Multi-Elemental Coatings on Zirconium Alloy for Corrosion Resistance Improvement

Abstract

Zirconium alloys are commonly used as a cladding material for fuel elements in nuclear reactors. This application is connected with zirconium alloy’s good resistance to water corrosion and radiation resistance under normal working conditions. In the case of severe accident conditions, the possibly very fast oxidation of zirconium alloys in steam or/and air atmosphere may result in the intense generation of hydrogen and explosion of the hydrogen oxide mixture. The development of a solution to minimize the aforementioned risk is of interest. One of the actual concepts is to improve the oxidation resistance of Zr alloy cladding with protective coatings. This study aimed to develop, form, and investigate new coatings for zirconium alloy Zry-2. Multi-elemental Physical Vapour Deposition (PVD) coatings with Cr, Si, and Zr were considered for Institute of Nuclear Chemistry and Technology) INCT as corrosion protective coatings for nuclear fuel claddings. Heat treatment at 850–1100 °C/argon, air oxidation processes at 700 °C/1–5 h, and a long-term corrosion test in standard conditions for Pressure Water Reactor (PWR) reactors (360 °C/195 bar/water simulating the water used in PWR) were carried out. Initial, modified, and oxidized materials were characterized with Scanning Electron Microscopy (SEM) (morphology observations), Energy Dispersive Spectroscopy (EDS) (elemental composition determination), and X-ray Diffraction (XRD) (phase composition analysis). Slower oxidation processes and a smaller oxidation rate, in the case of modified material investigations, were observed, as compared with the unmodified material. The obtained results displayed a protective character against the oxidation of formed layers in the defined range of parameters in the process.

1. Introduction

Water-cooled nuclear reactors (WCNR) have played a significant role in the nuclear industry. More than 400 of the currently operating reactors (c. 96%) were constructed as water-cooled reactors. The most common water-cooled reactors are light water reactors (LWR). This type of reactor requires special fuel that is enriched with the U-235 isotope. Nuclear fuel for water-cooled reactors is produced in the form of monolithic ceramic uranium dioxide pellets. Claddings for theses pellets are manufactured from zirconium-based alloys. Many improvements have been made to cladding and pellets since the first use of this type of fuel (1960s and 1970s). The severe accident at the Japanese Fukushima Daiichi took place in 2011, due to an earthquake and tsunami. During this accident (affecting three reactors), cooling was lost for many days. As a result, the overheating cladding reacted with water, and a high amount of hydrogen was produced. This causes explosions and nuclear fuel melting. This event highlighted the vulnerability of zirconium clad fuel under severe accident conditions.

In response to this event, it is considered important to investigate other fuel and cladding designs that would be more resistant to such severe accidents, whilst retaining the reliability and burn-up potential of current fuel designs.

Nuclear reactors present a harsh environment for component services, regardless of the type of reactor. Components within the reactor core must tolerate exposure to coolant (high-temperature water, liquid metals, gas, or liquid salts), stress, vibration, an intense field of high energy neutrons, and gradients in temperatures. Degradation of materials in this environment can lead to reduced performance and, in some cases, sudden failure [1]. Cladding has been a particular focus of accident tolerant fuels because of the rapid high-temperature oxidation of zirconium-based cladding with the evolution of H₂ when water is a reactant [2]. They are many requirements for all nuclear reactor structural materials. The material must have adequate availability, fabrication, and joining properties, as well as favourable neutronic and thermal properties. It must have good mechanical properties, good creep resistance, and long-term stability. The majority of the world’s commercial nuclear power plants are light water reactors (LWR). Current commercial LWR use a core of zirconium alloy clad UO₂ fuel [1,3].

Zirconium alloys are commonly used as a cladding material for the fuel elements in nuclear reactors. This is due to the good resistance of zirconium alloys to water corrosion and radiation resistance under the normal working conditions of nuclear reactors. Different Zr-based alloys are used for fuel claddings. Differences are connected with composition of commercial alloys as well as advanced Zr alloys [1,2,4].

Table 1. Typical commercial Zr-based alloys used as claddings in water-cooled reactors.

Alloy/Alloy Composition (wt.%)	Nb	Sn	Fe	Cr	Ni	O
Zircalloy-2 (Zry-2)	-	1.50	0.15	0.1	0.05	0.1
Zircalloy-4 (Zry-4)	-	1.50	0.2	0.1	-	-
M5	1.0	-	-	-	-	0.14
ZIRLOTM	1.0	1.0	0.1	-	-	0.1
E365	1.0	1.2	0.35	-	-	-

summarizes the compositions of the typical commercial Zr alloys used as claddings in water-cooled reactors [4].

In the case of severe accident conditions, including LOCA (loss of coolant accident) and SBO (station blackout accident), the extremely fast oxidation of zirconium in a steam-containing atmosphere at temperatures above 800 °C takes place. This process results in high-temperature oxidation under steam environment conditions, which leads to significant embrittlement, a large release of hydrogen, and the destruction of the zirconium cladding material [5].

In the case of intense hydrogen build-up and creation of explosives, a hydrogen oxide mixture is possible. Allen (2010) and Zinkle (2013, 2014) in their works proposed the evolution model for the degradation of fuel rods under LOCA and SBO conditions:

(i)

For the cladding: steam oxidation, ballooning and burst, and internal oxidation;

(ii)

For the fuel: relocation and dispersion, oxide eutectic formation (cladding, fuel, and oxygen), and rod melt [1,6,7].

A solution leading that minimizes the risk related to unforeseeable situations is urgently needed. Regarding the safety of nuclear reactors, strong improvements are required in the areas of nuclear fuel composition, cladding integrity, and fuel–cladding interaction.

Four key performance features of accident tolerant fuel (ATF) and accident tolerant material (ATM) concepts for improved safety margin were proposed. The ideas and concepts are presented: (i) reduction of the core enthalpy input, i.e., the steam–cladding oxidation rate and heat of oxidation of cladding have to be reduced; (ii) reduction of the combustible hydrogen generation, i.e., the steam–cladding oxidation rate has to be reduced, and the hydrogen sequestration and chemical conversion have to be enhanced; (iii) improvement of the cladding to maintain the core cooling ability and retain fission products, i.e., high-temperature cladding strength, fracture resistance, and burst margins have to be improved, melting temperature has to be increased, and thermal shock resistance and resistance to hydrogen embrittlement are also required; (iv) improvement (control) of containment of fuel fission products, i.e., retention of fission products has to be enhanced, fuel relocation and dispersion have to be minimized, operating temperatures have to be lowered, clad internal oxidation has to be reduced, and the fuel melting safety margin has to be increased. The aforementioned classification was proposed by [7]. Other authors and teams presented similar points of view, as connected with ATF [1,5,8].

The investigations were focused on the following issues: (i) non-zirconium cladding with high oxidation resistance and required strength; (ii) improved high-temperature oxidation resistance and/or strength of Zr alloys cladding; (iii) alternative fuel forms with improved performance and fission product retention, compared to monolithic UO2 [2,3,4,5,8].

Highly adherent oxidation-resistant coatings can be an alternative to bulk materials for cladding. According to the works carried out, some important issues were identified: coating could be easy regenerated if a small portion spalled off during fuel assembly handling during normal reactor operation; the significance of matching the coefficient of thermal expansion to minimize interfacial tensions and delamination in normal operation (from room temperature to 300 °C) and above normal operation (from 300 °C up to a high temperature) conditions; coating must be strong enough to withstand the diametrical compression that results from reactor pressurization; the durability of coatings required to protect the exposed cladding for a sufficient period of time after burst.

There are some ideas and proposed methods for increasing the claddings’ corrosion resistance: (i) develop new alloys with special composition and microstructure (for example: M5 from AREVA or ZIRLO from Westinghouse [9]); (ii) application of new materials that are resistant to water corrosion (for example: SiC [10]); (iii) modification of the surface layer of Zry by the so-called “Fresh-Green” process [11]; (iv) modification of the surface layer by irradiation with ion beams [12], pulsed electron beams [13], and plasma beams [12,14]; (v) incorporation of rare earth elements to the surface of Zry [15]; (vi) a protective layer formed on the Zry surface (for example: on the base of silicon [16,17] and ceramics MAX, where MAX phases are layered, hexagonal carbides and nitrides, which have the general formula: Mn+1AXn (MAX), where n = 1 to 3, and M is an early transition metal, A is an A-group (mostly IIIA and IVA or groups 13 and 14) element, and X is either carbon and/or nitrogen [18] or FeCrAl alloys [19]).

There are a lot of different methods for surface layer material modifications and coating formations. Surface engineering may be defined in a number of ways. The subject concerns at least three separate, yet interlayered, activities: (i) the optimisation of surface properties (particularly concerned with the performance of surfaces and coatings, with respect to corrosion and wear); (ii) coating and modified surface characterizations (with respect to condition, composition, structure, and morphology, as well as the mechanical, electrical, and optical properties); (iii) coating and surfacing technologies (embracing the more traditional technologies of painting, electroplating, weld surfacing, and spraying, as well as thermal and thermochemical treatments, such as nitriding and carburising), as well as more recently emerging technologies, such as laser surfacing, physical and chemical vapour deposition, and ion implantation. At the same time, the lack of understanding at the basis level, particularly in the overall design, in terms of the advanced processes, e.g., the physics of the ion plating process and structure/property relationship of/within the surface treatment/coating (and associated substrate, i.e., the system), is a major handicap in optimisation. It is now possible to deposit almost any material on an appropriate substrate (which may itself be engineered) to produce a coating system for optimum performance under very specific working conditions [20]. Due to the used process and its parameters, the structure of the formed materials is different. The parameters that are crucial for the coatings formation process should be mentioned, for example: temperature, atmosphere, time, additives, and current (type, density, and frequency) [21]. Difference in microstructures result in different physical properties [20]. The properties that were investigated, measured, and studied should be mentioned, for example: (i): elastic modulus, fracture toughness, strength and adhesion, thermal conductivity, electrical and magnetic properties, corrosion and oxidation [20], and (ii) optical [22].

Multi-elemental coatings formed on the zirconium alloy surface can play the role of a protective barrier that connects with the different behavior of the elements used.

The zirconium silicide or zirconium silicate coatings are known as materials with good resistance in high-temperature conditions. For that reason, they are considered for application as environmental barrier coatings for high-temperature gas turbine components. Silicon-based coatings may offer excellent prospects in this field. Particularly, they may provide a more protective barrier than the native ZrO₂ films formed on alloy cladding during routine nuclear plant operations, as well as an exceptional protective barrier during high-temperature accident scenarios [2].

The development of an environmental barrier coating focused on the application of non-oxide ceramics for high-temperature gas turbine components. Ceramics with silicon can be used as protective materials for nuclear cladding [23]. The behavior of the zirconium silicide-coated LWR fuel cladding was investigated by Yeom (2016 and 2018). The authors investigated the microstructural evolution during the isothermal oxidation, at temperatures of 700, 1000, and 1200 °C. Different phases were identified, due to process conditions: (i) Si, ZrO₂, ZrSi₂, (ii) oxide layer; ZrSi₂, (iii) oxide layer; Si-rich oxide ZrSi₂ [24,25]. The thermal stability of zircon (ZrSiO₄) was investigated by [26]. The authors analyzed the phase diagram ZrO₂-SiO₂ and zircon decomposition conditions.

The equilibrium system (knows as phase diagram) for silicon–zirconium systems shows the presence of compounds with different stability regions, as well as the silicon–zirconium ratios (Figure 1). For example: Si2Zr is stable up to 1620 °C, SiZr is stable up to 2210 °C, SiZr₂ is stable up to 2215 °C, and Si4Zr5 is stable up to 2250 °C. The temperature range of stability for zircon (ZrSiO₄) extends to 1673 °C, where it thermally decomposes to ZrO₂ and SiO₂. In the oxidizing atmosphere, elements Zr and Si form their respective oxides at high temperatures and surface layers of ZrO₂ or SiO₂; ZrSiO₄ may even be formed, depending on the stoichiometry [27]. The inner layer of SiO₂ can serve as the barrier layer for oxygen and moisture. In a high-temperature oxidizing environment, the sandwich-type layers can be form with ZrO₂ or SiO₂. As the effect of this process is compositionally and functionally graded, a multi-layered system can be created. As a result, the necessary and required protection under accident conditions is expected [24,25,26].

Chromium metal in the air is passivated by oxidation, forming a thin, protective surface layer of Cr₂O₃. The layer is a spinel structure of only a few molecules thick. It is very dense and prevents the diffusion of oxygen into the metal. Chromium has a high melting point, high corrosion resistance in water and steam, and is similar to the zirconium coefficient of thermal expansion. Chromium-based coatings are promising materials among ATMs [4,28].

The presented research work aimed to identify the possibility of extending the lifetime of zirconium claddings and decreasing hydrogen gas generation during severe accident conditions. The goal was to develop multi-elemental coatings with silicon and chromium on zirconium alloy claddings and evaluate their properties in different temperatures and atmospheres.

2. Materials and Methods

2.1. Investigated Materials

The substrate material used for the investigations was zirconium alloy (Zry-2) from Westinghouse, in the form of plates, with a thickness of 0.5 µm. The elemental composition, according to the producer’s information, is presented in

Table 2. Elemental composition of Zry-2 used for investigations (producer’s information).

Element	Concentration [Wt.%]
Tin (Sn)	1.3–1.6
Iron (Fe)	0.07–0.20
Chromium (Cr)	0.05–0.16
Nickel (Ni)	0.03–0.08
Average (Fe + Cr + Ni)	0.23–0.32
IMPURITIES (no more)	[ppm]
Al	50
B	0.5
Cd	0.5
C	500
Co	10
Hf	200
Pb	100
Mg	20
Mn	50
Mo	50
Si	100
Ti	50
W	50
V	50

. The main elements here are zirconium and tin with iron, with a chromium and nickel addition. The producer also showed some impurities: aluminium, boron, cadmium, carbon, cobalt, hafnium, lead, magnesium, manganese, molybdenum, silicon, titanium, tungsten, and vanadium with a concentration below the defined amount.

The elemental composition of the materials for investigation was determined using instrumental neutron activation analysis (INAA) and presented in

Table 3. The elemental composition of Zry-2 used for investigations (INAA measurements).

Element	Concentration [ppm]	Element	Concentration [ppm]
Ag	0.86	Mn	99.09
As	1.3	Mo	1.1
Au	0.01	Na	176.46
Ba	69.12	Nd	30.69
Br	111.59	Ni(Co-58)	921.38
Cd	84	U-238	2.15
Ce	2.72	Th	0.36
Co	1.71	Pr	3.61
Cr	619.06	Rb	16.48
Cs	1.87	Sb	3.99
Cu	318.77	Sc	0.31
Dy	0.72	Se	6.23
Er	5.57	Si	1630.4
Eu	0.18	Sm	0.18
Fe	10,292.27	Sn	12,489.51
Ga	83.12	Ta	0.61
Gd	0.89	Tb	0.7
Hf	40.96	Tm	0.15
Hg	2.74	W	0.3
Ir	0.0003	Yb	0.46
K	38.45	Zn	552.23
La	1.1	Zr	887,060.49
Lu	0.1

.

The presence of the main elements was confirmed: zirconium, tin iron, chromium, and nickel. More elements, including trace elements, were identified with a comparison of the producer’s data, for example, Ag, As, Ba, Br, Ce, Cs, Cu, Dy, Er, Gd, Hg, K, La, Na, Nd, U, Pr, Rb, Sb, Se, and Zn; those identified in amounts below 1.0 ppm: Ag, Au, Eu, Ir, Lu, Th, Sc, Sm, Ta, Tb, Tm, and Yb. These data can be important in the interpretation of the results, especially the effects of the oxidation experiments.

The Zry-2 material was cut into samples of 20 and 20 mm as well as 20 and 10 mm. The technological 2 mm hole was bored in the sample for the possibility of properly putting them into the autoclave.

The sources of the elements predicted to be present in the protective coatings were targets for the physical vapour deposition (PVD) method: ceramic ZrSi₂ and metallic Cr.

2.2. Coating Method

Coatings on the zirconium alloys were formed using the PVD method. The physical vapour deposition (PVD) process has been known for over 100 years. According to this fact, there are a lot of literature positions on this subject. The information presented below is from the work on sputtering physical vapour deposition, which was written by Baptista et al. in 2018 [29].

The PVD technique is a thin film deposition process, in which the coatings grow on the substrate atom-by-atom. The two most common PVD operations are thermal evaporation and sputtering. In both, the resulting vapour phase is put onto the target substrate via condensation. The thermal physical process of releasing or colliding transforms the material to be deposited (the target) into atomic particles, which are directed to the substrates in conditions of gaseous plasma in a vacuum environment, generating a physical coating by condensation or the accumulation of projected atoms. Physical processes, such as sputtering and evaporation, are used in PVD to generate vapour in the form of atoms, molecules, or ions of the coating material supplied from a target. They are then transported to and deposited on the substrate surface, resulting in coating formation. The steps of the PVD method are as follows: (i) vaporization of the material from a source, (ii) transportation of the disintegrated material, and (iii) nucleation and development to create a film.

Sputtering is a plasma-aided process that produces vapour from the source material by bombarding it with high-speed plasma ions. The evaporated source material atoms, bunches of atoms, or molecules travel in a straight line. If a substrate material is put in the way of these streaming particles, it will be covered by a thin film of the source material. Source material atoms atomically bond to the substrate to develop a thin film. To generate a consistent thin film that is just a few atoms or molecules thick, a target item can be rotated on various axes. Single or multiple coatings can be proceeded/performed with the same deposition process. In the case of the sputtering process, fine layers of several materials are applied while using magnetron sputtering. The raw material for this vacuum coating process takes the form of a target. A magnetron is placed near the target in the sputtering processes. Then, in the vacuum chamber, an inert gas is introduced, which is accelerated by a high voltage that is applied between the target and substrate in the direction of the magnetron, producing the release of atomic size particles from the target. These particles are projected as a result of the kinetic energy transmitted by gas ions, which have reached the target by going to the substrate and creating a solid thin film [29].

PVD can produce coatings with excellent adhesion, homogenous layers, designed structures, graduated properties, controlled morphology, and a high diversity of materials and properties. The PVD process allows for deposition in mono-layered, multi-layered, and multi-graduated coating systems, as well as special alloy compositions and structures.

Coatings formation processes with Zr, Si, and Cr in the composition on zirconium alloys were carried out using the PVD method with the magnetron sputtering processes in the Łukasiewicz Research Network—Institute for Sustainable Technologies ITS (Radom, Poland). The Balzers system, i.e., the multisource plasma magnetron sputtering system with three magnetron plasma sources, three power sets, and control panels, was applied. Three magnetron plasma sources allowed for depositing materials from separate targets and obtaining multi-elemental coatings. Before deposition, the chamber was evacuated to 3 × 10⁻³ Pa, and the samples were heated up to about 300 °C and etched with glow discharge in Ar-reduced pressure. The working pressure during deposition was 0.5 Pa, and the distance from the samples to the plasma source was 150 mm. During the process, the temperature was constantly monitored using a pyrometric temperature measurement system. The coatings on Zry-2 were prepared using two separate targets: ceramic as sintered ZrSi₂ and metallic Cr.

2.3. Materials Characterization

Materials: initial, modified, and heat-treated zirconium alloys were characterized in each step of the conducted investigations.

Surface and cross-section morphology observations were carried out with the OM Bresser Science ADL-601 P optical microscope (Bresser, Rhedo, Germany), the DSM 942 scanning electron microscope (SEM) (Zeiss, Jena, Germany), and a high-resolution scanning electron microscopes (HR SEM) ULTRA (Zeiss, Jena, Germany)

The cross-section of the investigated materials was prepared as follows. A small part of the investigated material was cut using the precision cutting machine, i.e., the Labotom 5 (Struers, Krakow, Poland), using the diamond cut-off wheel MOD 25 for cutting the ceramics (>HV 800) and minerals. The material was included in the conductive resin PolyFast (Struers, Krakow, Poland), using the mounting press CitoPress-1. PolyFast is a phenolic hot mounting resin with a carbon filler that is used for edge retention and examination in SEM. The included sample was treated using the Tegramin 25 Preparation System for high-quality specimen preparation and integrated specimen mover head and automated process control grinding/polishing machine. Four steps, with different grinding/polishing discs (Struers, Krakow, Poland), were applied: (i) MD Piano 220+ water, force 50N, time 3.0 min. MD Piano 220: resin-bonded diamond grinding disc for plane grinding of materials HV150-2000. Surface finish comparable to SiC paper grit 220. (ii) MD-Largo + Dia-Pro Allergo Largo 9, force 40 N, time 5 min. MD-Largo: fine grinding disc for fast material removal. Dia-Pro 9 µm: water-based diamond suspension containing a unique mixture of high-performance diamonds and cooling lubricant. (iii) MD-Dac + DiaPro Dac 3. MD-Dac: universal polishing cloth for all materials. DiaPro Dac 3 µm: water-based diamond suspension containing a unique mixture of high-performance diamonds and cooling lubricant. (iv) MD-Nap + OP-S, force 40 N, time 5 min. MD-Nap: cloth for final polishing of all materials with a short synthetic nap. OP-S nonDry: colloidal silica suspension for final polishing. Suitable for mixing with chemical reagents. For very ductile materials, such as refractory metals and other non-ferrous metals.

The elemental composition (surface and elemental depth profile) was investigated with:

-

The energy dispersive spectroscopy (EDS) method, using the Quantax 400 (Bruker, Billerica, MA, USA) system. The Quantax 400 library (database) was built for a high voltage (HV) of 15 kV. The system was calibrated for Cu reference, with a registered standard supplied by Micro-Analysis Consultant Ltd. (MAC, St. Ives, UK). The samples were subjected to an electron beam with HV 15 kV acceleration. The identification of elements was performed by an analysis of the energies of the X-ray emitted by excitation of specimen atoms with a focused electron beam. Emitted X-rays have an energy that is characteristic of excited elements. Area, point, and line scan analyses were applied.

-

The instrumental neutron activation analysis (INAA) method was used to determine the concentration of trace and major elements in matrices. A sample was subjected to a neutron flux; then, radioactive nuclides were produced. As these radioactive nuclides decay, they emitted gamma rays, whose energies were characteristic for each nuclide. Comparison of the intensity of these gamma rays with those emitted by a standard permits quantitative measurement of the concentrations of the various nuclides.

Crystallographic phases were identified by the X-ray diffraction method (XRD) in Bragg–Brentano geometry and grazing X-ray diffraction method (GXRD) in small-angle diffraction using CuKα (wavelength of 1.5406 Å) with an Advanced 8 diffractometer (Bruker). Phases were identified according to:

Zr—PDF 05-0665 (hex. P63/mmc SPGR = 194 a = 3.232 A, c = 5.147A);

Zr₂Si–PDF 73-2164 (tetr. I4/mcm SPGR = 140 a = 6.612 A, c = 5.2943 A);

ZrO₂—PDF 78-1807.

Thermal treatment and oxidation investigations in the form of:

-

Tests: 700 °C, up to 5 h, with a 1-h interval air chamber furnace;

-

Thermal treatment: 800 and 1100 °C, 4h, argon, tube furnace;

-

Long-term corrosion tests were performed in standard conditions for a pressurized water reactor (PWR), which means: 360 °C/195 bar/water simulating water used in PWR for 21, 42, and 63 days, using an autoclave Parr 4653 with a volume of 1 dm³.

Mass changes were determined with the precision balance Mettler Toledo EXCELLENCE XS 105. The kinetics of processes were also determined for chosen samples.

Water chemistry investigations were performed at 23 °C, in the form of:

-

pH (quantitative measure of the acidity or basicity of aqueous or other liquid solutions), DO (dissolved oxygen), and conductivity measurements with a ProLab 2500 digital meter for IDS sensors (SI Analytics, Mainz, Germany);

-

The presence of ions (anions and cations) was determined using the inductively coupled plasma mass spectrometry (ICP-MS) method, with the following spectrometers: Elan DRC II (Perkin Elmer, Waltham, MA, USA) and Thermo Electron Corporation Solar M6-Mk II.

3. Results and Discussion

3.1. Formed Coatings/Surface Layers

The view of Zry-2 coated with ZrSi₂ and Cr samples are shown in Figure 1. The samples were coated uniformly. Scratches on the initial material were visible. Agglomerates of the deposited material could also be distinguished (Figure 2a,b). The border between the base material and coatings was clearly seen. The thickness of the obtained layer was about 2.5 µm (Figure 2c).

The mass (wt.%) and atomic (at.%) fractions of the identified elements were used to define the composition of the formed coatings. Elemental composition—the presence of the identified elements were detected using the EDS method. Identification was determined using the Quantax 400 library built for HV of 15 kV. The elemental composition of formed coatings was determined. Totals of 40 wt.% Zr, 24 wt.% Si, and 36 wt.% Cr were in the formed coatings. So, authors, knowing the elemental composition of the formed coatings, decided to name it Zr₄₀Si₂₄Cr₃₆.

The presence of zirconium, silicon, and chromium elements was confirmed by the elemental analysis of the formed coatings. The depth elemental profiles were conformable to the surface elemental analysis results (Figure 3). The line scan analysis confirms the presence of a clearly defined border between the base material and formed coating. This observation is in agreement with the PVD method theory of coatings formation, as described in Section 2.2 of this work.

One important point/parameter of the formed coatings is their homogeneity. The difference in the elemental compositions of different points of the coatings may result in a difference in the areas of the sample/coatings behaviors. The homogeneity of the formed coating was checked using a point elemental analysis with EDS. The results of the elemental composition analysis at nine different chosen areas of the surface showed that the obtained layer was homogeneous (

Table 4. Homogeneity of the Zr-Si-Cr coatings in nine areas of the modified sample.

1A	1B	1C	Elements Concentration
2A	2B	2C
3A	3B	3C	wt.%
Sample Area			Zr	Si	Cr
1A			35.62	24.24	39.05
1B			35.47	24.25	39.19
1C			35.23	24.27	39.34
2A			35.72	24.32	38.96
2B			35.35	24.20	39.18
2C			35.31	24.26	39.41
3A			35.51	24.31	39.59
3B			36.14	24.22	40.13
3C			35.37	24.07	38.97

). The table’s cell data correspond to the elemental composition of the investigated area of the sample.

The elemental compositions of the formed coatings were known. There were 40 wt.% Zr, 24 wt.% Si, and 36 wt.% Cr in the formed coatings. Finally, it can be concluded that the homogeneous coatings of Zr₄₀Si₂₄Cr₃₆, with thicknesses of around 2.5 µm, were formed on Zry-2 surface.

3.2. Material Oxidation Behavior

Various types of tests were performed, in order to determine the oxidation resistance of the formed Zr₄₀Si₂₄Cr₃₆ coatings.

3.2.1. Thermal Treatment: 800 °C and 1100 °C, 4 h, Argon

The thermal treatment experiment was performed at 850 °C and 1100 °C/argon/4 h, using the tube furnace.

In the case of heat treatment processes, the thickness of the visible formed layer increased, as compared to 2.5 µm of formed coatings. As for the process at 850 °C, a layer with a thickness of 4.16 µm (two sublayers, 2.24 and 1.92 µm thick) was observed (Figure 4a). As for the process at 1100 °C, a layer with a thickness of 5.79 µm (with two sublayers, 2.31 and 3.48 µm) was observed (Figure 4c). The thickness of the observed layers was bigger, as compared to the initial layer thickness of around 40% and 57% after 800 °C and 1100 °C, respectively. The authors suggest that these observations are the result of elemental diffusion from the formed coatings toward the core of the base material.

The investigated elemental profiles also showed changes, as a result of heat treatment.

In the case of treatment at 850 °C, the elemental depth profile showed: (i) the area with Zr and a small amount of O presence; (ii) the area enriched in Si, up to 36%, at the Zry-2 side, with or without a small presence of Cr, (iii) and enriched in Cr, up to 42%, of the layer near the surface, with the remaining volume of Si, as in the formed coating.

The silicon diffusion towards the base material and chromium diffusion towards the surface was proved by analyzed sublayers presence with different elemental compositions, as compared to the material before heat treatment (Figure 4b). In the case of treatment at 1100 °C, the elemental depth profile showed (i) the area with the presence of Zr and O in the amounts of 75% and 15%, respectively; (ii) the area with Zr and Si in the amounts of 70 at. % and 24% Si, respectively, and a small amount of O; (iii) the area with the presence of Zr and O in the amounts of 70% and 25%, respectively, and a small amount of Si. These results show that, after the discussed treatment, Si was present at the initial level. Its profile was shifted towards the base material. Cr was not detected in the analyzed cross-section (Figure 4d). The small amount of oxygen detected here can be explained by the presence of the passivation layer at the Zry-2 and oxygen presence as a contamination in the used gaseous atmosphere.

The detailed results of the observations and elemental compositions of different areas of the formed coatings are presented in Figure 5 and

Table 5. Elemental composition of the surface areas after thermal treatment at 1100 °C/argon/4 h in external and internal parts.

Element	Concentration (wt.%)	Concentration (at. %)	Error (1 Sigma)
external part
Zirconium	86.85	66.04	3.11
Silicon	12.31	30.39	0.52
Chromium	0.03	0.04	0.0
internal part
Zirconium	95.59	85.16	3.44
Silicon	1.76	5.11	0.10
Chromium	0.10	0.16	0.04
Tin	0.76	0.52	0.06

. In the external part of the area, the silicon concentration was at a level of around 12.5 wt.%, and chromium concentration was about 0.04 wt.%. These values were about 52% and 0.1% Si and Cr, respectively, as compared with the elemental composition of the formed coatings. The small amount of chromium could be the result of chromium diffusion close to the surface of the coatings. In the internal part of the area layer, the silicon concentration was at the level of around 1.8 wt.%, and the chromium concentration was at about 0.1 wt.%. These values were around 7.5% Si and 0.03% Cr, respectively, as compared with the elemental composition of the formed coatings.

The question of such a small chromium concentration is still open. The composition of the internal part of the layer was close to the base material (here: Zry-2) composition, and it was the reason for the presence of Sn.

The Zr, ZrO₂ monoclinic phase and Zr₂Si phases were identified in the XRD spectra (Figure 6).

Zr with a hexagonal structure was identified (black markers). This fact was confirmed by the presence of peaks in the following positions: 2θ = 32, 36, 37, 48, 57, 63.5, 67, 68.5, 70, and 74 degrees.

ZrO₂ monoclinic phase was identified (red markers). This fact was confirmed by the presence of peaks in the following positions: 2θ = 28, 34, 51, 56, and 66 degrees.

The most important information is that, in the case of 850 °C and 1100 °C treatment in an ambient atmosphere, the Zr₂Si phase was formed. This phase was stable up to 1950 °C, with respect to the Zr-Si phase equilibrium phase diagram presented in the Introduction section [27].

The presence of high-temperature-resistant phases can improve the high-temperature resistivity of the system base material with coating. This fact allows us to underline the increase of the high-temperature resistivity of the modified cladding material, as compared to an unmodified one.

3.2.2. Oxidation Experiment: 700 °C/1, 2, 3, 4, and 5 h/air

The weight gain of oxidized uncoated Zry-2 was systematic grown after 1, 2, and 3 h, and it was drastically faster after 3 h of the oxidation process. After 5 h, the weight gain was 11.9 mg/g. In the case of Zry-2 coated with Zr₄₀Si₂₄Cr₃₆, the weight gain systematically grew during the whole duration of the oxidation process. Finally, this parameter obtained the value of 1.9 mg/g. After 5 h of oxidation, the weight gain was around 63% smaller, as compared to the oxidized Zry-2 base material (Figure 7). The observed difference was significant. The presence of the formed Zr₄₀Si₂₄Cr₃₆ layer slowed the oxidation process down at 700 °C and protected the base material against oxygen migration inside the material.

The depth profiles of elements identified in the Zr₄₀Si₂₄Cr₃₆ layer, after the oxidation process showed changes in the element profiles (Figure 8).

The character of elemental distribution in the coating (better here: system coating/base material) was changed in the oxidation time. In the case of materials just after modification, the border between the base material and coating was clearly visible (as was described in Section 3.1). After the oxidation process, the amount of Zr, Si, and Cr elements remained at the same level as they were inside the formed coatings. This means that the formed Zr₄₀Si₂₄Cr₃₆ coatings were stable in this experiment. Oxygen was not detected inside the formed coatings. Simultaneously, a small amount (only about 3%) of oxygen was detected beyond the coatings. This means that the oxidation of the base material occurred.

Taking the weight gain results and elemental profile distributions into consideration, the authors conclude that the presence of the formed Zr₄₀Si₂₄Cr₃₆ layer slowed the oxidation process down at 700 °C and protected the base material against oxygen migration inside the material. This means that it played an oxidation protective role in the range of these test conditions.

3.2.3. Long-Term Corrosion Tests

Long-term corrosion tests of Zry-2 and Zry-2 coated with the Zr₄₀Si₂₄Cr₃₆ were performed with the parameters: 360 °C/195 bar/water, at 21, 42, and 63 days, using an autoclave PARR 4653, with a volume of 1 dm³. The water used in autoclave experiments simulated water for PWR reactors [30].

The surface of the initial Zry-2 material was plain with visible longitude grooves, as a result of plate production. During the corrosion process, different structures were formed at the material’s surface. The presence of plates formed in different directions was visible. The SEM micrographs of the Zry-2 samples showed that surface roughness increased during the time that the samples were tested. Fiber-, needle-, and plate-like grains were visible. Higher magnification images showed needle structures and plates in the shape of grain agglomerates (Figure 9).

The surface of the modified material was plain, with longitudinal grooves connecting with the mapping of the initial material. In the course of the oxidation processes, it became rough, with irregular structures and visible morphological objects, such as grains and agglomerates. Fiber, needle, and plate grains were visible (Figure 10).

Cross-section of materials Zry-2 and Zry-2 coated with Zr₄₀Si₂₄Cr₃₆ after 21-, 42-, and 63-day tests show the presence of oxidized layers (Figures 14 and 15, respectively). The oxide layer was formed at the sample surfaces of the initial Zry-2 and Zry-2 materials coated with Zr₄₀Si₂₄Cr₃₆. The thickness of these layers was as follows (in µm):

(i)

For Zry-2: 1.5488 (21 days), 2.074 (42 days), and 2.67 µm (63 days);

(ii)

For Zry-2 coated with Zr₄₀Si₂₄Cr₃₆: 0.5166 (21 days), 1.090 (42 days), and 1.32 µm (63 days).

In the case of Zry-2 coated with Zr40Si24Cr36, the formed oxide layer was thinner by 35% after the 21-days test, by 53% after the 42-days test, and by 51% after the 63-days test, as compared to the unmodified material.

The depth elemental profile changed, due to the corrosion tests carried out (Figure 11 and Figure 12). The oxidation process of the initial Zry-2 and Zry-2 materials coated with Zr₄₀Si₂₄Cr₃₆ materials occurred.

In the case of Zry-2 coated with Zr₄₀Si₂₄Cr₃₆, the presence of oxygen was confirmed in the external part of the formed coatings. This means that the formed coating was oxidized. The base material (here: Zry-2) remained unoxidized. This means that the presence of Zr₄₀Si₂₄Cr₃₆ coatings slowed Zry-2 oxidation down. So, the protective character of formed Zr₄₀Si₂₄Cr₃₆ was observed and shown. The most important observation was the confirmed presence of Zr and Cr in the oxide layer in the case of Zry-2 coated with Zr₄₀Si₂₄Cr₃₆. An analysis of the silicon and chromium depth elemental profile showed silicon diffusion towards the base material and chromium diffusion to the surface.

Masses of initial and coated with were changed during performed long-term corrosion tests of initial Zry-2 and Zry-2 coated with Zr₄₀Si₂₄Cr₃₆. Mass gain [g], relative mass gain [%], surface mass gain [g/m²], and surface oxidation rate [g/m²/day] were analyzed in both cases.

The mass of the investigated materials increased as a result of the corrosion/oxidation process. The mass gain was rather small (in the range of mg), as Zry-2 was the nuclear material used for long-term normal operation in PWR conditions. The relative mass gain of modified Zry2 was not worse. The relative mass gain showed a lower oxidation rate of the oxidized layer formation than in the Zry-2 case. These results can be clearly observed at 21 and 63 days of testing (Figure 13). The deposited coating Zr₄₀Si₂₄Cr₃₆ played a protective role during the initial oxidation time.

The set of XRD spectra of Zry-2 initial material and after the 21-, 42- and 63-day autoclave tests is presented in Figure 14. Zr with a hexagonal structure was identified (red markers). This fact was confirmed by the presence of clearly visible 10 peaks in the following positions: 2Θ = 32, 36, 37, 48, 57, 63.5, 67, 68.5, 70, and 74 degrees.

One oxidized phase ZrO₂ was identified in the spectra (blue markers). This fact was confirmed by the presence of characteristic peaks in the following positions: 2Θ = 28 and 34 degrees. The authors predicted the formed passivation layer at the surface.

The set of XRD spectra of the initial material and after the autoclave tests of Zry-2 coated with Zr₄₀Si₂₄Cr₃₆ is presented in Figure 15. Zr with a hexagonal structure was identified—red markers. This fact was confirmed by the presence of 10 clearly visible peaks in the following positions: 2θ = 32, 36, 37, 48, 57, 63.5, 67, 68.5, 70, and 74 degrees. These peaks belonged to the base material (Zry-2) spectrum. They can be recorded and identified, due to the thin coating—about 2.5 µm. The XRD spectra of materials after the autoclave tests of Zry-2 coated with Zr₄₀Si₂₄Cr₃₆ showed peaks characteristic of the ZrO₂ monoclinic phase (blue markers). After the 21-days test at points 2θ = 28, 34, and 51 degrees and 42-days test at the points 2θ = 28, 34, 42, 51 (stronger than after 21), 56, and 66 degrees °. The peaks mentioned here were present in the bigger amount, as in the case of uncoated materials; the peaks in positions 2θ = 28 and 34 degrees were stronger, as compared to the unmodified material. The observed peaks widening was connected with the presence of dispersive phases (grains). The bulge visible at the position of about 2θ = 40 deg at the spectrum of coated material belongs to the amorphous phase present in the coated Zry-2.

The displayed comparison of the two sets of XRD spectra allows the authors to confirm that the oxidation process took place during the autoclave test in the described conditions.

The water used in the autoclave experiments simulated water from PWR reactors [30]. The PWR water chemistry contains boric acid (H₃BO₃) for fuel management and lithium hydroxide (LiOH) to adjust the pH between 6.9 and 7.4. The standard conditions for this water are: Li = 2–2.2 ppm and B = 600–1000 ppm. Highly deionized water from the Millipore system was used for preparing water for the autoclave. The procedure of making reactor water was performed in two steps. The first step was to add boric acid (H₃BO₃) to get the concentration B = 800 ppm. A powder of H₃BO₃, 99%, extra pure from Acros Organics (Poznań, Poland), was used. The second step was to add lithium hydroxide (LiOH) to get the concentration Li = 2.1 ppm. A powder of LiOH*H₂O from POCH (Dywity, Poland) was used. After this, the initial water was obtained.

Changes in water chemistry were investigated, and the obtained results are presented in

Table 6. Characterization of the water used for autoclave long-term corrosion tests.

Parameter Ions Concentrations [mg/L]	Initial	After 21 Days	After 42 Days
pH	6.4	5.68	6.51
DO [mg/L]	8.88	7.41	7.32
σ [μS/cm]	11.90	62.3	66.1
Cl−	0.45	7.5	2.18
NO3−	0.033	1.38	0.183
SO42−	0.429	9.44	5.35
Na+	4.4	3.52	2.14
K+	0.4	0.46	0.95
Ca2+	2.7	7.09	5.08
Li+	0.3	2.08	1.674
Mg2+	0.3	0.25	0.35

. The observed parameters, meaning the chemistry changes, for example, DO, σ, and ion concentrations, were connected with the solution of substances inside the autoclave during the corrosion process. For example, a DO decrease of around 20% can be explained by using oxygen diluted in the water for the oxide layer formation, especially in the first part of the process (21 days). A significant increase in the conductometric parameter was connected with the solution in water materials, such as the investigated samples and autoclave’s construction materials.

4. Conclusions

The presented work aimed to determine the possibility of extending the lifetime of the claddings formed using zirconium alloys. The goal was to develop multi-elemental coatings with silicon and chromium on zirconium alloy and evaluate of their properties in different conditions (temperatures and atmospheres).

The zirconium coatings with compounds of Zr₄₀Si₂₄Cr₃₆ were obtained using the PVD (physical vapour deposition) method and two separate targets: ZrSi₂ and Cr. Surface layers with 2.5 µm thickness and the presence of Zr, Si, and Cr were obtained.

The main coating property that was taken into consideration was corrosion resistance. Three experiments were performed, and the obtained results can be grouped as follows:

(i)

Thermal treatment: 800 °C and 1100 °C, 4 h, argon

-

Formation of the Zr₂Si phase, stable up to 1950 °C.

-

Elemental diffusion: silicon towards the base material and chromium towards the surface.

(ii)

Oxidation experiment: 700 °C/1, 2, 3, 4, and 5 h/air

-

Significantly lower weight gain (mg/g), in the case of Zr₄₀Si₂₄Cr₃₆ coated Zry-2, as compared with the unmodified material.

-

The formed coating was stable in the experiment conditions.

(iii)

Long-term corrosion tests: 360 °C/195 bar/water simulating water used in PWR reactors, for 21, 42, and 63 days

-

The oxide layer formed on the coated samples was thinner by 35% after the 21-days test, by 53% after the 42-days test, and by 51% after the 63-days test, as compared to the unmodified material.

-

The relative mass gain (%) displayed a lower oxidation rate of oxidized layer formation in the case of Zr₄₀Si₂₄Cr₃₆-coated Zry-2, as compared to the unmodified material.

The deposited Zr₄₀Si₂₄Cr₃₆ coating protected the base material (here: Zry-2) from oxygen migration and slowed the oxidation processes down. The protective mechanism is assumed to be due to two facts: Cr diffusion to the surface and the presence of stable high-temperature (Si and Zr) compounds.

The presence of a stable multi-elemental Zr₄₀Si₂₄Cr₃₆ coating played a protective role for zirconium alloys in the range of the conducted investigations. This means that the lifetime of zirconium claddings can be extended.

The developed and formed multi-elemental Zr₄₀Si₂₄Cr₃₆ coating is a new proposal. This material can be considered an oxidation protective coating for the claddings produced from zirconium alloy.