Cr Coatings Research Articles

Neutronic Performance Analysis of Inner-Side Chromium Coatings for Accident Tolerant Fuel

To address concerns about the degradation of unprotected internal surfaces of nuclear fuel claddings, inner-side coatings have been proposed as a complementary approach to the accident tolerant fuel protective coating concept. In addition to the increased coping time during severe events, these coatings are expected to provide enhanced protection in normal operation conditions. This study analyzes the neutronic performance of chromium (Cr) inner-side coatings in VVER-type fuel assemblies. Different aspects, such as reactivity and cycle length penalties, enrichment requirements, neutron flux, and the associated isotopic concentration changes are discussed, considering both the coating thickness and position. The results show that for the same thickness, reactivity penalties due to the use of inner-side Cr coatings will be (~30% on average) less compared to external coatings. The fuel assembly operating cycle showed reductions by ~ 5.5 effective full-power days when a 10-µm-thick internal Cr coating is introduced, while a 10-µm two-sided coated assembly possessed an ~13.6-day shorter operating cycle compared to an uncoated fuel assembly of the same specifications. The neutron flux showed slight shifts and hardening in the thermal energy region. The analysis of nuclide inventories showed relative increases in these inventories, which were proportional to the thickness. For the fissile plutonium isotope 239Pu, this relative increase reached a peak of 0.25% and 0.42% (for the 10-µm and 20-µm internal Cr coatings) at a fuel burnup of 18 MWd/kg heavy metal (HM). While for 241Pu, the observed highest relative increases for the 10-µm- and 20-µm-thick internal Cr coatings were 0.74% and 1.03%, respectively. The 135Xe isotopic concentration showed a relative increase that reached 0.2% and 0.4% for the 10-µm and 20-µm internal Cr coatings at a burnup of 34 MWd/kgHM, while the 149Sm concentration increased by 0.2% and 0.5% for the 10-µm and 20-µm internal Cr coatings, respectively. While these observed isotopic concentration changes were generally small for the studied inner-side coatings, the results showed that the changes remain subject to further increases as the amount of coating material gets higher. Therefore, it is important for the coating thickness to be optimized, taking into account the impact of such nuclide inventory changes. Possible fuel-clad gap reductions and the associated effects on heat transfer, as well as gap tolerance to fission products and fuel relocations, will require further studies, especially so that additional enrichments may be applied.

Oxidation behavior and microstructural evolution of Cr coatings prepared by multi-arc ion plating on Zry-4 in steam environments up to 1400 °C

This study aimed to investigate the effectiveness of the multi-arc ion plating method in depositing Cr coatings on Zry-4 substrate and to study the oxidation behavior and microstructural evolution of the Cr coatings in steam environments up to 1400 ℃. The results showed that the Cr-coated sample exhibited excellent high-temperature resistance at 1100-1200 ℃, but accelerated oxidation occurred at 1300 ℃, leading to the formation of bubbles and voids in the Cr2O3 layer and the development of a new Cr-Zr-O layer between the residual Cr layer and the ZrCr2 layer. At 1400 ℃, the coating failed completely. These findings provide valuable insights into the oxidation behavior and degradation mechanism of Cr-coated zirconium alloys under accident conditions. The novelty of this study lies in the investigation of the microstructural evolution and failure mechanism of Cr coatings on Zry-4 using multi-arc ion plating method in high-temperature steam environments, which has not been extensively explored in previous research.

Improvement of the high-temperature steam oxidation resistance of Zirlo tubes by multilayer Cr/CrAlSiN coatings

Chromium (Cr) has been recognized as a promising candidate material for ATF coatings. Herein, multilayer Cr/CrAlSiN coatings were prepared by magnetron sputtering to improve the high-temperature steam oxidation resistance of Zirlo tubes. The key performances of specimens in terms of high-temperature steam oxidation were investigated by using X-ray diffractometer (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Compared with Cr coatings, the oxidation resistance time of the Cr/CrAlSiN coatings increased from 1 h to 4 h in 1100 °C steam. A “thin Cr2O3 scale + thick Cr-rich layer” structural arrangement formed during the oxidation of the Cr/CrAlSiN coatings. The thin Cr2O3 scale means less growth stress produced in the oxidation process, and the thick residual Cr-rich layer can play a leading role in relieving the great stress and bearing the plastic deformation from the Cr2O3 layer.

High-Temperature Oxidation Behaviour of CrSi Coatings on 316 Austenitic Stainless Steel.

In this study, a closed-field unbalanced magnetron sputtering system, which is environmentally friendly and has high deposition efficiency, was used to deposit CrSi coatings on 316 austenitic stainless steel. This system utilised separate Cr and Si targets, and the appropriate content of Cr and Si of the coatings was adjusted by changing the currents applied to the targets. A series of CrSi coatings with different Si/Cr ratios were produced, and their oxidation behaviour at elevated temperatures was investigated. By analysing the weight gain, surface morphology and microstructure, composition and phase constituents, the oxidation behaviour at 600 °C, 700 °C and 800 °C was investigated and the optimized coating to protect the stainless steel has been identified. The outcome of the research indicated that a small amount of Si (between 4-7 at.%) in Cr coatings is effective in protecting the austenitic stainless steel against oxidation at high temperatures, while a high Si content (around 10 at.% or more) makes the coating more brittle and prone to cracking or delamination during oxidation at 800 °C.

An innovative method for building the model of oxidation resistance of Cr coatings

The oxidation resistance of Cr coating is influenced by the coating thickness and oxidation time. A high-throughput method is proposed to build a model reflecting the thickness effect. The wide range of thickness gradient coating was deposited by magnetron sputtering. Oxidation behavior of series of Cr coatings demonstrate that coating thickness greater than 10.7 µm are partially protective and 16.0 µm are fully protective after being oxidized in 1200 °C for 50 min. The predictions are in good agreement with the experimental results and the reported literature.

Comparative study on protective Cr coatings on nuclear fuel cladding Zirlo substrates by AIP and HiPIMS techniques

After the Fukushima nuclear accident, the concept of accident-tolerant fuel (ATF) cladding coating was proposed with the aim of increasing the rescue time in the case of an accident. Cr coatings are considered to be promising ATFs cladding materials for loss-of-coolant-accident conditions. The oxidation behavior of a Cr coating is dominated by the microstructure and defects formed during the deposition process. To endow the Cr coatings with excellent oxidation resistance under a high-temperature steam environment, we deposited Cr coatings on Zirlo substrates by high-impulse-power magnetron sputtering (HiPIMS), a technique that can optimize the microstructure of the coating compared to the traditional arc ion plating (AIP) technique. The results revealed that the HiPIMS-Cr coatings exhibited a better steam oxidation resistance, and their oxidation weight gains were 14.6% and 42.4% lower than those of AIP-Cr coatings at 1100 °C and 1200 °C, respectively.

MAX phase forming mechanism of M–Al–C (M = Ti, V, Cr) coatings: In-situ X-ray diffraction and first-principle calculations

• The phase evolution of MAX phases dependent on temperature were clarified using in-situ XRD. • First-principle calculations correlated with the experience revealed the phase forming mechanism. • The relationship between phase composition and mechanical properties was established. • This study provided a deeper understanding to guide the preparation of high-purity MAX phase coatings. The interesting hybrid properties of ceramics and metals induced by unique nano-laminated structures make the Mn+1AXn (MAX) phase attractive as a potential protective coating for vital structural components in harsh systems. However, an extremely narrow phase-forming region makes it difficult to prepare MAX phase coatings with high purity, which is required to obtain coatings with high-temperature anti-oxidation capabilities. This work describes the dependence of the phase evolution in deposited M–Al–C (M = Ti, V, Cr) coatings as a function on temperature using in-situ X-ray diffraction analysis. Compared to V2AlC and Cr2AlC MAX phase coatings, the Ti2AlC coating displayed a higher phase-forming temperature accompanied by a lack of any intermediate phases before the appearance of the Ti2AlC MAX phase. The results of the first-principle calculations correlated with the experience in which Ti2AlC exhibited the largest formation energy and density of states. The effect of the phase compositions of these three MAX phase coatings on mechanical properties were also investigated using ex-situ Vickers and nano-indenter tests, demonstrating the improved mechanical properties with good stability at high temperatures. These findings provide a deeper understanding of the phase-forming mechanism of MAX phase coatings to guide the preparation of high-purity MAX phase coatings and the optimization of MAX phase coatings with expected intermediate phases such as Cr2C, V2C etc., as well as their application as protective coatings in temperature-related harsh environments.

Cr-containing diamond-like carbon coatings deposited on 316 stainless steel substrates: Characterization and interfacial fracture toughness measurements

A series of Cr-containing hydrogenated amorphous carbon (a-C:H:Cr) coatings were deposited onto 316 stainless steel (316SS) substrates using inductively coupled plasma assisted reactive sputter deposition. Elemental Cr interlayers with different thicknesses of 100, 200, and 300 nm were deposited between a-C:H:Cr and 316SS. Detailed composition, structure, and mechanical behavior characterization of deposited a-C:H:Cr/Cr/316SS specimens were performed. Fracture toughness values of the a-C:H:Cr layer, the Cr layer, and the a-C:H:Cr/Cr/316SS interfacial regions were measured by bending of microcantilever beams with focused ion beam milled pre-notches in-situ a scanning electron microscope. Measured fracture toughness of a-C:H:Cr/Cr/316SS interfacial regions exhibits an approximately linear correlation with the area fraction of the fracture surface occurring in the Cr interlayer, indicating that the interfacial fracture toughness depends on the detailed path of crack propagation and suggesting that interfacial toughness can be engineered through materials design of the interfacial region.

Oxidation behavior of CrAl-Mo coated Zircaloy-4 in DB and BDB scenarios

Cr coatings have been widely applied on Zircaloy for accident tolerant fuel materials. However, it has long been a great challenge in the nuclear community for Cr coatings surviving in the beyond design basis conditions (BDB) because of the unstable chromia layer and eutectic reactions between Cr and Zr. In this study, bi-layered CrAl-Mo coatings have been designed to improve the performance of Cr coatings in the BDB conditions. Results showed that the CrAl-Mo coated samples can tolerant in steam at temperatures as high as1400 °C because of the formation of a more stable alumina layer on the surface and the inhibition of eutectic reactions near the coating/substrate interface. Moreover, diffusion of Mo facilitated the formation of a CrAlMo layer, which stabilized the outer alumina layer and reserved the Al inventory within the coating.

Effect of Power on Structural and Mechanical Properties of DC Magnetron Sputtered Cr Coatings

Cr coatings were deposited on the surface of PCrNi1MoA steel by DC magnetron sputtering. The effects of power (100~250 W) on the structure and mechanical properties of the coating were systematically studied. The results show that all Cr coatings have strong (110) preferred orientation and anisotropic surface morphology, and the cross sections are columnar structures. With the increase in power, the deposition rate increases linearly, the surface roughness and grain size increase gradually, and the nanomechanical properties decrease first and then increase slightly. At 100 W, the columnar grain of the coating is compact and continuous, and the wear resistance and plastic deformation resistance are the best, with the highest binding force of 27.04 N. At 200 W, due to grain growth and tensile stress, internal defects increase, and mechanical properties are poor. The combination of coating and substrates is not tight enough, and it is easy to fall off and fail under the action of external force. When the power reaches 250 W, the higher bombardment rate helps the coating release stress, and reduce porosity.

Effect of bias voltage on the microstructure and high‐temperature steam oxidation behavior of Cr coatings prepared by multi‐arc plating on Zircaloy‐4 alloy

AbstractCr coatings were deposited on the Zircaloy‐4 alloy by multi‐arc ion plating under different bias voltages from −50 to −110 V, and the microstructure, mechanical properties, and high‐temperature steam oxidation behavior of these coatings were systematically investigated. The results indicate that the bias voltage significantly affects the crystal orientation, surface morphology, deposition rate, nanohardness, and bonding strength of Cr coating on the Zircaloy‐4 alloy. The high‐temperature oxidation behavior of the Cr coating is closely related to its crystallographic orientation. Compared with the Cr coating with relatively random grain orientations, the Cr coating with (211) preferred orientation has a dense and uniform oxide scale and has the best oxidation resistance in the steam environment at 1200°C.

STUDIES ON PHYSICAL, MICRO-STRUCTURAL, AND SLURRY EROSION BEHAVIOR OF COLD-SPRAYED Ni–20Cr+TiC+Re COATINGS ON SA516 STEEL FOR HIGH-TEMPERATURE APPLICATIONS

In this paper, three kinds of Ni–20Cr coatings were deposited on SA516 substrate steel by cold-sprayed coating technique. Physical properties (such as hardness, surface roughness, and residual stress) and slurry erosion behavior (with impingement angles of 30∘ and 90∘) of cold-sprayed substrates have been evaluated. Moreover, a scanning electron microscope (SEM) examination has been performed to evaluate the morphological characterization of various coatings. It has been found that the residual stresses induced in the coated specimens exhibited compressive nature. Further, micro-hardness and surface roughness was observed to proliferate with the incorporation of titanium carbide (TiC) and rhenium (Re) in Ni–Cr coatings. Micro-hardness for Ni–20Cr+TiC and Ni–20Cr+TiC+Re was observed at 233.67 and 278.9 Hv, respectively, where Surface roughness for Ni–20Cr+TiC and Ni–20Cr+TiC+Re was observed at 9.86 and 11.68[Formula: see text][Formula: see text]m, respectively. All types of Ni–20Cr coatings were quite efficient in reducing the erosion rate of the SA516 steel as compared to uncoated SA516 and most prominent of all was Ni–20Cr+Tic+Re coating. It was observed that at 30∘, weight loss for Ni–20Cr, Ni–20Cr+TiC and Ni–20Cr+TiC+Re was up to 0.00027, 0.00015 and 0.00012 g/mm2, whereas at 90∘, weight loss was for Ni–20Cr, Ni–20Cr+TiC and Ni–20Cr+TiC+Re was up to 0.00024, 0.00014 and 0.00012 g/mm2.

Effects of Cr/Zircaloy-4 coating qualities for enhanced accident tolerant fuel cladding

Cr-coated zirconium alloys represent a modern approach to enhance cladding safety during accident scenarios. Two high-power impulse magnetron sputtered Cr-coated Zry-4 systems were subjected to simulated loss-of-coolant accident conditions to investigate cladding performance. The first Cr-coating (4.8 µm thick) was deposited onto Zry-4 cladding and exhibited through-thickness cracking while the second Cr-coating (6.8 µm thick) was deposited with improved deposition parameters onto polished Zry-4 and exhibited no cracking. During burst testing, the coating with a higher density of defects failed to consistently reduce oxidation and exhibited similar burst behavior as Zry-4. In contrast, the second Cr-coating reduced ZrO2 formation through formation of Cr2O3 and displayed enhanced burst temperatures by ∼80 °C compared to Zry-4. Utilizing an empirical relation for burst behavior of zirconium alloys, the 6.8 µm Cr/Zry-4 system displayed enhanced burst temperatures equivalent to an effective 0.464 mm increase in Zry-4 wall thickness, highlighting the value of continuous Cr coatings for accident scenarios.

High temperature steam oxidation behavior of textured Cr coatings with different grain structures

In order to improve the oxidation resistance of Zr-4 alloy in high temperature steam environment, Cr coatings with the 〈001〉 fiber texture were prepared on Zr-4 substrates by the high-power pulsed magnetron sputtering (HIPPMS) and direct current magnetron sputtering (DCMS) methods, and their structure and high temperature steam oxidation behavior at 1200 and 1300 °C were systematically investigated. The results showed that the fine grains of Cr coatings combined together to form large grains while the texture of the coatings was not changed during oxidation. Furthermore, cracks or defects formed during the combination of grain boundaries in the grain growth of DCMS Cr coating, providing paths for short-circuit diffusion of the oxidizing medium, which accelerated the oxidation of the Cr coating.

Polycrystalline diamond and magnetron sputtered chromium as a double coating for accident-tolerant nuclear fuel tubes

Controlling the hot steam corrosion of nuclear fuel tubes is crucial for power plant efficiency and safety. We demonstrate an effective method of protecting ZIRLO ® (ZIRLO) fuel tubes against corrosion at accident temperatures in a water-cooled nuclear reactor environment. The tubes surfaces were coated by double layers consisting of the polycrystalline diamond (PCD) and magnetron-sputtered Cr. Coated and bare ZIRLO tubes were subjected to hot steam tests for 30 min at 900 °C followed by 40 min at 1000 °C. Surprisingly, Cr and PCD double layers ability to protect ZIRLO fuel tubes against hot steam corrosion strongly depends on the order of the layers in coatings. ZIRLO coated with Cr as the bottom layer and PCD as the top layer exhibited the best ability to slow down the oxidation process. This double layer coating reduced ZIRLO tube corrosion by 47% compared to unprotected ZIRLO. On the contrary, the double layer coating with PCD as bottom and Cr as the top layer reduced the ZIRLO tube corrosion by 20–25% compared to unprotected ZIRLO. To explain the specific roles of Cr and PCD coatings experimental and theoretical procedures were performed. Our results pointed to two basic parameters, significantly affecting the corrosion of coated ZIRLO tubes: 1. the amount of Cr carbides in the surface part of the coated ZIRLO fuel tubes and 2. adhesion of the coatings to the ZIRLO substrate.

Enhanced corrosion resistance properties of Fe–Al–Cr laser cladding coatings on 1045 carbon steel substrates

Fe–Al–Cr coatings with different content of Cr additive were prepared on 1045 carbon steel substrates by a laser cladding process. The incorporation of Cr atoms can effectively enhance the corrosion resistance of the coatings. In particular, the Fe–28Al–5Cr laser cladding coating exhibits the best film quality without phase segregation. In addition, the interfacial adhesion between the Fe–28Al–5Cr coating and the 1045 carbon steel substrate is improved. As a result, the Fe–28Al–5Cr laser cladding coating exhibits the best corrosion resistance in a 3.5 wt% NaCl solution under both immersion and electrochemical conditions. However, excessive Cr additive lead to the formation of Al8Cr5 in the grain boundaries, resulting in inferior corrosion resistance. Therefore, the new findings demonstrated in this work may inspire the design of high-quality coatings with excellent corrosion resistance.

Improved thermal stability and irradiation tolerance of oxide dispersion strengthened chromium coatings

A novel Cr coating with uniformly dispersed yttrium-based nano-oxides (i.e., ODS-Cr coating) was synthesized. Structural analyses show that, after heat treatments, the grain coarsening was not detected, and mechanical properties remained almost intact in ODS-Cr coatings while the grain size increased six times and mechanical properties degraded remarkably for pure Cr coatings. Irradiation-induced cavities in ODS-Cr coatings were also greatly suppressed compared with those in pure Cr coating. These results indicate that the ODS-Cr coating has a better mechanical properties and irradiation resistance than traditional pure Cr, and highlight its potential application as protective coatings for ATF solutions.

Direct and pulse electroplating effects on the diffusion barrier property of plasma-nitrided Cr coatings on HT9 steel

The introduction of a Cr and chromium nitride multi-coating barrier between the nuclear metallic fuel and the cladding is a potential candidate to mitigate fuel cladding chemical interaction (FCCI). Cr coatings were electroplated on HT9 disks using direct and pulse currents followed by plasma nitriding for synthesizing additional nitride coating. The pulse current induced Cr coatings revealed severe microstructural changes from crack-free to the abundance of cracks during the plasma nitriding due to phase transformation during the plasma nitriding. The phase transformation of hexagonal CrH to cubic Cr occurred at a temperature as low as 110 °C resulting in the liberation of hydrogen pores agglomerated at Cr grain boundaries and internal stress. Finally, the diffusion barrier property of the pulse current induced Cr coatings was significantly reduced, while the plasma-nitrided Cr coating produced by a direct current exhibited a crack-closure effect, and an additional nitride layer successfully hindered the elemental diffusion of Ce and Nd at 650 °C for 25 h. This current study points to the critical effect of current waveforms on the final barrier property of Cr-chromium nitride coatings for nuclear cladding applications.

Effect of long-term aging treatment on the structure and oxidation resistance of Cr coatings under high-temperature steam

Cr coatings were deposited on Zr-4 tubes by arc ion plating to explore the effect of different aging treatment times at low temperature on the steam oxidation performance of Cr coatings. The intrinsic relationship between the coating grain evolution and oxidation behavior after different aging times was analyzed by EBSD. The mid-term aging treatment improve the oxidation resistance of the Cr coating. After long-term aging treatment, a significant amount of Cr2O3 in the grain boundaries close to the Zr-4 substrate preferentially reacts with Zr, which is the reason for the serious degradation of the performance of the Cr coating.

Electrodeposition of high-quality Cr coatings with solid solution Al from Cr2+ electrolyte

Smooth and dense nanocrystallline Cr coatings with solid solution Al have been electrodeposited using the proposed Cr2+ electrolyte without any additives, i.e., AlCl3–1-ethyl-3-methyl-imidazolium chloride (AlCl3–EMIC) ionic liquid dissolving CrCl2. Systematic study demonstrates that 2 to 50 at.% Al is in solid solution with body-centered-cubic (BCC) Cr. The solid solution alloys deposited at relatively positive potentials are attributed to the underpotential deposition (UPD) of Al on Cr deposits, which is confirmed by cyclic voltammetry (CV), potentiostatic deposition and X-ray photoelectron spectroscopy (XPS). The coating microstructure, surface morphology and composition are dependent on the deposition potential. The Vickers hardness is 850HV0.01, which is comparable to that of Cr coatings electrodeposited from aqueous solution. The corrosion current density is as low as 2.8 × 10−8A/cm2, which is an order of magnitude lower than that of pure Cr sheet and pure Ni sheet. In addition, the formation process and mechanism of divalent chromium ions are discussed in detail. Our work will have significant impact on engineering application for the excellent performance and the avoidance in hydrogen embrittlement.