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.     

铁素体-马氏体钢P92在超临界水中的腐蚀性能

研究了P92钢在550和600℃超临界水中的腐蚀特性,采用扫描电镜、X射线能谱仪和X射线衍射仪分析了氧化膜的表面形貌、组织结构和元素分布。结果表明：P92钢在超临界水中的氧化动力学大致服从立方生长规律,600℃下P92钢的腐蚀增重和氧化膜厚度均为550℃时的3倍。P92钢在超临界水中形成的氧化膜为双层结构,氧化膜外层富Fe,而内层富Cr。600℃时P92钢氧化膜发生了开裂和剥落,其原因主要在于降温过程中基体与氧化物间的热膨胀系数不相匹配而产生的较大热应力。     

Cellular automaton modeling on the corrosion/oxidation mechanism of steel in liquid metal environment

Corrosion and oxidation of structure material in liquid lead alloys, which have been strongly associated with the nuclear industry, are specific but important areas of corrosion in liquid environments. In high temperature liquid metal environment, the corrosion rate of stainless steels is controlled by the solid diffusion of iron along grain boundaries in the oxide layer and a duplex oxide layer is formed with the inner layer growing by infiltration of corrosive solution along oxide micropores and the outer layer growing by the diffusion of metal ions. To interpret the role of diffusion and reaction process in the process of oxidation, a cellular automaton model, which combines the surface growth and internal oxidation, was created to explain the oxidation mechanism of steels in high temperature corrosive liquid metal environment. In this model, three main processes, which include the corrosion of the substrate, the diffusion of iron species across the oxide layer and precipitation of iron on the oxide layer, are simulated. The diffusion process is simulated by random walk model. Mapping between present model and experimental data has been created. The gross features concerning the evolution of the involved process were explored.     

Contribution to the understanding of the ZrNb(l%)O(0.13%) oxidation mechanism at 500 °C in dry air

The oxidation of ZrNb(l%)O(0.13%) at 500 °C in dry air was investigated in situ by thermogravimetric analyses and electrochemical impedance spectroscopy. Sheets of the alloy were coated with different noble metals (Pt, Au, Ag) as electrode material. After an initial sub-parabolic rate law, the kinetics of ZrNb(l%)O(0.13%) oxidation are characterized by a transition to another decreasing rate law for different times and thicknesses. Noble metals were observed to clearly modify the oxidation rate, even when a pre-oxidized zirconia film was formed before the deposit and the increase in the oxidation rate was always monitored for thick oxides (30 μm). The kinetic transition is hypothesized to be associated with the microstructural degradation of the oxide film. Localized oxidation rate increases were revealed by scanning electron microscopy at the tip of radial cracks distributed on more than 2% of the total area of the sample. Catalytic effects observed on the oxidation rate after the noble metal deposition suggest that the mechanism controlling the oxidation rate is not a solely one of oxygen diffusion through the oxide layer. The reaction of oxygen reduction at the oxide/metal/gas interface partially controls the oxidation kinetics of ZrNb(l%)O(0.13%). Complex electrical signatures monitored during the oxide growth corroborate this assumption and hence indicate that oxygen reduction is still partially controlling the oxidation rate when noble metal are present on ZrNb(l%)O(0.13%) surface. Finally, a mixed process of interfacial-diffusion mechanism is proposed to be the rate determining step for ZrNb(l%)O(0.13%) oxidation in this environment.     

Nodular Corrosion of Alpha-Annealed Zircaloy-4 in Steam at 500°C

The early stage of nodular corrosion was investigated on α-annealed Zircaloy-4 in 500°C steam under various pressures. The preoxide film thicker than 1 μm, which was formed in atmospheric pressure, markedly restrained the nodule nucleation. The restraining effect was reduced when the preoxidation pressure increased. The careful observations of the initial stage of nodule nucleation revealed that accelerated corrosion appeared selectively on certain grain groups, and small oxide crystallites were formed at the metal/oxide interface underneath the selective grains and they induced the cracking of surface oxide film. The crystallites in the selective grains were enlarged rapidly after the oxide film was cracked. When the crystallites grew a certain size, the nodules were appeared on those grain groups with large crystallitic oxides at metal/oxide interfaces. Cold working increased the resistivity against the nodular corrosion and showed the maximum resistivity at 30% cold working. Conclusively, the nodular corrosion may be explainable as a locally accelerated corrosion due to the combined effect of the preoxide, the oxidation rate difference among grains, and the oxide shape.     

Pyrochemical reduction of uranium dioxide and plutonium dioxide by lithium metal

The lithium reduction process has been developed to apply a pyrochemical recycle process for oxide fuels. This process uses lithium metal as a reductant to convert oxides of actinide elements to metal. Lithium oxide generated in the reduction would be dissolved in a molten lithium chloride bath to enhance reduction. In this work, the solubility of Li₂O in LiCl was measured to be 8.8 wt% at 650 °C. Uranium dioxide was reduced by Li with no intermediate products and formed porous metal. Plutonium dioxide including 3% of americium dioxide was also reduced and formed molten metal. Reduction of PuO₂ to metal also occurred even when the concentration of lithium oxide was just under saturation. This result indicates that the reduction proceeds more easily than the prediction based on the Gibbs free energy of formation. Americium dioxide was also reduced at 1.8 wt% lithium oxide, but was hardly reduced at 8.8 wt%.     

Oxidation of zircaloy cladding in air

Non-irradiated, stress-relieved Zircaloy-4 claddings were oxidized at 350, 400, 450 and 500 °C in air. Some samples were dipped in sea water, dried and heated. Weight gain measured as a function of time revealed that the oxidation has pre- and post-transition regions just like the reaction in high temperature steam and water, and that sea water salts enhanced the weight gain after the transition point. Optical microscopy was carried out on the post-test oxide layer and substrate metal structure. Rate constants of the oxidation were determined and proved to be close to those of the previous studies including the experiments in high temperature steam and water.     

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.     

Residual stresses and tetragonal phase fraction characterisation of corrosion tested Zircaloy-4 using energy dispersive synchrotron X-ray diffraction

To improve the understanding of the oxidation mechanism in zirconium alloys for fuel clad applications, detailed residual stress and phase fraction analysis was carried out for the oxides formed on Zircaloy-4 after autoclave exposure at 360 °C for various times by means of synchrotron X-ray diffraction. In a post-transition sample (220 days), significant stress variation through the oxide thickness was found for the monoclinic phase in individual oxide layers, with maximum in-plane compressive stresses located towards the metal–oxide interface and a discontinuity in the residual stress profile. The depth of this discontinuity matched well with the depth at which electron microscopy analysis showed an interface between two distinct oxide layers. Analysis of the tetragonal phase with exposure time demonstrated changes of the total volume of tetragonal phase before and during transition. These observations are put into the context of residual stress evolution presented previously, to provide further insight into the importance of phase transformations and residual stresses in determining the corrosion kinetics of Zr alloys.     

The effect of crystal textures on the anodic oxidization of zirconium in a boiling nitric acid solution

The effects of crystal textures and the potentials in the anodic oxidation of zirconium in a boiling nitric acid solution were investigated to study the stress corrosion cracking of zirconium in nitric acid solutions. The test specimen was machined such that the specimen surface was parallel to the rolling surface, arranged with a (0002) crystal texture. The potentials applied for the anodic oxidation of zirconium were set at 1.2, 1.4, and 1.5 V against a saturated KCl–Ag/AgCl electrode (SSE) in boiling 6 M HNO₃. The growth of the zirconium oxide film dramatically changed depending on the applied potential at a closed depassivation potential (1.47 V vs. SSE in this study). At 1.5 V, the zirconium oxide film rapidly grows, and its growth exhibits cyclic oxidation kinetics in accordance with a nearly cubic rate law. The zirconium oxide film grows according to the quantity of electric charge and the growth rate does not depend on the crystal texture in the pretransition region before the cyclic oxidation kinetics. However, the growth and cracking under the thick oxide film depend on the crystal texture in the transition region. On the normal direction side, the oxide film thickness decreases on average since some areas of the thick oxide film are separated from the specimen surface owing to the cracks in the thick oxide. On the rolling direction (RD) side, no cracks in the thick oxide film are observed, but cracks are found under the thick oxide film, which deeply propagate in metal matrix along the RD without an external stress. The cracks under the thick oxide film propagate to the center of the oxide layer. The crystal orientation relationship between the oxide layer and the zirconium matrix is (0002)_Zr//(111)_ZrO2, and the cracks in the oxide layer propagate in the (0002)_Zr plane in the zirconium matrix. The oxide layer consists of string-like zirconium oxide and zirconium hydride. The string-like zirconium oxide contains orthorhombic ZrO₂ in addition to monoclinic ZrO₂. It is not well known why the cracks propagate along the (0002)_Zr plane under the thick oxide film. As one assumption for the mechanism of crack initiation and propagation without an external stress, it is considered that the oxidizing zirconium hydrides precipitated in the (0002)_Zr plane near the interface of the thick oxide film and the matrix. Then, the phase transformation from orthorhombic ZrO₂ to monoclinic ZrO₂ in the oxide layer causes the crack propagation in the (0002) plane.     

Stress measurements during thin film zirconium oxide growth

Stress accumulation during thin film zirconium oxide growth was successfully measured using new curvature measurement technique and stress of up to 5.1 GPa was observed in an approximately 50 nm thick oxide film. Experimental results also show that steam and air oxidation make little difference in the stress profile on the oxide film thickness, especially during the early stage of oxidation. This result possibly supports the theory that zirconium corrosion kinetics crucially depends on the oxide phase transformation at the metal-oxide interface. Apparent discrepancies between previous studies were interpreted in terms of stress relaxation effects on the measurements.     

Corrosion Resistance of Fe-Al-Alloy-Coated Ferritic/Martensitic Steel under Bending Stress in High-Temperature Lead-Bismuth Eutectic

The corrosion test was performed for ferritic/martensitic steel HCM12A with and without Fe-Al alloy coating in LBE at temperatures of 550 and 650°C under loading for an immersion time up to 500 h. After the corrosion test at 550°C for 500 h, both of the uncoated and Fe-Al-coated HCM12A showed a good corrosion resistance without the influence of the tensile stress on the LBE corrosion. On the other hand, after the corrosion test at 650°C, the Fe-Al coating layer on the specimen surface exhibited no LBE dissolution corrosion because of the formation of a stable oxide protection film on the coating layer surface, although the coating layer cracked. The LBE penetrated into the cracks and corroded the base metal and the precoating layer. The uncoated HCM12A exhibited the double oxide layers of FeO and Fe- Cr spinel. The FeO was damaged in the bent zone by the stress, and the Fe-Cr spinel layer was not destroyed by the influence of cracking and the tensile stress. The cracking and stress did not have a large influence on the overall oxidation corrosion rate.     

Influence of oxide layer morphology on hydrogen concentration in tin and niobium containing zirconium alloys after high temperature steam oxidation

The influence of the oxide layer morphology on the hydrogen uptake during steam oxidation of (Zr,Sn) and Zr-Nb nuclear fuel rod cladding alloys was investigated in isothermal separate-effect tests and large-scale fuel rod bundle simulation experiments. From both it can be concluded that the concentration of hydrogen in the remaining metal strongly depends on the existence of tangential cracks in the oxide layers formed by the tetragonal - monoclinic phase transition in the oxide, known as breakaway effect. In these cracks hydrogen is strongly enriched. It results in very local high hydrogen partial pressure at the oxide/metal interface and in an increase of the hydrogen concentration in the metal at local regions where such cracks in the oxide layer exist. Due to this effect the hydrogen uptake of the remaining zirconium alloy does not depend monotonically on temperature. Differences between (Zr,Sn) and Zr-Nb alloys are caused by differences in the hydrogen production due to different oxidation kinetics and in the crack forming phase transformation in the oxides as well as in the mechanical stability of the oxides.     

Etude de la reaction du magnesium massif avec l'air

The reaction of magnesium with air follows very irregular kinetics; periods of slow and rapid growth alternate in random fashion. We show that initially the metal reacts simultaneously with nitrogen and oxygen. A nitride layer is formed next to the metal and an oxide layer next to the gas. This double layer becomes temporarily protective, which leads to a substantial slowing down of the rate of weight increase. Sudden cracking entails a rapid acceleration of both oxydation and nitridation, giving rise to two surface layers. An irregular sequence of several fast and slow stages leads to the development of a sharply layered film. The cracks formed when the oxide layer is thicker are gradually sealed by the products of vapour-phase reactions. The total layer then acquires a distinct protective power, and the gases can no longer reach the metal surface. The reaction then starts again by the oxidation of the strata of nitride formed earlier and proceeds at a slow but regular pace.     

Thermal properties of plasma-sprayed tungsten deposits

Tungsten powder was plasma-sprayed onto a graphite substrate in order to examine the microstructures, porosities, and thermal conductivities of tungsten deposits. Tungsten was partially oxidized to tungsten oxide (WO₃) after plasma spraying. Most pores were found in the vicinity of lamellar layers in association with oxidation. It was revealed that both tungsten oxide and the lamellar structure with pores have a significant influence on the electrical and thermal conductivity.     

Uranium oxidation: Characterization of oxides formed by reaction with water by infrared and sorption analyses

Three different uranium oxide samples have been characterized with respect to the different preparation techniques. The results show that the water reaction with uranium metal occurs cyclically forming laminar layers of oxide which spall off due to the strain at the oxide/metal interface. Single laminae are released if liquid water is present due to the prizing penetration at the reaction zone. The rate of reaction of water with uranium is directly proportional to the amount of adsorbed water on the oxide product. Rapid transport is effected through the open hydrous oxide product. Dehydration of the hydrous oxide irreversibly forms a more inert oxide which cannot be rehydrated to the degree that prevails in the original hydrous product of uranium oxidation with water. Inert gas sorption analyses and diffuse reflectance infrared studies combined with electron microscopy prove valuable in defining the chemistry and morphology of the oxidic products and hydrated intermediates.     

Corrosion behavior of Zr alloys with a high Nb content

The corrosion behavior of the Zr alloy with a high Nb content was evaluated in the water loop system containing 2.2 wppm Li and 650 wppm B. The characteristics of the precipitates were analyzed by transmission electron microscopy (TEM) and the oxide was characterized by an X-ray diffraction method using a synchrotron radiation source. On the basis of the results obtained by these measurements, the relationship among the oxidation behavior, the precipitate characteristics and the oxide properties was discussed. It was shown that the Cu addition was of benefit to the corrosion resistance of the Zr alloy with a high Nb content and the corrosion resistance of the Cu-containing alloy (Zr–1.5Nb–0.5Sn–0.2Fe–0.1Cu) was superior to that of the Cr-containing alloy (Zr–1.5Nb–0.5Sn–0.2Fe–0.1Cr). The fine β-Nb precipitates were found more frequently in the Cu-containing alloy than the Cr-containing alloy when heat-treated in the same condition. The fraction of the tetragonal zirconia in the region of the metal/oxide interface was higher in the Cu-containing alloy than the Cr-containing alloy, suggesting that the stabilization of the tetragonal phase in the oxide was promoted more when the smaller precipitates are incorporated into the oxide. It is concluded that the fine distribution of β-Nb is desirable for stabilizing the tetragonal phase in the oxide, thereby increasing the corrosion resistance of the Zr alloy with a high Nb content.     

水化学对Zr-4合金氧化膜形貌的影响

为了研究LiOH水溶液影响Zr-4合金耐腐蚀性能的机理,用扫描电子显微镜和扫描探针显微镜观察Zr-4合金样品在不同介质中腐蚀后氧化膜内外表面的形貌。实验结果表明：由于氧化过程体积膨胀,氧化膜中存在平行于界面的压应力。在压应力的作用下,氧化膜向外鼓起,并在垂直于平面的方向上产生张应力,导致氧化膜破裂,从而使腐蚀加速。LiOH水溶液浓度越高,这个过程越快。     

Influence of oxide films on primary water stress corrosion cracking initiation of alloy 600

In the present study alloy 600 was tested in simulated pressurised water reactor (PWR) primary water, at 360 °C, under an hydrogen partial pressure of 30 kPa. These testing conditions correspond to the maximum sensitivity of alloy 600 to crack initiation. The resulting oxidised structures (corrosion scale and underlying metal) were characterised. A chromium rich oxide layer was revealed, the underlying metal being chromium depleted. In addition, analysis of the chemical composition of the metal close to the oxide scale had allowed to detect oxygen under the oxide scale and particularly in a triple grain boundary. Implication of such a finding on the crack initiation of alloy 600 is discussed. Significant diminution of the crack initiation time was observed for sample oxidised before stress corrosion tests. In view of these results, a mechanism for stress corrosion crack initiation of alloy 600 in PWR primary water was proposed.     

铝热自蔓延还原氧化铪实验研究

本文采用化学分析、X射线衍射（XRD）与扫描电子显微镜（SEM）等手段和方法,对铝热自蔓延还原氧化铪方法进行实验研究。结果表明,在物料单位发热量大于3 000 kJ/kg的条件下,可实现氧化铪的直接还原,原料加入CaO后,产物的渣金分离效果明显。XRD分析结果表明,还原产物为Al3Hf金属间化合物;SEM和能谱仪（EDS）分析结果表明,金属产物呈现片状,不规则线性排列,由Al3Hf相区和Al相区组成;化学分析结果表明,金属产物中氧含量低至020%,铪含量高达4120%。