; CPEox is the constant phase element for the oxide layer and Rox resistance of the formed oxide layer; CPEcoat and Rcoat are constant phase element and, respectively, the resistance of the Cr-EBPVD coating.
As indicated in Table 3, an adequate fit of the experimental data to this model was achieved.
Table 3 shows that Rcoat increased as the autoclaving period increased. As a result, the alloy is protected from corrosion by the coating. The rate of corrosion decreases as layer thickness increases. Because the values for all constant phase elements were close to 1, the behavior is almost capacitive.

3.3.3. Potentiodynamic Polarization Tests

The potentiodynamic curves recorded in LiOH electrolyte, at 10.5 pH and room temperature (22 ± 2 °C), for the Cr-EBPVD-coated Zy-4 samples and autoclaved for different periods under primary circuit conditions of the CANDU nuclear reactor are shown in Figure 13. Based on these, a decrease in the corrosion current density can be observed simultaneously with the movement toward more electropositive values of the corrosion potential values along with the increase in the autoclaving time for all studied samples.
The values of the electrochemical parameters determined following the potentiodynamic polarization tests on the Cr-EBPVD-coated Zy-4 samples are included in Table 4. Also, applying relations (1) and (2), the two factors used in evaluating the integrity of a coating, the porosity coefficient, and the protection efficiency, were calculated.
The corrosion kinetic parameters reported in Table 4 were determined by applying two methods to the polarization curves from Figure 13: the Tafel slope extrapolation and polarization resistance method. The main calculated kinetic parameters are corrosion potential (Ecorr), corrosion current density (icorr), corrosion rate (Vcorr), polarization resistance (Rp), protection efficiency (Pi), and porosity (P). All parameters were subjected to statistical analysis, and the results are shown as mean ± 1 standard deviation. According to the data in the table above, for the autoclaved samples, the values of the corrosion potentials, as well as the corrosion currents, are located towards more electropositive values, compared to the non-autoclaved sample. It can also be observed that the values of the corrosion rates decreased with the duration of autoclaving. An increase in the value of the resistance to polarization (Rp) can also be observed, with the increase in the autoclaving time, due to the formation of a compact oxide film over the chromium layer deposited by the EBPVD method, which determines the corrosion resistance to increase for the Zy-4 alloy studied in LiOH solution at pH 10.5. These data are corroborated by the values obtained for the porosity coefficient and the protection efficiency, which demonstrate the maintenance of the anticorrosive properties of the coating, as well as their strengthening.
If we compare the results obtained for the uncoated alloy studied under the same autoclaving conditions, we can say that the coating greatly improves the performance of the material [40]. Moreover, this type of chromium coating, deposited by Electron Beam Physical Vapor Deposition on Zy-4 has better properties in terms of corrosion resistance under primary circuit conditions, in LiOH solution, at 310 °C and 10 MPa, than chromium coatings obtained by other methods [43,59].

4. Conclusions

For the first time, the morphological and electrochemical behaviors of chromium-EBPVD-coated Zy-4 samples after different periods under simulated primary circuit conditions of a CANDU reactor (lithiated water, 310 °C, 10 MPa, pH = 10.5) were studied.
Based on weight gains it was determined that the corrosion kinetics follow a typical power rate. It was also observed that after 2000 h of corrosion testing, the weight gain showed a plateau, which indicates a stabilization of the corrosion process.
A qualitative evaluation of hydrogen absorption made by optical microscopy showed a relatively uniform distribution of hydrides along the sample in the horizontal direction. An increase in density of hydrides was noticed after 1512 h of autoclaving, but after 3024 h of autoclaving, a stabilization of density appeared.
As autoclaving time increases, a slight increase (from 229 to 257) in microhardness values was calculated. This effect may be ascribed to the tendency of the material to harden over time due to the working conditions or because of the hardening effect of the zirconium hydrides.
By SEM investigations was observed that the coatings are still adherent and the integrity of the coatings was not affected. The results of SEM measurements indicate layer thickness between 2 and 3 µm, which is in good correlation with gravimetric measurements.
EDS spectra showed higher variations of the element’s concentration after 504 h of autoclaving than for the next autoclaving cycles. Thus, it can be considered that the coating showed good stability over time under testing conditions. Also, the results of the EDS line scan cross-sectional measurements indicate that there are not considerable variations of the elements during the autoclaving process.
A high degree of crystallinity with a high intensity of chromium peaks has been observed from the diffraction patterns. A decrease in the intensity of the chromium peak was recorded after 3024 h of autoclaving.
The Nyquist plots revealed just one open capacitive semicircle for non-autoclaved samples and two capacitive semicircles for all autoclaved samples. Greater impedance was obtained for all Cr-coated samples; thus, it can be concluded that these samples have better corrosion resistance. The resistance of coating has a higher value as the autoclaving time increases.
For the autoclaved samples, the values of the corrosion potentials are located towards more electropositive values (+25 mV, for the sample autoclaved for 3024 h), compared to the non-autoclaved sample (−167 mV). It may also be seen that the values of the corrosion rates decreased with the duration of autoclaving. A decrease by an order of magnitude, from 0.586 to 0.059 nA/cm2, was observed in the corrosion current densities in the case of the sample coated with Cr deposited by the EBPVD method and autoclaved for 3024 h compared to the non-autoclaved sample. These data are corroborated by the values obtained for the porosity coefficient and the protection efficiency, which demonstrate the maintenance of the anticorrosive properties of the coating, as well as their strengthening.

Author Contributions

Conceptualization, D.D., M.F. and F.G.; methodology, D.D., B.B., P.P.D. and F.G.; software, D.D.; validation, M.F. and F.G.; formal analysis, D.D., B.B. and P.P.D.; investigation, D.D., B.B. and P.P.D.; resources, F.G.; data curation, D.D. and F.G.; writing—original draft preparation, D.D., M.F., B.B. and P.P.D.; writing—review and editing, D.D. and F.G.; visualization, F.G.; supervision, F.G. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available upon request.

Conflicts of Interest

The authors declare no conflict of interest.

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Coatings 13 01739 g001
Figure 1. Research diagram.
Figure 1. Research diagram.
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Coatings 13 01739 g002
Figure 2. Weight gain evolution for chromium-EBPVD-coated Zy-4 samples autoclaved under primary circuit conditions.
Figure 2. Weight gain evolution for chromium-EBPVD-coated Zy-4 samples autoclaved under primary circuit conditions.
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Coatings 13 01739 g003
Figure 3. Optical micrographs indicating the hydride formation on Cr-EBPVD-coated Zy-4 sample after different autoclaving periods: (a) 0 h; (b) 504 h; (c) 1512 h; (d) 3024 h.
Figure 3. Optical micrographs indicating the hydride formation on Cr-EBPVD-coated Zy-4 sample after different autoclaving periods: (a) 0 h; (b) 504 h; (c) 1512 h; (d) 3024 h.
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Coatings 13 01739 g004
Figure 4. Surface morphologies of Cr-EBPVD-coated Zy-4 alloy after different autoclaving periods: (a) 0 h; (b) 504 h; (c) 1512 h; (d) 3024 h.
Figure 4. Surface morphologies of Cr-EBPVD-coated Zy-4 alloy after different autoclaving periods: (a) 0 h; (b) 504 h; (c) 1512 h; (d) 3024 h.
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Coatings 13 01739 g005
Figure 5. Variation of Vickers microhardness for and Cr-EBPVD-coated Zy-4 samples of autoclave for different periods of time.
Figure 5. Variation of Vickers microhardness for and Cr-EBPVD-coated Zy-4 samples of autoclave for different periods of time.
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Figure 6. SEM surface morphologies for Cr-EBPVD-coated Zy-4 alloy after different autoclaving periods: (a) 0 h: (b) 504 h; (c) 1512 h; (d) 3024 h.
Figure 6. SEM surface morphologies for Cr-EBPVD-coated Zy-4 alloy after different autoclaving periods: (a) 0 h: (b) 504 h; (c) 1512 h; (d) 3024 h.
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Coatings 13 01739 g007
Figure 7. The SEM cross-section micrographs for autoclaved Cr-EBPVD-coated Zy-4 alloy after different autoclaving periods: (a) 504 h; (b) 1512 h; (c) 3024 h.
Figure 7. The SEM cross-section micrographs for autoclaved Cr-EBPVD-coated Zy-4 alloy after different autoclaving periods: (a) 504 h; (b) 1512 h; (c) 3024 h.
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Figure 8. EDS line scan analysis of the Cr layer deposited on the Zy-4 substrate by the EBPVD method, after (a) 504 h, (b) 1512 h, and (c) 3024 h of autoclaving.
Figure 8. EDS line scan analysis of the Cr layer deposited on the Zy-4 substrate by the EBPVD method, after (a) 504 h, (b) 1512 h, and (c) 3024 h of autoclaving.
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Coatings 13 01739 g009
Figure 9. The diffraction patterns of Cr-EBPVD-coated Zy-4 samples autoclaved for (a) 0 h; (b) 504 h; (c) 1512 h; (d) 3024 h.
Figure 9. The diffraction patterns of Cr-EBPVD-coated Zy-4 samples autoclaved for (a) 0 h; (b) 504 h; (c) 1512 h; (d) 3024 h.
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Figure 10. OCP variation for Cr-EBPVD-coated Zy-4 alloy tested in LiOH electrolyte, after different autoclaving periods.
Figure 10. OCP variation for Cr-EBPVD-coated Zy-4 alloy tested in LiOH electrolyte, after different autoclaving periods.
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Coatings 13 01739 g011
Figure 11. Nyquist (a) and Bode (b) diagrams for Cr-EBPVD-coated Zy-4 alloy samples, after different autoclaving periods.
Figure 11. Nyquist (a) and Bode (b) diagrams for Cr-EBPVD-coated Zy-4 alloy samples, after different autoclaving periods.
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Figure 12. The electric equivalent circuit proposed for Cr-EBPVD-coated Zy-4 samples, after different autoclaving periods.
Figure 12. The electric equivalent circuit proposed for Cr-EBPVD-coated Zy-4 samples, after different autoclaving periods.
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Figure 13. Potentiodynamic polarization curves for Cr-EBPVD-coated Zy-4 samples in LiOH electrolyte, at 10.5 pH and room temperature (22 ± 2 °C).
Figure 13. Potentiodynamic polarization curves for Cr-EBPVD-coated Zy-4 samples in LiOH electrolyte, at 10.5 pH and room temperature (22 ± 2 °C).
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Table 1. Kinetic parameters for Cr-EBPVD-coated Zy-4 samples.
Table 1. Kinetic parameters for Cr-EBPVD-coated Zy-4 samples.
Kinetic EquationkpnR2
y = 1.919 × t 0.14331.9190.14330.996
Table 2. Elemental composition, wt.%.
Table 2. Elemental composition, wt.%.
Sample Oxidized ForCrOZrFeSn
0 h92.273.34.43
504 h89.299.670.350.130.56
1512 h89.179.330.350.111.04
3024 h89.059.060.680.181.03
Table 3. The values of equivalent electrical circuit elements for Cr-EBPVD-coated Zy-4.
Table 3. The values of equivalent electrical circuit elements for Cr-EBPVD-coated Zy-4.
Oxidation
Period		Rs, CPEdl-TCPEdl-PRctCPEox-TCPEox-PRoxCPEcoat-TCPEcoat-PRcoatChi-Squared
hΩ × cm2μF × cm−2 MΩ × cm2nF × cm−2 Ω × cm2μF × cm−2 Ω × cm2
0200.90.180.872.18---0.150.80648 × 1063.3 × 10−4
504143.51.220.750.4630.450.8891670.990.742.06 × 10104.9 × 10−5
1512149.31.180.850.4510.150.9185970.950.785.81 × 10101.1 × 10−5
3024154.41.230.760.5812.890.9383080.980.881.17 × 10114.3 × 10−5
Table 4. Electrochemical parameters related to Cr-EBPVD-coated Zy-4 samples tested in LiOH solution at 10.5 pH and room temperature (22 ± 2 °C).
Table 4. Electrochemical parameters related to Cr-EBPVD-coated Zy-4 samples tested in LiOH solution at 10.5 pH and room temperature (22 ± 2 °C).
Autoclaving Time, hTafel Slope MethodPolarization Resistance MethodPi
(%)		P
(%)
Ecorr,
mV		icorr,
nA×cm−2		Vcorr
nm×year−1		Rp
MΩ×cm2		icorr
nA×cm−2
0−167
± 0.5		0.586
± 0.003		7.06
± 0.03		48.1
± 0.2		0.543
± 0.003		--
504−75
± 0.7		0.269
± 0.002		3.24
± 0.02		110
± 0.5		0.231
± 0.002		54.09
± 0.01		0.041
± 0.001
1512−33
± 0.5		0.091
± 0.001		1.12
± 0.02		350
± 0.5		0.073
± 0.001		84.33
± 0.01		0.0045
± 0.001
3024+25
± 0.5		0.059
± 0.001		0.71
± 0.01		620
± 0.5		0.050
± 0.001		89.92
± 0.02		6.98 × 10−4
± 0.02
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Diniasi, D.; Fulger, M.; Butoi, B.; Dinca, P.P.; Golgovici, F. Accident-Tolerant Barriers for Fuel Road Cladding of CANDU Nuclear Reactor. Coatings 2023, 13, 1739. https://doi.org/10.3390/coatings13101739

AMA Style

Diniasi D, Fulger M, Butoi B, Dinca PP, Golgovici F. Accident-Tolerant Barriers for Fuel Road Cladding of CANDU Nuclear Reactor. Coatings. 2023; 13(10):1739. https://doi.org/10.3390/coatings13101739

Chicago/Turabian Style

Diniasi, Diana, Manuela Fulger, Bogdan Butoi, Paul Pavel Dinca, and Florentina Golgovici. 2023. "Accident-Tolerant Barriers for Fuel Road Cladding of CANDU Nuclear Reactor" Coatings 13, no. 10: 1739. https://doi.org/10.3390/coatings13101739

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