Oxidation Of Zircaloy-2 Research Articles

Plasma electrolytic oxidation of Zircaloy-2 alloy in potassium hydroxide/sodium silicate electrolytes: The effect of silicate concentration

This study mainly investigated the effects of silicate concentration at the range of (16g/L to 56g/L) on the plasma electrolytic oxidation of Zircaloy-2 in potassium hydroxide/sodium silicate electrolytes in detail, including the growth behavior, wear and corrosion resistance of as-obtained ZrO2/SiO2 alloyed coatings. It was found that the coating thickness increased continuously on increasing the silicate concentration in electrolyte. Besides, the amount of t-ZrO2 in the coatings increased with increase of silicate concentration in electrolyte, while the amount of m-ZrO2 decreased. Comparative studies have shown that the coatings formed in electrolyte with high silicate concentration possessed superior wear and corrosion performance, which could be ascribed to heavy silica deposition associated with the presence of t-ZrO2 stabilized by SiO2. The results may provide guidance for obtaining high performance of ZrO2/SiO2 alloyed coatings. Besides, it is believed that the presence of Si species in zirconia endows the coatings with enhanced bioactivity like bioactive glasses and ceramics coatings and we envision that the as-prepared ZrO2/SiO2 alloyed coatings have great potential for biological applications.

Wear-resistant coatings formed on Zircaloy-2 by plasma electrolytic oxidation in sodium aluminate electrolytes

Plasma electrolytic oxidation of Zircaloy-2 has been investigated in dilute and concentrated aluminate electrolytes, under a pulsed-bipolar current regime, in order to develop coatings of high wear resistance. Coating growth kinetics, cell potential-time responses and discharging behaviours depend significantly on the electrolyte concentration. The coatings formed in dilute aluminate electrolyte reveal a three-layered structure, with pancake structures at the coating surfaces. “Soft sparks” occur during PEO in dilute aluminate electrolyte, causing a relatively fast growth of the inner layer and resulting in a large amount of alumina-enriched material beneath the pancake structures, and hence an increased wear resistance of the coating. In contrast, more homogenous coatings, free of pancakes, result with the concentrated electrolyte. The main phase in the coatings is t-ZrO2, with γ-Al2O3 also present in coatings formed in the latter electrolyte. The coatings formed in the concentrated electrolyte display a high wear resistance, even for thin coatings formed for short times, which is attributed to the relatively high alumina content of the coatings.

Comparison of plasma electrolytic oxidation of zirconium alloy in silicate- and aluminate-based electrolytes and wear properties of the resulting coatings

Plasma electrolytic oxidation of Zircaloy-2 at constant rms current is examined in silicate and aluminate electrolytes, revealing significantly different behaviors in the growth kinetics and properties of the coatings. Coatings thicken continuously in the silicate electrolyte, while in the aluminate electrolyte, the thickness reaches a relatively constant value. The latter coincides with changing appearances of discharges and detachment of an outer coating layer. Dissolution of zirconium is faster in the silicate electrolyte in the early stage of PEO, but is faster in the aluminate electrolyte following coating breakdown. The pre-spallation coating formed in the aluminate electrolyte shows superior wear resistance, which can be ascribed to its relative compactness, associated with the presence of tetragonal zirconia stabilized by aluminium species.

Oxidation of Zircaloy-2 studied using a wedge-shaped specimen

Experiments have been carried out to vary the stress in the oxide layer during oxidation of Zircaloy-2. This was achieved by varying the thickness of the metal substrate, using a specimen with a tapered wedge-shaped cross-section. X-ray diffraction measurements confirmed that the compressive stress level in the oxide was reduced when the metal substrate was thinner. The rate of oxidation was also slower for conditions where the stress was reduced. The results can be interpreted such that transitions in the growth result from sequential cracking of the oxide when sufficient elastic strain energy accumulates and that the cracks then enhance access of oxygen to the metal interface.

Thermal expansion of corium prepared from UO2 and Zircaloy-2

ABSTRACTIn this study, sintered pellets were prepared from Zircaloy-2 oxide and UO2 as a parameter of content ratio (Zr contents were 0, 24.3, 49.0, 73.4, and 97.9 at% in metal). The sintered pellets were heated in 5%H2/Ar gas. UO2 pellets underwent simple thermal expansion caused by thermal vibration while Zircaloy-2 oxide pellets underwent thermal expansion and volume change with phase transformation. Finally, the 24.3, 49.0, and 73.5 at%Zr-UO2 pellet specimens showed both phenomena. However, phase transformation temperatures were lower than that of Zircaloy-2 oxide, and volume changes were much smaller. X-ray diffraction patterns obtained after thermal expansion measurements showed that the 24.3 at%Zr-UO2 specimen contained tetragonal and cubic (Zr, U)O2 while the 73.5 at%Zr-UO2 specimen contained mainly monoclinic ZrO2.

Oxidation of Zircaloy-2 and Zircaloy-4 in water and lithiated water at 360°C

Zircaloy-2 and Zircaloy-4 samples were oxidised in an autoclave in water and in lithiated water (70 ppm) in order to evaluate the influence of both the alloy compositions and the effect of lithium on the oxidation kinetics. Cross-sectional TEM and electrochemical impedance spectroscopy were used to analyse the samples. The pre-transition oxidation rate is not affected by the presence of LiOH, but the transition occurs earlier and the post-transition oxidation rate is increased. The oxidation rate correlates with the density of cracks in the oxide layer and the morphology of the oxide grains. The impedance measurements show that the oxides formed have a layered structure and for samples oxidised in LiOH solution the inner protective layer is thin. The hydrogen pickup ratio follows the weight gain, not the oxidation rate, up to the second transition. When the protective oxide layer is degraded the hydrogen pickup ratio increases strongly. The effect of LiOH is suggested to be due to partial dissolution of the oxide and subsequent incorporation of lithium ions during a dissolution–precipitation process. Newly formed oxide is probably more hydrous and the grain boundaries are particularly liable to dissolution. The up-concentration of LiOH within cracks and pores could cause the detrimental levels necessary for dissolution. This is supported by the insensitivity in the pre-transition region, to both the compositions of the alloy and to the environment.

In situ measurement of electrical conductivity of Zircaloy oxides and their formation mechanism under electron irradiation

Insitu electrical conductivity of Zircaloy-2, improved Zircaloy-2 and Zircaloy-4 oxide films has been measured with 1 MeV electron irradiation using gold, copper and zirconium electrodes during beam-on and -off in the temperature range of 296–680 K in order to find the rate controlling factor of oxidation of the alloy. Current–voltage characteristics of all oxides during irradiation show almost ohmic behavior for Zircaloy-2 and Zircaloy-4 oxides, and non-ohmic for improved Zircaloy-2 oxide where the current is electronic in nature and may be associated with ohmic and Schottky and/or Poole–Frenkel processes, respectively. The electrical conductivity of such films under irradiation, called radiation induced conductivity (RIC), is proportional to the irradiation flux and it is predominantly due to electronic excitation. Subsequent subjection of the specimens under irradiation with beam-on and -off shows RIC by electron excitation from the valence to conduction band and annealing of the conductivity at temperature, respectively. It is concluded that the electron motion dominates the electrical conductivity of Zircaloy oxides. Therefore, the slow-diffusing negative oxygen ions control the oxidation process of Zircaloys.

Oxidation-induced creep in Zircaloy-2: III. The average stress in the oxide layer

Oxidation of Zircaloy-2 at high temperatures results in the formation of a surface oxide, growth of an underlying layer of oxygen-stabilised α-phase and tensile deformation of the metal substrate core. A model, developed to account for this oxidation-induced deformation, is used to calculate the mean stress generated within the oxide during the course of oxidation.

Oxidation-induced creep in Zircaloy-2: II. Stress distribution in the oxygen-stabilised α-phase layer

Steam oxidation of Zircaloy-2 produces tensile extension; at a given temperature the strain increases parabolically with time. Oxygen dissolution in Zircaloy-2 during oxidation stabilises the α-phase and causes lattice dilatation. These two observations are used to develop a model of the oxidation-induced deformation behaviour of the stabilised α-layer and hence to calculate the stress distribution across the layer.

High-temperature oxidation of zircaloy-2 and zircaloy-4 in steam

At temperatures above the (α + β) β transformation temperature for zirconium alloys, steam reacts with β-Zr to form a superficial layer of zirconium oxide (ZrO 2) and an intermediate layer of oxygen-stabilized α-Zr. Reaction kinetics and the rate of growth of the combined ( ZrO 2 + α- Zr) layer for Zircaloy-2 and Zircaloy-4 oxidation in steam were measured over the temperature range 1050–1850°C. The reaction rates for both alloys were similar, obeyed parabolic kinetics and were not limited by gas phase diffusion. The parabolic rate constants were consistently less than those given by the Baker and Just correlation for zirconium oxidation in steam. A discontinuity was found in the temperature dependence of both the reaction rate and the rate of growth of the combined ( ZrO 2 + α- Zr) layer. The discontinuity is attributed to a change in the oxide microstructure at the discontinuity temperature, an observation which is consistent with the zirconium-oxygen phase diagram.

The effects of deformation on the high-temperature steam oxidation of zircaloy-2

An experimental study was made of the effects of deformation on the oxidation of Zircaloy-2 reactor fuel cladding in flowing steam in the temperature range 700 to 1300°C. The kinetics and mechanism of oxygen penetration and embrittlement were examined using gravimetric and metallographic methods. Tensile deformation during oxidation resulted in cracking of the growing oxide and local oxidation of the metal at the base of the crack. The number of points of local attack was inversely related to the temperature of oxidation. The depth of local penetration of the metal (in excess of the uniform depth of penetration for undeformed cladding) was greatest when the deformation rate was slow. The maximum depth of local attack found in these experiments was equivalent to an increase in total oxygen penetration of about twice that due to uniform diffusion. The possible relevance of these results to the analysis of reactor loss-of-coolant accidents is discussed.

高温水蒸気中におけるジルカロイ-2の酸化

高温水蒸気中におけるジルカロイ-2の酸化

Heat Transfer Characteristics of Zircaloy-2 Oxide Film

Oxide film was made to form on the outside surface of zircaloy-2 tube (inside diameter=12.672mm, wall thickness=0.826mm) under the following three different conditions, (a) 300°C saturated LiOH solution, (b) 450°C and 15 ata steam and (c) 450°C air. The resulting oxide presented colors ranging from white to black, depending on the oxidation temperature and duration of exposure.Each of these oxidized specimens, together with a control specimen (unoxidized) was heated directly by alternating current in high temperature coolant water sealed in an autoclave.The apparent thermal conductivity of the oxide film was obtained by comparison of the temperatures inside tube measured on the oxidized and clean specimens under conditions as similar as possible. The experiment was conducted under 110°, 150°, 200°, 250° and 300°C of coolant temperature.At near 110°C of coolant temperature, the apparent thermal conductivity was not affected significantly by difference in the method adopted for oxidization, and the values obtained were distinctly lower than that of sintered ZrO2 stabilized with CaO. At higher temperatures, the oxide films formed under different conditions each showed individual heat transfer characteristics. The apparent thermal conductivity of white oxide film was smaller than that of black oxide film at near 300°C of coolant temperature.

Heat Transfer Characteristics of Zircaloy-2 Oxide Film

Oxide film was made to form on the outside surface of zircaloy?2 tube (inside diameter=12.672 mm, wall thickness=0.826 mm) under the following three different conditions, (a) 300°C saturated LiOH solution, (b) 450°C and 15 ata steam and (c) 450°C air. The resulting oxide presented colors ranging from white to black, depending on the oxidation temperature and duration of exposure. Each of these oxidized specimens, together with a control specimen (unoxidized) was heated directly by alternating current in high temperature coolant water sealed in an autoclave. The apparent thermal conductivity of the oxide film was obtained by comparison of the temperatures inside tube measured on the oxidized and clean specimens under conditions as similar as possible. The experiment was conducted under 110°, 150°, 200°, 250° and 300°C of coolant temperature. At near 110°C of coolant temperature, the apparent thermal conductivity was not affected significantly by difference in the method adopted for oxidization, and the values obtained were distinctly lower than that of sintered ZrO2 stabilized with CaO. At higher temperatures, the oxide films formed under different conditions each showed individual heat transfer characteristics. The apparent thermal conductivity of white oxide film was smaller than that of black oxide film at near 300°C of coolant temperature.

The influence of oxide stress on the breakaway oxidation of zircaloy-2

The stresses in oxide films on Zircaloy-2 and zirconium crystal bar were measured after oxidation in the temperature range 500–700 °C. The stresses in films on Zircaloy-2 reached a maximum at an oxide thickness approximately equal to that at which breakaway oxidation occurs. The stresses in films on zirconium crystal bar remained low, and breakaway did not occur under the conditions of the experiment. The results show that breakaway oxidation in Zircaloy-2 is probably due to the mechanical failure of a protective inner layer of oxide under the compressive stresses which arise during growth.

A study of charge transport processes during the oxidation of zirconium alloys

It has been confirmed that the oxidation of crystal-bar zirconium is limited by electron transport through the oxide and across the outer oxide surface between electronic flaws in the oxide and the sites of oxygen reduction. Two thirds of the total resistance is due to the surface resistance. Electronic conduction does not appear to be a limiting factor in the oxidation of Zircaloy-2 or of zirconium-rich binary alloys with chromium, iron, tin and nickel. A decrease in the oxidation rate of these alloys on short circuiting the electronic current cannot be explained on the basis of either simple charge transport processes or the small morphological changes caused by changes in the field across the oxide. An hypothesis based on space-charge accumulation at a p-n junction near the outer oxide surface is proposed. An explanation of differences in the early pre-transition kinetics of electropolished and pickled Zircaloy-2 is proposed on the basis of the above hypothesis.

Low temperature (< 300 ° c) oxidation of Zircaloy-2 in water

Measurements of the oxidation of two batches of Zircaloy-2 at 200, 250, 275 and 300 °C in demineralised water, combined with earlier unpublished data at intermediate temperatures, have shown that published extrapolations of the oxidation rate downward in temperature gave inaccurate predictions. Neither the kinetic rate law, nor the temperature coefficient was regular, the latter being low for temperatures between 260 and 290 °C, and for exposure times below ~ 50 days. The irregularities indicated successive changes in the balance between a series of oxidation controlling processes. Interferometer measurements, optical and electron microscopy have shown that up to 40% of the weight gain resulted from the formation of local regions of thicker oxide (together with some oxygen dissolution in the metal). Nucleation of spinel-like crystals (possibly of magnetite) on the oxide surface also contributed to the weight gain. Their deposition was probably controlled by the local electrical properties of the oxide film.

The kinetics and mechanism of the oxidation of zircaloy-2 at 350–500 °C

An analysis of the kinetics of reaction of Zircaloy-2 with oxygen and water vapour in the ranges 20 to 200 Torr and 350 to 500 °C is reported, based upon continuous measurements of the progress of oxidation by both gravimetric and manometric techniques. The study included the transient effects of changing some of the conditions of oxidation (temperature, pressure), the measurement of surface area at various stages of oxidation and microscopy. The kinetics could be divided into four distinct ranges of weight gain, progressing from parabolic through cubic to linear relationships, the weight gains at the transitions being dependent upon temperature. A model is developed which describes the mechanism of oxidation and in particular offers a simple explanation of the cubic rate law in terms of cracking along or near the oxide/metal interface.

Some effects of pressure on the oxidation of zircaloy-2 in steam and oxygen

Experiments under constant pressure conditions show that the oxidation of zircaloy-2 in steam or oxygen is relatively insensitive to pressure. When the pressure is changed during the course of post-transition oxidation, however, the rate of oxidation is observed to change proportionately with the change in pressure. Subsequently the rate of oxidation recovers and eventually attains a rate close to that prior to the changing of the gas pressure. This behaviour is difficult to reconcile with the concept of an oxide diffusion barrier of constant thickness which controls the linear post-transition oxidation rate. Alternative mechanisms are discussed and it is concluded that the post-transition oxidation rate at any temperature may be determined by the rate of oxidation of non-stoichiometric ZrO 2 at that temperature. The possibility of performing experiments to test these suggestions is considered.