Barrier Properties of Cr/Ta-Coated Zr-1Nb Alloy under High-Temperature Oxidation

Abstract

In this paper, Cr (8 μm)/Ta (3 μm) bilayer coatings deposited on a Zr-1Nb alloy substrate were investigated and compared with a Cr-coated alloy under high-temperature steam oxidation at 1200–1400 °C. The bilayer coatings with α- and β-Ta interlayers were obtained by magnetron sputtering. The Cr/Ta-coated samples were studied using scanning electron microscopy (SEM), X-ray diffraction (XRD), and optical microscopy (OM). The coating with an α-Ta interlayer can suppress the interdiffusion of chromium and zirconium more effectively up to 1330 °C in comparison with the coating having a β-Ta interlayer. The weight gain of the α-Ta-coated samples after oxidation at 1200 °C for 2000 s was 5–6 times lower than that of the Cr-coated Zr alloy samples. Oxidation at 1400 °C for 120 s showed no significant difference in the weight gain of the Cr- and Cr/Ta-coated Zr-1Nb alloy samples. It was shown that the effect of suppression of Zr-Cr interdiffusion by the barrier coating (α- and β-Ta) is only short-term.

1. Introduction

Nuclear accidents such as the one that occurred at Fukushima in 2011 have demonstrated the serious danger of loss-of-coolant accident (LOCA) conditions [1]. Under LOCA, a reaction of zirconium with steam can occur resulting in a release of hydrogen and heat and, as a consequence, embrittlement of the fuel cladding due to zirconium oxidation [2]. The reaction of Zr and water steam rapidly starts at 950 °C and becomes self-sustaining at 1200 °C [2].

Accident-tolerant fuel cladding (ATF) is currently receiving a lot of attention in view of the zirconium–steam interaction. ATF cladding should not affect reactor economy under normal operating conditions (360 °C and 18.6 MPa) and should slow down or fully prevent high-temperature oxidation in steam. Several approaches are considered for the development of ATF claddings. The first one is the replacement of zirconium alloys by other materials, for example, Mo [3,4], SiC/SiC [5,6], or some structural steels [7,8]. Obviously, such an approach requires a lot of time and financial investment to realize. An alternative way is the deposition of protective coatings, which are capable of preventing oxygen diffusion into the Zr alloy. The same requirements are applied to protective coatings as for the main fuel cladding material. The coatings should not significantly affect the neutron economy, and they should be resistant to high-temperature steam oxidation, have a low hydrogen absorption rate, have a low thermal neutron capture cross-section, have good cohesion, and have interface adhesion to the zirconium alloys.

Among the many investigated coating materials, most attention is given to chromium-based coatings [9,10,11,12,13]. Chromium has good corrosion resistance in water steam due to the protective Cr₂O₃ scale formed under high-temperature oxidation [14]. In addition, chromium has good adhesion to Zr alloys and a relatively small thermal neutron capture cross-section, and the elastic modulus of Cr is about twice that of Zr, which helps to improve the overall stiffness of the coated cladding [15,16,17]. However, a Zr-Cr interdiffusion accelerates with increasing oxidation temperature leading to the formation of a Cr₂Zr Laves phase and an eutectic phase at 1332 °C [18]. Moreover, the lattice parameters of Cr₂Zr are much larger than those of pure chromium and zirconium. Thus, the formation of the eutectic phase results in the loss of the protective ability of the chromium coating and rapid oxidation of the Zr alloy to ZrO₂.

The intermediate barrier (interlayer) can be applied to reduce interdiffusion at the Cr/Zr interface and improve the high-temperature oxidation behavior of the Cr-coated Zr alloy interlayer. The diffusion barrier material should not form eutectics (above 1332 °C) and other compounds with either zirconium or chromium. Many papers are devoted to the study of the application of some ceramic interlayers such as CrN and ZrO₂. For example, the CrN/Cr coating system was considered in ref. [19]. The CrN/Cr intermediate layer allows for the prevention of Cr-Zr interdiffusion for a short period; nevertheless, the protective effect was not observed at 1330 °C. The deposition of a multilayer Cr/CrN coating was described in ref. [20]. The simultaneous decomposition of a thin CrN layer in each part of the multilayer structure resulted in the formation of pores between multilayers in the coating. Hence, neither oxygen diffusion inward nor Zr diffusion outward can be limited. In ref. [21], an approach for Cr-CrN-Cr multilayer coating was described, where the Cr-Zr interdiffusion was successfully limited by the CrN layer. Typical features of Cr-Zr interdiffusion, including the Cr₂Zr layer, are that Cr precipitates in the Zr substrate, there are Kirkendall voids under the Cr₂Zr layer, and Zr diffusion along grain boundaries in the Cr coating are not observed in the Cr-CrN-Cr-coated samples after high-temperature annealing. CrN-based coatings are currently among the most promising. However, the radiation resistance of these coatings has not been sufficiently studied. The bilayer coatings with ZrO₂/FeCrAl were obtained in ref. [22]. The deposition of an intermediate ZrO₂ layer can slow down the diffusion of FeCrAl and Zr, but this study was performed at temperatures lower than the BDBA (beyond design basis accident). In ref. [23], the ZrO₂/Cr-coated Zr alloys were investigated at a temperature range of 1200–1400 °C. The pores were found at the ZrO₂/Cr interface due to the Kirkendall effect. Thus, the thinner the ZrO₂/Cr multilayer coating, the more cavities were observed in the cross-section of the samples after steam oxidation. It is assumed that Mo, Re, Nb, and Ta are potentially applicable as barrier interlayers due to their high melting points and low diffusion coefficients in Zr [13,24,25]. However, Nb has high solubility in zirconium [26], and rhenium has a rather high thermal neutron capture cross-section of 89.7 barn [27]. At high temperatures, the diffusion of molybdenum leads to the formation of Cr₃Mo and Mo₃Zr phases [28]. In ref. [29], it is noted that the limitation of Cr-Zr interdiffusion using a Mo barrier interlayer occurs only up to 1330 °C. Thus, niobium and rhenium are not suitable as barrier layers. The use of molybdenum does not significantly improve the oxidation resistance properties. In addition, the enhanced oxidation of the Mo-Zr layer may lead to local oxidation of the Zr alloy.

The use of tantalum is theoretically reasonable as a barrier interlayer because no formation of intermediate phases with zirconium can be observed. The solubility of Ta in β-Zr is 13.3 at.% at 1200 °C and 7.7 at.% at 1000 °C [29,30]. Tantalum can form the Laves phase Cr₂Ta with chromium. In this case, the solubility of Ta in Cr is about 1 at.%, when the solubility of Cr in Ta is 5 at.% [31,32]. It is well known that Ta has two phases: an α-Ta with a body-centered cubic lattice and a metastable β-Ta phase with a tetragonal lattice. In the coating industry, the deposition of brittle and rigid β-Ta coatings is mostly considered [33]. Nevertheless, no studies of the high-temperature corrosion resistance of Cr/α-Ta and Cr/β-Ta coatings were performed under DBA and beyond DBA temperatures. Thus, this work is devoted to the investigation of the behavior of the Zr-1Nb alloy with Cr/Ta coatings with a different crystal structure of Ta during high-temperature steam oxidation in the temperature range of 1200–1400 °C.

2. Materials and Methods

2.1. Sample Preparation

Before coating deposition, the alloy samples were mechanically grinded and polished by a MP-1B machine (TIME Group Inc., Beijing, China) using a SiC abrasive paper (P600→P4000). Then, the samples underwent additional ultrasonic cleaning and degreasing with technical soap and isopropyl alcohol (99.9%). After reaching the base pressure in a vacuum chamber (at least of 3 × 10⁻³ Pa), the alloy substrates were bombarded by Ar ions for 20 min using an ion source with Hall electron drift. Ion treatment was carried out at the following parameters: ion current I = 45 mA, voltage U = 3 kV, and argon pressure P = 0.11 Pa.

2.2. Coating Deposition

The coatings were deposited using the multi-cathode magnetron sputtering system developed at Tomsk Polytechnic University (Russia) [34].

At first, the experiments were carried out to analyze the phase structure and preferential orientation of Ta coatings depending on their deposition conditions. Tantalum interlayers with different α- and β-phases were deposited by using single and dual configurations of direct current (DC) magnetron sputtering by varying a bias potential (from grounded to −300 V) and substrate temperature (150–350 °C). The disks of Ta (99.95%) were used as the targets. Next, two deposition modes were selected to obtain α- and β-Ta interlayers on Zr-1Nb alloy substrates. After this, Cr coatings were deposited using the multi-cathode magnetron sputtering with a DC power supply. The disks of Cr (99.95% and 90 mm in diameter) were used as magnetron targets. The deposition mode of Cr coatings was chosen based on previously published results in ref. [24].

The selected parameters for deposition of the protective chromium coating and the tantalum interlayers are presented in

Table 1. Deposition parameters of Cr and Ta layers.

Layer	Q, W/cm2	Ubias, V	js, mA/cm2	Tmax, °C
Cr	39	−50	65	320
Ta (α-phase)	39	−100	16	210
Ta (β-phase)	39	floating	–	180

Note: Q—target power density; U_bias—bias potential; j_s—ion current density to a substrate; T_max—maximal temperature.

.

Figure 1 shows the diffraction patterns of Zr-1Nb alloy with tantalum interlayers deposited at different conditions.

According to XRD (Figure 1), the samples are significantly different from each other. The coating deposited at a bias potential of −100 V contains an α-Ta phase with a body-centered cubic lattice. The Ta layer deposited at a floating potential is characterized by a β-Ta phase with a tetragonal modification. Hereafter, the series of the samples deposited at a floating potential and chromium protective coatings are denoted as «Cr/β-Ta», and the series of the samples deposited at a bias potential of −100 V and chromium protective coatings are denoted as «Cr/α–Ta». According to refs. [35,36], applying substrate biasing can modify the crystal structure of Ta coatings. The application of a negative bias voltage to a substrate during deposition has a dramatic effect on the structural properties of the Ta films. This can be caused by the energetic coating deposition by the modification of the atomic arrangement due to higher momentum transfer to the coating [36].

The total thickness of Cr/Ta coatings was equal to ~11 μm (Figure 2), where the outer layer of chromium was ~8 μm and the Ta interlayer was ~3 μm. It should be noted that one part of the surface of Zr samples did not have a coating (the uncovered area was about 5–10 mm² due to the fixing of samples in the holder during coating deposition). The temperature of the samples during coating deposition was measured by an infrared pyrometer Optris CT laser 3MH1CF4 (Berlin, Germany).

2.3. Oxidation Tests in Water Steam

High-temperature oxidation tests in water steam were carried out at temperatures of 1200–1400 °C. The tests at 1200 °C simulating LOCA conditions were carried out by LOCA345 installation (JSC “VNIINM”, Moscow, Russia). The samples were kept in the preheating zone (at 300 °C for 300 s). Then, they were moved to the high-temperature zone of the furnace and heated up to 1200 °C with a rate of 20 °C/s. During the tests, the flow rate of steam was 4.0 mg/cm² and the temperature of the samples was maintained at 1200 ± 3 °C. When the oxidation duration was reached, the samples were quenched with distilled water. The duration of such tests was 1000–2000 s.

The high-temperature oxidation in water steam at 1250–1400 °C (at subcritical BDA conditions) was performed using GASPAR unit (JSC “LUCH”, Moscow, Russia) consisting of a ceramic tube Ø28 × 2 mm² installed in a furnace with a graphite heater. Water steam (40 mg/s) was introduced into the ceramic tube. In the experiments, the samples were directly placed in the tube and heated at a rate of 33 °C/s. The samples were oxidized for 120–300 s. Then, they were moved to the cold zone of the furnace and cooled up to 900 °C in steam with a rate of 20 °C/s. After reaching 900 °C, the samples were quenched in distilled water.

2.4. Sample Characterization

The weight gain of the samples was measured using analytical balance CP 124S (Sartorius, Goettingen, Germany) with an accuracy of 10⁻⁴ g. The evaluation of the weight gain was conducted taking into account the uncovered part (without protective coating at the fixed place). The weight gain calculations were performed according to our previous work [27].

The microstructure and elemental compositions of the samples after oxidation were analyzed using an MIRA3 scanning electron microscope (Tescan, Brno, Czech Republic) and Ultim Max 40 energy dispersive analysis attachment (Oxford Instruments, High Wycombe, UK). Also, optical microscopy (AXIOVERT 200MAT, Zeiss, Jena, Germany) was additionally used to analyze the microstructure of the cross-sections of the samples after steam oxidation.

To describe interdiffusion behavior of Cr/Ta/Zr system, in situ diffraction studies were performed under linear heating in a vacuum up to a temperature of 1250 °C. In situ experiments were performed by synchrotron radiation at the “Precision Diffractometry II” station at Siberian Synchrotron and Terahertz Radiation Center of the Budker Institute of Nuclear Physics of the Siberian Branch of Russian Academy of Science. The experimental samples were investigated at a wavelength of 1.0084 Å during linear heating in vacuum in the temperature range of 25–1250 °C with a rate of 50 °C/min. Bilayer Cr/Ta samples with size dimensions of 10 × 10 × 0.5 mm³ were specially prepared for these studies. The thicknesses of the Ta interlayer and the outer Cr coating were selected so that the intensities of reflected X-rays in the diffraction patterns from Zr, Cr, and Ta were comparable. The evaluation of the radiation penetration depth at a wavelength of 1.0084 Å demonstrated that the thicknesses of the barrier layer and protective coating should be 0.5 and 1 μm, respectively, and the total thickness was equal to 1.5 μm.

3. Results

3.1. Weight Gain Measurements

The analysis of the weight gains of the samples with Cr/α-Ta and Cr/β-Ta coatings showed the high effectiveness of the Ta interlayers under high-temperature oxidation in steam at 1200 °C (Figure 3). The Cr/α-Ta coatings have a lower weight gain (~1.73 mg/cm² at 1000 s) compared to the samples with single-layer Cr coatings (~2.2 mg/cm² at 1000 s) and almost the same values of weight gains for the Cr- and Cr/α-Ta-coated Zr alloys at 2000 °C [28]. In the case of the Cr/β-Ta coatings, the weight gain is higher (~8.6 mg/cm² at 2000 s).

The weight gain in the temperature range of 1250–1400 °C for the Cr/α-Ta and Cr/β-Ta samples is shown in Figure 4. It is very evident that the weight gains of the Cr/α-Ta samples are lower compared to samples with the Cr/β-Ta coatings. The difference between the samples becomes more pronounced with increasing oxidation temperature. The weight gains of the samples differ by more than 1.5 times when the temperature was increased up to 1330 °C. In the case of the Cr/β-Ta coatings, the weight gain (~36.2 mg/cm²) is significantly higher compared to the single Cr coatings [37].

It is assumed that the significant increase in weight gain is directly related to the phase transformations of β-Ta during oxidation. β-Ta has a tetragonal structure with a large unit cell (a ≈ 10.2 Å and c ≈ 5.3 Å), whereas α-Ta has a cubic lattice with a much smaller unit cell (a ≈ 3.3 Å). High-temperature oxidation leads to β-Ta→α-Ta transformation [38,39]. This transformation results in compressive stresses (strains) in the tantalum layer. In turn, tensile stresses act on the side of the chromium coating, creating new oxygen diffusion pathways, resulting in accelerated oxidation of the zirconium alloy with the protective coating. In the case of α-Ta, no phase transformation occurs during oxidation, which has a positive effect on the high-temperature oxidation in steam. Thus, the corrosion weight gain values of the Cr/α-Ta samples are lower than those of the Cr/β-Ta samples under the same oxidation conditions.

3.2. Scanning Electron Microscopy of Samples after Oxidation

The analysis of the SEM images of the Cr/α-Ta and Cr/β-Ta samples after high-temperature oxidation in steam at 1200–1400 °C (Figure 5, Figure 6 and Figure 7) showed that the formation of a layered microstructure consisted of an outer oxide (Cr₂O₃) layer, a residual α-Cr layer, an interdiffusion Cr-Ta layer, a residual Ta layer, an interdiffusion Ta-Zr layer, and a zirconium alloy It should be noted that cavities are formed at the Cr₂O₃/Cr, Cr/Ta, and Ta/Zr interfaces in both Cr/α-Ta and Cr/β-Ta during oxidation at 1200 °C. These cavities can promote active oxygen diffusion into the Zr alloy.

No significant changes in the microstructure of the oxidized samples were found when increasing the oxidation temperature up to 1250 °C. The thickness of the outer oxide layer is even lower than at 1200 °C due to a shorter oxidation time. The cross-sectional microstructures of the samples after oxidation at 1250 and 1330 °C are similar to the samples oxidized at 1200 °C. The Zr alloy sample with the Cr/β-Ta coating was strongly oxidized under 1400 °C for 120 s, so there is no possibility to show its cross-section in this work. The significant change in the microstructure occurs after oxidation at 1400 °C for 120 s. The SEM images of the Cr/α-Ta-coated sample show a thin outer Cr₂O₃ layer and a thick inner ZrO₂ layer indicating an oxygen diffusion to the Zr alloy and a loss of protective properties of the coating under the given temperature.

3.3. Optical Microscopy of Samples after Oxidation

Figure 8 shows the cross-sectional optical images of Cr/α-Ta- and Cr/β-Ta-coated samples oxidized at 1200 °C for 1000 and 2000 s. The multilayered structure of the formed oxide is easy to see, which was described in the previous section. The Cr/α-Ta coating is more protective than the Cr/β-Ta coating. The OM micrograph analysis showed that the coating of the Cr/β-Ta sample completely delaminated (Figure 8d), while the coating of the Cr/α-Ta sample (Figure 8b) oxidized uniformly maintaining adhesion with the zirconium alloy after oxidation at 1200 °C for 2000 s. The optical images allowed us to see the places of coating delamination, as well as how the thicknesses of the layers change. Thus, Figure 8d shows that the Cr/β-Ta coating has almost peeled off, while the Cr/α-Ta coating (Figure 8c) remains unchanged.

Figure 9 and Figure 10 show the optical images of Cr/α-Ta- and Cr/β-Ta-coated samples oxidized at 1250–1400 °C for 120 and 300 s. In both cases, there is an increase in the thickness of the Cr₂O₃ outer layer and a corresponding thinning of the residual Cr/Ta layer with increasing temperature. In addition, Figure 9 and Figure 10 show a change in the structure of the coatings: there is a thinning of the Cr/Ta layer with a corresponding increase in the outer oxide layer and the Ta/Zr interdiffusion layer. The deep oxidation of the Zr alloy sample with the Cr/α-Ta coating is observed after oxidation at 1400 °C. In this case, two layers can be distinguished: the outer Cr₂O₃ layer and the thick ZrO₂ layer. Unfortunately, it was not possible to analyze the cross-section of the Cr/β-Ta-coated sample oxidized at 1400 °C due to its complete delamination after the test.

3.4. In Situ XRD Study

Figure 11 shows the phase composition of the Cr/Ta-coated Zr alloy sample during linear heating up to 1250 °C by in situ synchrotron X-ray diffraction. The as-received sample is represented by the following phases: α-Zr with hexagonal close-packed (hcp lattice), α-Cr, and α-Ta with a body-centered cubic (bcc). No significant changes in the phase composition of the sample are observed up to 900 °C. There is only some shift of the reflections of α-Zr, α-Cr, and α-Ta intensities toward smaller 2θ angles due to the increase in the lattice parameters of materials due to its thermal expansion. The phase transition of α-Zr→β-Zr is observed at a temperature of about 900 °C.

The increase in temperature up to 1250 °C is accompanied by a growth in the intensity of the β-Zr phase, indicating an increase in its content. The β-Zr phase becomes dominant over the α-Zr phase at temperatures above 1200 °C. The isothermal exposure of the sample at 1250 °C for 2 min leads to asymmetry of the (110) reflection of the α-Cr phase, which indicates the formation of the Cr₂Zr phase. Further exposure results in partial melting of the Cr/Ta-coated zirconium alloy and, as a consequence, disruption of the crystal structure of the investigated sample. In the case of the single-layer chromium coating, the formation of the Cr₂Zr (Figure 12) phase occurs at lower temperatures (1200 °C) [19].

4. Discussion

A serious problem in high-temperature oxidation of Cr-coated Zr alloy is the fast diffusion at the interface between the coating (Cr) and fuel cladding material (Zr). Interdiffusion is mainly observed for metallic coatings due to the higher diffusion coefficients in metals compared to ceramic materials [40]. Thus, metal coatings can be consumed by oxidation (due to a growth of the outer oxide layer) and the formation of an interdiffusion zone with a substrate material underneath the residual metal coating. Diffusion can be accelerated because the forming eutectic phases having low melting points, especially under accident conditions and other conditions. Thus, in the case of single-layer Cr coatings for Zr alloys, the melting point of the eutectic Cr-Zr layer is about 1332 °C [41]. This temperature is the limit of the protective behavior of the Cr coating under BDBA conditions [37].

To improve the protective properties of Cr coatings onto Zr alloys, several materials are suggested to use as a barrier interlayer for preventing Cr-Zr interdiffusion. This critical paper considers the effect of the deposition of a 3 µm thick Ta barrier interlayer on the oxidation behavior of a Cr/Ta-coated Zr alloy under LOCA and beyond LOCA temperature conditions. Based on the analysis of the cross-sectional microstructures of the Cr/Ta-coated samples after oxidation in steam and in an in situ XRD study during heating, the following description of the oxidation and interdiffusion behavior of the Cr-Ta-Zr system can be made.

At the initial stage of oxidation, the outer Cr layer can be oxidized to form Cr₂O₃, reducing the thickness of residual Cr. At the Cr₂O₃/Cr interface, the formation of cavities by the Kirkendall mechanism is well observed due to the diffusion of Cr to the surface from the residual Cr layer, which causes a back diffusion of vacancies formed during the oxidation of the chromium layer [18,28]. Similarly, cavities are formed at other interfaces of the Cr/Ta-coated Zr alloy: interdiffusion is well observed at the interfaces of Cr-Ta and Ta-Zr that results in the formation of interdiffusion zones. The thickness of the residual Ta layer decreases due to the interdiffusion at their interfaces. The eutectic Cr-Zr layer enriched in Ta is being formed at the Cr/Cr-Ta-Zr interface, which can melt at 1332 °C. In these regions, capillary effects can arise, which cause the displacement of the eutectic Cr-Ta-Zr layer to the material surface. The mentioned reasons contribute to the acceleration of oxygen diffusion into the samples. The interaction of oxygen with the melted Cr-Zr-Ta regions leads to ZrO₂ and Cr precipitations. Cr and Ta formed during the reaction will diffuse into the Zr alloy resulting in the appearance of Cr- and Ta-enriched layers in the zirconium alloy. Also, due to the oxygen diffusion underneath the outer Cr coating, a Ta2O5 phase with orthorhombic crystalline modification can be formed during oxidation. The lattice parameters of this phase are much larger compared to Cr, Zr, and their oxides, which results in the generation of high internal stresses leading to cracking and partial delamination of the protective coating [42].

SEM and OM images of the oxidized samples show partial delamination of the Cr/Ta coating during long-term treatment in Ar and high-temperature oxidation tests (Figure 5 and Figure 6). It is assumed that the chromium coating loses its protective properties due to the capillary effect and Ta₂O₅ formation underneath the coating.

Long-term oxidation is accompanied by the formation of pores at the boundaries of where, according to EDS analysis, oxygen, tantalum, and zirconium are localized (Figure 5). Such pores were found only in the samples tested at 1200 °C for 1000 and 2000 s. The mechanism of formation of these pores remains unexplored, and more detailed studies are required.

Nevertheless, the complex analysis of the obtained data showed that the suppression of the Cr-Zr interdiffusion by the Ta interlayer can be possible at high-temperature oxidation in steam up to 1330 °C, but this effect is only over the short term. Considering the obtained results, it can be assumed that the thin metal coatings cannot protect the zirconium alloy from oxidation under BDBA conditions during long-term oxidation.

5. Conclusions

The high-temperature steam oxidation of the Cr/Ta-coated Zr-1Nb alloy samples simulating the loss-of-coolant accident conditions showed the following conclusions:

• Corrosion resistance of the Cr/α-Ta-coated Zr alloy samples is better than Cr/β-Ta under all considered conditions. The weight gain of the Cr/β-Ta samples is 3 times higher compared to Cr/β-Ta (36.17 mg/cm² and 14 mg/cm², respectively) under steam oxidation at 1400 °C. All other experiments further confirm the better performance of samples having an α-Ta interlayer compared to a β-Ta one.
• Up to 1330 °C, the protective properties of the Cr/Ta coating are preserved, and in the case of the Cr/α-Ta samples, the thickness of the residual chromium decreased in comparison with the samples studied at 1250 °C. At 1400 °C, a thin layer of Cr₂O₃ and a thick layer of ZrO₂ are observed, which indicates a complete loss of protective properties of the coating. The coating delamination is observed during long-term oxidation due to the formation of many cavities at the interfaces, which may coalesce into large pores.
• The Ta barrier interlayer deposited between the protective Cr coating and the Zr-1Nb zirconium alloy can limit Cr-Zr interdiffusion due to a deceleration of the Cr₂Zr phase formation under high-temperature oxidation. This limitation is realized up to 1330 °C only for a short period, and this affect was not observed during oxidation at 1400 °C.