Oxidation Of Zirconium Alloys Research Articles

Simulation of high-temperature zirconium alloy oxidation in a nitrogen, oxygen and steam mixture

The paper presents a correlation model of high-temperature zirconium alloy oxidation in a mixture of gases (steam, nitrogen and oxygen) for the weight gain calculation of the oxidized sample. The essence of the model consists in using the linear interpolation between the kinetic constants of oxidation in pure gases to describe the oxidation kinetics in gas mixtures. The approach allows one to describe the oxygen weight gain with different contents of gases in steam, nitrogen and oxygen mixtures. The model was validated against experimental data on oxidation in an air-steam mixture in a proportion of 50% to 50% of the volume. For temperatures of 1200 °C and 1400 °C, prior to the 625th second, the deviation between the experimental data and the simulation results is not more than 18%; for 800 °C, prior to the 14th hour, the deviation between the experimental data and the simulation results is not more than 40%. Besides the air-steam mixture, the paper presents the results of the weight gain calculation using the proposed model for oxidation in an oxygen-nitrogen mixture and in a steam-oxygen mixture at 800 °C and 1400 °C.

Study on Growth Mechanism and Characteristics of Zirconium Alloy Micro-Arc Oxidation Film

Ceramic coatings on R60705 zirconium alloy were prepared on the surface by the micro-arc oxidation (MAO) technique in electrolytes containing Na2SiO3, NaOH, and Na2EDTA. The growth behavior of the MAO ceramic coatings at different stages, including growth rate, microstructure, and phase composition, was investigated using the method of direct observation of the boundary area. The results showed that the growth of the MAO coatings on R60705 zirconium alloy occurred in both inward and outward directions. At an oxidation time of 5 min, the thickness of the oxidation layer increased fastest, reaching 103.43 μm, with a growth rate of 0.345 μm/s. After 5 min, the growth rate decreased and tended to level off around 15 min, with a thickness and growth rate of 162.7 μm and 0.181 μm/s, respectively. The total thickness of the coatings continuously increased throughout the process, with the outward growth thickness always higher than the inward growth thickness. The composition of the zirconium alloy micro-arc oxidation coatings mainly consisted of monoclinic zirconia (m-ZrO2), tetragonal zirconia (t-ZrO2), and a small amount of SiO2. The main elements in the coatings were Zr, O, and Si. The corrosion resistance of the zirconium alloy micro-arc oxidation coatings increased first and then decreased with increasing oxidation time, with a corrosion current density of 8.876 × 10−9 A·cm−2 at 15 min, indicating the best corrosion resistance.

Long-Term Oxidation of Zirconium Alloy in Simulated Nuclear Reactor Primary Coolant—Experiments and Modeling

Oxidation of Zr-1%Nb fuel cladding alloy in simulated primary coolant of a pressurized water nuclear reactor is followed by in-situ electrochemical impedance spectroscopy. In-depth composition and thickness of the oxide are estimated by ex-situ analytical techniques. A kinetic model of the oxidation process featuring interfacial reactions of metal oxidation and water reduction, as well as electron and ion transport through the oxide governed by diffusion-migration, is parameterized by quantitative comparison to impedance data. The effects of compressive stress on diffusion and ionic space charge on migration of ionic point defects are introduced to rationalize the dependence of transport parameters on thickness (or oxidation time). The influence of ex-situ and in-situ hydrogen charging on kinetic and transport parameters is also studied.

High-temperature oxidation of zirconium alloys with magnetron sputtered chromium coatings

Loss-of-coolant accidents (LOCAs) often lead to dire consequences. That’s why the global community are actively engaged in research aimed at creating accident tolerant fuels. One of the areas of such research is preserving zirconium alloys as the material for fuel-element claddings by creating corrosion-resistant coatings that would be in contact with the coolant. Chromium could be a promising material to be used in such protective coatings as chromium oxide (Cr2O3) serves as an effective barrier for oxygen both in normal operation and in case of LOCA. Chromium coating hinders oxygen diffusion into the metal substrate and thus prevents embrittlement and failure of the fuel-element cladding. This paper describes the results of research studies that have been carried out in recent years and that look at the resistance of magnetron sputtered chromium coatings to high-temperature oxidation in water vapour up to 1,500 oC. The paper demonstrates advantages and drawbacks of chromium coatings in hightemperature oxidation conditions. The focus is on understanding how regimes of magnetron sputtering influence the resistance of chromium coatings to high-temperature oxidation and how the structure and phase state of chromium coatings are related to their properties. The authors describe the optimal regimes of magnetron sputtering for obtaining dense coatings and examine the effect of the substrate temperature and the bias voltage on the structure and density of resulting coatings. High Power Impulse Magnetron Sputtering (HiPIMS) serves as an effective technique that helps enhance the density of coatings. The conclusion drawn is that in order to broaden the temperature range in which chromium coatings can effectively protect zirconium alloys from failure up to 1500 oC, there should be a diffusion barrier between the surface of the fuel-element cladding and the chromium coating. The authors would like to thank B. A. Kalin, supervisor of this project, who had passed away before this publication was made. The staff of the Laboratory of Ion-Plasma and Ion-Beam Machining of Materials, a part of Department No. 9 at the National Research Nuclear University MEPhI, devote this paper to his memory.

Effects of steam concentration and flow rate on the high temperature oxidation of PEO-coated zirconium alloy at 1000 °C and 1200 °C

A compact oxide coating of 7 μm thick on ZIRLO zirconium alloy was fabricated by plasma electrolytic oxidation (PEO) in phosphate electrolyte. The effects of steam concentration and flow rate on the steam oxidation behavior of PEO-coated ZIRLO alloy at 1000 °C and 1200 °C were evaluated by thermogravimetric analysis (TGA). The morphologies, compositions, phase constituents before and after the steam oxidation were characterized. It is found that the critical steam concentration without steam starvation for PEO-coated ZIRLO alloy at 50 ml/min flow rate is 35 % at 1000 °C. Meanwhile, the critical flow rate at 50 % steam concentration is 35 ml/min at 1000 °C and 150 ml/min at 1200 °C. A dimensionless number (P = 39) is suggested corresponding to the critical steam concentration and flow rate. It means that the minimum mass of steam injection is about 39 per unit mass gain of sample, in order to avoid the steam starvation for Zr alloys after 3600 s steam exposure. The steam oxide layers consist of monoclinic zirconia (M-ZrO2) and a little tetragonal zirconia (T-ZrO2) phase at different temperatures, steam concentrations and flow rates. The hydrogen has not diffused into the steam oxide layer and ZIRLO substrate due to the hindrance of ZrO2 coating to the hydrogen diffusion.

Multi-Elemental Coatings on Zirconium Alloy for Corrosion Resistance Improvement

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.

Isothermal experiments on steam oxidation of magnetron-sputtered chromium-coated zirconium alloy cladding at 1200 ℃

The isothermal steam oxidation behavior of chromium-coated zirconium alloy cladding at 1200 ℃ is studied. The cladding substrate is Zr-1 Nb alloy with outer diameter of 9.5 mm and thickness of 0.57 mm. The Cr coating is prepared by magnetron sputtering method with thickness of about 11 µm. The oxidation durations are 300 s~14,400 s. After oxidation, blisters are formed on the surface due to the compressive stress in coating, and the coating surfaces are porous or exhibit a morphology with ravines and granular crystals. A four-layer structure (Cr2O3, Cr, Zr(Cr,Fe)2 and Zr from outside to inside) is formed in the early stage of oxidation. However, Zr4+ can migrate into Cr coating through grain boundaries and reacts with Cr2O3. The redox reaction between Zr and Cr2O3 leads to the coating failure, resulting in the oxidation of Zr alloy substrate. Due to selective oxidation, a certain thickness of Cr coating is retained during the thickening of outer ZrO2. The emergence of outer ZrO2 layer is significantly delayed by Cr coating, and Cr coating is able to reduce the growth rate of outer ZrO2 layer. Therefore, Cr coating has a protective effect in 1,200 ℃ steam environment and can improve the oxidation resistance of Zr alloy cladding.

Investigation and characterization of the plasma electrolytic oxidation of zirconium alloy: effect of negative duty cycle

Investigation and characterization of the plasma electrolytic oxidation of zirconium alloy: effect of negative duty cycle

INFLUENCE OF CURRENT DENSITY ON THE PROCESS OF FORMATION OF OXIDE COATING ON ZIRCONIUM ALLOY E110

Today, binary, triple and quadruple zirconium alloys containing iron, chromium and copper are known for corrosion protection, which have a clear advantage over circal and niobium binary alloys in terms of corrosion resistance. However, insufficient attention is paid to studies aimed at studying the process of formation and morphology of oxide films on the zirconium alloy E110. This area of research has a high scientific and technical interest in the development of oxide coatings on metal surfaces, given the widespread use of zirconium alloys in nuclear energy as a structural material in the construction of nuclear reactors. The paper considers the advantages of the method of microarc oxidation in comparison with other methods of protection of zirconium alloys from different types of corrosion damage. The main task of the work is to study the influence of current density on the formation and morphology of oxide films on zirconium alloy. It was found that the obtained dependences of the formation stress on the time of microarc oxidation of zirconium alloy in 1 mol/l potassium pyrophosphate at different current densities have a characteristic classical appearance, characterized by three sections, such as spark, spark and microarc. The dependence of the rate of change of the voltage dU / dt on the molding voltage is constructed for a more detailed study of the process of formation of oxide films and determination of the moment of the beginning of sparking and microarcs. It was found that when the current density increases to 6 A/m2, the spark voltage increases by 5 V, and when it increases to 10 A/m2 - by 15 V, but the voltage of the microarcs remains unchanged. Mostly solid, well-bonded to the base oxide films of light gray colour on a zirconium alloy are obtained. The morphology of oxide coatings obtained on the surface of the zirconium alloy E110 by the method of microarc oxidation in potassium pyrophosphate at different current densities was studied. It is shown that oxide films without cracks and surfaces have irregularities with a cellular texture.

Role of microchannels in breakaway oxidation of Zr alloy under high-temperature steam oxidation at 1000 ℃

In the present study, to determine the mechanism of the breakaway phenomenon, high-temperature steam oxidation of zirconium alloys was carried out at 1000 ℃. Additionally, the oxide microstructures of samples were characterized using multiple techniques. The results show that the microchannels caused an undulated structure of the oxide/metal (O-M) interface, which is a function of time. Meanwhile, after breakaway oxidation, a formation mechanism of cracks, based on vacancies, was proposed according to the crack morphologies. Finally, taking the microchannels as the center, a mechanism is established to explain the high-temperature oxidation behavior of the Zr alloy.

A comparative study on high-temperature air oxidation of Cr-coated E110 zirconium alloy deposited by magnetron sputtering and electroplating

Chromium coatings were deposited either by magnetron sputtering and electroplating on E110 zirconium alloy. The as-deposited Cr coatings showed dense microstructure, but different mechanical properties and adhesion behavior depending on the deposition technology. High-temperature oxidation was performed in air at 1100 °C for 40 min. The magnetron-deposited coatings had a stable oxidation behavior which was improved by increasing coating thickness. Surface activation of E110 alloy prior to Cr electroplating resulted in formation of the interlayer with a thickness of 7–12 μm between the coating and the alloy containing hydride and fluoride phases. It was shown that cracks and defects in this interlayer of the electroplated Cr coatings affected the coating adhesion and resulted in lower oxidation resistance compared to the magnetron-deposited coatings.

Oxidation of Zirconium Alloy Processed with Compression Plasma Flows

Oxidation of Zirconium Alloy Processed with Compression Plasma Flows

Raman imaging in corrosion science: High‐temperature oxidation of the zirconium alloys used in the nuclear industry as an example

AbstractSuccessive technological improvements have made Raman imaging systems very efficient. Raman imaging applications now cover a wide range of scientific fields, including high‐temperature materials corrosion, also known as dry corrosion. The analysis of the phenomena occurring during corrosion requires knowledge in metallurgy, thermodynamics, chemical kinetics, solid‐state chemistry, and physics. Part of this information (crystallographic phases, stress state, oxygen transport, etc.) can be obtained, at least qualitatively, from the analysis of Raman images of the oxidation layer. Throughout this paper, we consider as a prototypal example the oxidation of a particular zirconium alloy, Zircaloy‐4, used in the nuclear industry as cladding materials in light‐water reactors. Through some examples, the aim of the present paper is to describe what kind of information this method can bring, to review its merits and weaknesses. The analysis protocols methods described herein are not exhaustive, but most probably represent a simple way to read and understand the Raman images. These few examples show that rather subtle details that concern phase distribution, strain, and oxygen transport can effectively be captured by this method. Most probably, Raman imaging may be applied to other alloys or systems as well.

A Model of the Relationship between the Parameters of Oxidation of Zirconium Alloys in Water Steam at 1000°C and the Alloy Composition: II. Model Selection, Comparison with Experiment

A Model of the Relationship between the Parameters of Oxidation of Zirconium Alloys in Water Steam at 1000°C and the Alloy Composition: II. Model Selection, Comparison with Experiment

Rapid assessment of structural and compositional changes during early stages of zirconium alloy oxidation

A multimodal chemical imaging approach has been developed and applied to detail the dynamic, atomic-scale changes associated with oxidation of a zirconium alloy (Zircaloy-4). Scanning transmission electron microscopy, a gas-phase reactor chamber attached to an atom probe tomography instrument, and synchrotron-based X-ray absorption near-edge spectroscopy were employed to reveal morphology, composition, crystal, and electronic structure changes that occur during initial stages of oxidation at 300 °C. Oxidation was carried out in 10 mbar O2 gas for short exposure times of 1 and 5 min. A multilayered oxide film with a cubic ZrO adjacent to the oxide/metal interface, a nanoscopic transition region with a graded composition of ZrO2−x (where 0 < x < 1), and tetragonal ZrO2 in the outermost oxide were formed. Partitioning of the major alloying element (tin) to the oxide/metal interface and heterogeneously within the oxide accompanied the development of the layered oxide. Our work provides a rapid, high-throughput approach for detailed characterisation of initial stages of zirconium alloy oxidation at an accelerated time scale, with implications for several other alloy systems.

Breakaway oxidation of zirconium alloys exposed to steam around 1000 °C

The breakaway oxidation of Zircaloy-4 and M5Framatome alloys oxidized in flowing steam around 1000 °C was studied. The influences of oxidation temperature between 950 and 1050 °C, one-side versus two-side oxidation and chemical variations around the nominal composition of M5Framatome were investigated. Breakaway oxidation is sensitive to the oxidation temperature, the material and the oxidation type. A phenomenological mechanism for breakaway oxidation is postulated on the basis of microstructural and microchemical analyses.

Characterization of metal oxide micro/nanoparticles elaborated by plasma electrolytic oxidation of aluminium and zirconium alloys

The formation of fine particles during the Plasma Electrolytic Oxidation process (PEO) was investigated. Particles were produced by carrying out PEO treatments on two different grade of aluminium alloy (Al1050, Al2024) and a low-alloyed zirconium grade (Zr-M5). They were characterized at different scales mainly using transmission electron microscopy. Whatever the processed material, results show that particles exhibit a hollow spherical shape with outer diameter ranging from 100 nm to 10 μm. They consist of polycrystalline alumina (η- and α-Al2O3) for the PEO of aluminium alloys and zirconia (m-ZrO2) for the zirconium alloy. A correlation is drawn between the microstructure of the produced particles and the “pancake”-like structure that usually develops over PEO coatings. On this basis, a descriptive mechanism of the formation of these metal oxide particles is proposed. The ignition of a micro-discharge inside the “pancake” cavity induces the propagation of a pressure shockwave that pushes the molten oxide through the narrow orifice of the “pancake” structure. Like a rubber balloon, the molten oxide forms a hollow sphere and is rapidly quenched by the surrounding electrolyte and ejected into the electrolyte.

Oxygen potential calculations for conventional and Cr-doped UO2 fuels based on solid solution thermodynamics

The effect of Cr-doping on the oxygen potential of UO2 fuels was studied through a comprehensive thermodynamic treatment. Berman-type sublattice solid solutions were formulated and calibrated for UO2 (with Pu, Am, Cm, Np, La, Nd, Ce and Cr as solutes) and for a five-component noble-metal phase (Mo, Pd, Ru, Rh, Tc). Ideal gas, a binary (Sr,Ba)ZrO3 ideal solid solution and a number of stoichiometric solids were also included. The equilibrium calculations were carried out using the GEM-Selektor code with the in-house Heracles database.Ellingham diagrams were calculated for burnups between 1 and 60 GWd/tiHM over the temperature range 400 °C–1400 °C. They indicate no significant effect of Cr-doping on the oxygen potential, except at very low burnup. Oxygen potentials appear however to depend sensitively on mole fraction and non-ideality of the Mo component in the noble-metal phase. In contrast, the effect of solutes in UO2 is small. Above about 40 GWd/tiHM, the fuel oxygen potential curves converge to values exceeding by 5–30 kJ/mol the pure Mo–MoO2 buffer. Further calculations indicate that oxidation of zirconium alloy at the inner side of the cladding could considerably lower the oxygen potential.

Модель связи параметров окисления циркониевых сплавов в парах воды при 1000°С с составом. II. Выбор модели, сравнение с экспериментом

In the framework of the previously developed model for the oxidation of zirconium alloys in water stream during the LOCA (Loss of Coolant Accident), the role of each element in the alloy composition in the oxidation kinetics is estimated. The contribution of each component to oxidation resistance was determined by the method of solving the “inverse problem”. Algorithm, flowchart, tables of step-by-step solution of “inverse” and “direct” problems, and the results of calculations of the parabolic oxidation constant K and the time t to transition to the linear stage of oxidation for each alloy are presented. The contribution of each alloying and impurity element to the oxidation resistance of the alloy is determined by the stoichiometric coefficients and corresponds to the thermodynamic sequence of the formation of oxides of these elements in water vapor at 1000°C. The thermodynamic meaning of the parameters included in the kinetic equations is determined, which are proportional to the standard formation energy ∆Gi of the oxide of the corresponding alloy component.

Short communication on “self-crack-healing behavior of oxide formed on a zirconium alloy cladding tube”

The oxidation of zirconium alloys is largely influenced by cracks in the oxide layers. It was reported that cracks formed in ZrO2/Al2O3 can be healed by the phase transformation of ZrO2. Here, we conducted several experiments to determine whether this also holds true for zirconium alloys. The experimental results showed that the cracks at the oxide-metal interface were healed after oxidation at 1200 °C, but not healed after oxidation at 1000 and 1100 °C. It appears that this crack healing behavior is related to the phase transformation of ZrO2.