Zircaloy-4 Specimens Research Articles

Structure and Properties of Chromium- and Zinc-Oxide Layers Formed on Zircaloy-2 in High-Temperature Water

The corrosion behavior of Zircaloy-2 in the presence of Zn was investigated. Zinc is a possible technological additive to be injected in the coolant to reduce the 60Co buildup. However, its influence on the cladding corrosion, alone or in combination with some typical corrosion impurities, as, for example, Cr, has not been considered so far. Because of this, the surface composition and electrochemical properties of Zircaloy specimens were investigated after their exposure to Zn2+, CrO42-, and CrO42- + Zn2+ aqueous solutions at 250°C. It was found that zinc-containing phases did not deposit from solutions containing on Zn2+ ions. Amorphous Cr3+-oxide and ZnCr2O4 ferrite phases were found on the surface of the samples after their exposure to CrO42- and CrO42- + Zn2+ environments, respectively. The amounts of the deposited Cr and Zn + Cr strongly depended on the times of the preconditioning of the Zircaloy specimens in high-temperature water. The rate of the oxide precipitation declined with increasing exposure time to both the CrO42- and CrO42- + Zn2+ solutions. The electrochemical measurement showed that the limiting factor of the Cr and Zn + Cr deposition reaction was the reduction of Cr(VI) to Cr(III). The reduction completely depended on the resistance of ZrO2, Cr, and Zn + Cr oxides, which increased with the time of preconditioning and exposure. A thermodynamic analysis based on oxide solubilities was applied to explain the different deposition pathways in the CrO42- and CrO42- + Zn2+ environments. In view of the decreasing deposition rate of the Zn - Cr-oxide phases, it could be concluded that their limited precipitation and presence do not have a significant adverse effect on the fuel cladding corrosion.

Dissolution of oxygen-enriched Zircaloy-2

When metal is removed from oxidised Zircaloy specimens in order to study the morphology of the oxide at the metal–oxide interface needle-like features are frequently observed. Since similar features are not observed in cross-section TEM examinations it has been questioned whether or not the needles are a result of the dissolution process. In particular it has been proposed that reprecipitation of oxide may take place when the metal is enriched with oxygen. In the present work oxygen-enriched Zircaloy has been dissolved and the resulting structures examined. The results indicate that the needles are in fact artefacts of the specimen preparation procedure. However, there are no significant differences between oxygen-enriched and normal Zircaloy below an oxide layer grown in steam at about 400 °C. In view of the differences between the needle structure observed after metal removal with bromine or by electropolishing and removal by HF–HNO 3 pickling solution it is speculated that the needles consist of a hydrous zirconium oxide which is unstable in the pickling solution.

Effect of Ar ion irradiation on electrochemical behaviors of zircaloy-4

In order to investigate the effect of the damage on the electrochemical behavior, zircaloy-4 specimens were irradiated by Ar ions using an accelerator at an energy of 50 keV, with doses from 3×10 14 to 3×10 16 ions/cm 2 at liquid nitrogen temperature. Potentiodynamic polarization measurements were employed to evaluate the corrosion resistance of zircaloy-4 in a 0.5 M H 2SO 4 water solution at room temperature. A damage efficiency function on the passive current density of the polarization curve was defined and deduced theoretically.

The air oxidation of yttrium ion implanted zircaloy-4 at 500°C

The beneficial effect of yttrium ion implantation on the oxidation behavior of zircaloy-4 at 500°C has been investigated. Zircaloy-4 specimens were implanted by yttrium ions with a dose range from 1×10 16 to 1×10 17 ions/cm 2 at about 120°C, and then oxidized in air at 500°C for 100 min. The valence of the oxides in the scale was analyzed by X-ray photoemission spectroscopy (XPS). Glancing angle X-ray diffraction (GAXRD) was employed to examine the phase transformation due to the yttrium ion implantation in the oxide films. The addition of yttrium changed the phase in the scale from monoclinic zirconia to tetragonal zirconia. The measurement of weight gain showed that a significant improvement was achieved in the oxidation behavior of yttrium ion implanted zircaloy-4 compared with that of the as-received zircaloy-4. The mechanism of the improvement was discussed.

A hydrogen uptake micro-mechanism for Zr alloys

Past attempts to develop a micro-mechanistic model for the hydrogen uptake process during zirconium oxidation have concentrated on solid state diffusion through a barrier oxide. Experimental results have often been invalidated by artefacts. Recently it has become evident that the uptake may be via flaws in the oxide film which penetrate, at least briefly, up to the metal–oxide interface. An attempt to characterise and locate such flaws in oxide films on specimens from a number of batches of several zirconium alloys is reported here, using cathodic polarization to deposit small Cu balls on the active cathodic sites and scanning electron microscopy to identify and characterize these sites. In oxides on Zircaloy-2 samples, the cathodic sites were usually small cracks in the oxide, sometimes (but not always) at pits where intermetallic particles had been etched out during the initial surface preparation. The same cathodic sites could be decorated during successive cathodic polarizations, but even a short subsequent oxidation (1 day, 400°C steam) repassivated all previous sites, and created new ones. In one batch of Zr–2.5%Nb alloy, that contained an array of large carbides, cracks in the oxide at the sites of these particles provided the cathodic sites. In the absence of such particles, far fewer cathodic sites were found in oxides on Zr–2.5%Nb alloy specimens than on Zircaloy-2 specimens. These few sites were always small cracks in the oxide film. Exposure to a low pressure hydrogen atmosphere created many more cracks and cathodic sites in the oxide on Zr–2.5%Nb.

Transport mechanism of hydrogen through oxide film formed on zircaloy-4

The diffusion behavior of hydrogen in the oxide films of zircaloy-4 specimens containing different size of Zr(Fe,Cr)2 precipitates was examined. In the case of the specimen containing fine precipitates, hydrogen diffused uniformly through the zirconium oxide phase. The diffusion coefficient was 2·10−21 m2·s−1 at room temperature and 6·10−19 m2·s−1 at 673 K. The transport rate of hydrogen in the oxide film of the specimen containing coarse precipitates was significantly higher than that of the specimen containing fine ones at both room temperature and 673 K.

Influence of Ar ion bombardment on the uniform corrosion resistance of laser-surface-melted Zircaloy-4

Zircaloy-4 specimens were surface treated by laser surface melting and/or Ar ion bombarding. Corrosion tests were conducted in a 1N H 2SO 4 solution at room temperature and transmission electron microscope (TEM) was employed to analyze the change of microstructures in the surface layer. The potentiodynamic tests showed that the corrosion current density, i corr, was closely related to the surface condition, and it decreased with increasing the fluence of Ar ions. After Ar ion bombardment, a significant improvement was achieved in the uniform corrosion resistance of laser-surface-melted Zircaloy-4 compared with that of the as-received Zircaloy-4. At the end, Gibbs free energy was used to explain why the fine grain structure and the fine dispersion of alloy components always led to the better uniform corrosion resistance.

Determination of the β/β + γ eutectoid transition temperature in ZrO 2− x at variable heating/cooling rates

The temperature at which ZrO 2− x undergoes a eutectoid transition from tetragonal (β) to tetragonal + cubic (β + γ) structure has been measured using oxidized Zircaloy-4 specimens, a pyrometer-based differential thermal analysis technique, and heating/cooling rates in the range 19–569 K/min. The transition was found to be completely reversible, showed no hysteresis and was essentially independent of heating/cooling. The overall temperature uncertainty was obtained by combining calibration, pyrometer reproducibility, thermal lag and systematic errors. A transition temperature of 1786 ± 12 K is recommended for use in Zircaloy/steam oxidation-rate correlations employed in reactor safety codes.

A mechanism for the hydrogen uptake process in zirconium alloys

Hydrogen uptake data for thin Zircaloy-2 specimens in steam at 300–400°C have been analysed to show that there is a decrease in the rate of uptake with respect to the rate of oxidation when the terminal solid solubility (TSS) of hydrogen in the metal is exceeded. In order for TSS to be reached during pre-transition oxidation a very thin 0.125 mm Zircaloy sheet was used. The specimens had been pickled initially removing all Zr 2(Fe/Ni) particles from the initial surfaces, yet the initial hydrogen uptake rates were still much higher than for Zircaloy-4 or a binary Zr/Fe alloy that did not contain phases that dissolve readily during pickling. Cathodic polarisation at room temperature in CuSO 4 solution showed that small cracks or pores formed the cathodic sites in pre-transition oxide films. Some were at pits resulting from the initial dissolution of the Zr 2(Fe/Ni) phase; others were not; none were at the remaining intermetallics in the original surface. These small cracks are thought to provide the ingress routes for hydrogen. A microscopic steam starvation process at the bottoms of these small cracks or pores, leading to the accumulation of hydrogen adjacent to the oxide/metal interface, and causing breakdown of the passive oxide forming at the bottom of the flaw, is thought to provide the mechanism for the hydrogen uptake process during both pre-transition and post-transition oxidation.

The effect of hydride on the corrosion of Zircaloy-4 in aqueous LiOH solution

Commercial Zircaloy-4 sheets were charged with 230–250 wppm hydrogen by the gas-charging method and were homogenized at 400°C for 72 h in vacuum. The hydrogen charged specimens were corroded in pure water and aqueous LiOH solutions using static autoclaves at 350°C, and then characterized by measuring their weight gains with the corrosion time and observing their microstructures using an optical microscope (O/M) and a scanning electron microscope (SEM). The elemental depth profiles for hydrogen and lithium were measured using a secondary ion mass spectrometry (SIMS) to confirm their distribution at the oxide/metal interface. As the concentration of Li ions in the aqueous LiOH solution increased to more than 30 ppm, the normal Zircaloy-4 specimens corroded abruptly and heavily, forming more Zr-hydrides by the larger amount of absorbed hydrogen at higher Li ion concentrations. The specimens that had been charged with amounts of hydrogen greater than its solubility limit corroded early with a more rapid accerleration than normal specimens, regardless of the corrosion solutions. At longer corrosion times under 220 wppm Li ion concentration conditions, however, normal specimens showed a rather accelerated corrosion rate compared to the hydrogen-charged specimens. These slower corrosion rates of the hydrogen-charged specimens at longer corrosion times under 220 wppm Li ion concentration conditions may be due to the pre-existent Zr-hydride, which causes the hydrogen pick-up into the specimen to be depressed after the formation of the oxide with an appropriate thickness.

Stresses in an Oxidized Zircaloy Cladding under Quench Conditions: Recalculation of Single-Rod Quench Tests

About 70 single-rod quench tests with fresh and preoxidized Zircaloy specimens are carried out at Forschungszentrum Karlsruhe with water and steam as quench media. A number of mechanical effects are observed on the cladding of the test rods by posttest examination; the most important ones are the occurrence of through-wall cracks mainly for thick oxide scales and a severe degradation of the α-phase for water-quenched and steam-cooled tests.Calculations of some tests are done with the WSPAN one-dimensional rod mechanics code. This code calculates a very high stress pulse in the oxide scale occurring at the phase transition in the oxide phase. With the assumption of purely elastic behavior of the α-phase at the high strain rates occurring at the phase transition, the cracking in the α-phase and the formation of through-wall cracks can be understood.

Study on the precipitates in Zircaloy-4 by Mössbauer spectroscopy

Mössbauer spectroscopy of 57 Fe was used to study the content and structure of precipitates in different thermal-treated Zircaloy-4 specimens. The spectra of Zircaloy-4 have two sets of peaks. One of them can be ascribed to precipitates of intermetallic compound Zr(Cr,Fe) 2. The structure of the specimen is changed by thermal treatment. The area of the Mössbauer peak is also changed. Content of Zr(Cr,Fe) 2 in a water quenched sample is the same as that in an as-received (50% cold worked) sample. The content of alloying element-iron in matrix between precipitates is not an important factor which has an influence on corrosion resistance of Zircaloy-4.

Influence of size distribution of Zr(Fe, Cr) 2 precipitates on hydrogen transport through oxide film of Zircaloy-4

The hydrogen uptake behavior of two types of Zircaloy-4 specimens containing fine or coarse Zr(Fe,Cr) 2 precipitates was studied at 623–723 K in the pre-transition period of oxidation in steam. The amount of hydrogen uptake of the specimen containing fine precipitates was smaller than that of the specimen containing coarse ones, and the former was independent of the oxidation temperature while the latter increased with decreasing temperature. These results were successfully explained with the model that the Zr(Fe,Cr) 2 precipitates remaining unoxidized act as the short-circuiting route in the hydrogen transport through the oxide film. The amount of hydrogen uptake of the present Zircaloy-4 specimens containing coarse precipitates was larger than that of the Zircaloy-2 specimens containing comparable sizes of precipitates. This was attributed to the fact that the concentration of precipitates remaining unoxidized in the oxide film of Zircaloy-4 was higher than that of Zircaloy-2.

Hydrogen uptake during oxidation of zirconium alloys

By analysing hydrogen uptake data for Zircaloy specimens with the same chemistry, but different thicknesses, it is found that hydrogen uptake rates decrease when TSS in the metal is exceeded Hydride precipitation is usually uniform in the bulk metal, and could only affect hydrogen uptake if the tendency to precipitate in the bulk is overcome and precipitation occurs at the point of ingress. This requires very high local ingress rates and a very local uptake process (10 −4 of the surface area) to achieve.

Analysis of Small-Angle Scattering Data from Spherical Particles by both the Indirect Transform Method and the Maximum-Entropy Method

In this paper extensive comparison studies of the indirect transform method (ITM) and the maximum-entropy method (MEM) were made for recovering the size distribution of spherical particles. Simulated small-angle scattering (SAS) data with different noise levels for various size distributions are tested. It is found that both the MEM and ITM are capable of accurately recovering the original size distributions for various types of size distributions from simulated SAS data with a 1% noise level. When the noise level of the simulated SAS data is increased to 6%, the size distributions recovered by both the ITM and MEM begin to have many noisy oscillations. It is also found that the MEM is more vulnerable to termination effects, i.e. when low-Q or high-Q data are not available. On the other hand, even for cases with limited-Q-range data the ITM can still accurately recover the original size distribution. This was clearly shown in the analysis of the measured small-angle X-ray scattering data, with a limited Q range, from a β-quenched zircaloy-4 specimen.

Role of Intermetallic Precipitates in Hydrogen Uptake of Zircaloy-2

The hydrogen uptake behavior of two types of Zircaloy-2 specimens containing either fine intermetallic precipitates or coarse ones was studied at 623–723 Kin the pre-transition period of oxidation to clarify the role of the precipitates in the hydrogen transport through the oxide film. In the former case, the amount of hydrogen taken up was small and did not show the oxidation temperature dependence. In the latter case, the amount of hydrogen taken up was large and dependent markedly upon the oxidation temperature; it increased with decreasing oxidation temperature. These results were successfully explained with the model that the intermetallic precipitates remaining in a metallic state in the oxide film act as the fast transport route of hydrogen.

Role of Intermetallic Precipitates in Hydrogen Uptake of Zircaloy-2.

The hydrogen uptake behavior of two types of Zircaloy-2 specimens containing either fine intermetallic precipitates or coarse ones was studied at 623–723 Kin the pre-transition period of oxidation to clarify the role of the precipitates in the hydrogen transport through the oxide film. In the former case, the amount of hydrogen taken up was small and did not show the oxidation temperature dependence. In the latter case, the amount of hydrogen taken up was large and dependent markedly upon the oxidation temperature; it increased with decreasing oxidation temperature. These results were successfully explained with the model that the intermetallic precipitates remaining in a metallic state in the oxide film act as the fast transport route of hydrogen.

The development of porosity in thick zirconia films

The pore structures of posttransition oxide films on specimens of Zircaloy-4 fuel cladding oxidised in 340–360°C (613–633 K) water or 400°C (673 K), 10 MPa steam, have been examined by impedance spectroscopy, scanning electron microscopy, and replica transmission electron microscopy of the oxide surfaces, the oxide-metal interfaces (after dissolution of the metal) and fractured oxide cross sections. The results were compared with measurements of the total oxide thickness derived from weight gains, FTIR interferometry, impedance measurements with mercury contacts, and metallographic sections. The various measurements of total thickness were in good general agreement. Impedance measurements during soaking with aqueous ammonium nitrate solution showed two characteristically different types of behaviour, while the impedance spectra of the soaked oxides also showed a dichotomy of behaviour, but for different groups of specimens. The impedance spectra showed surprising similarities for specimens from similar cladding batches but with very different oxide thickness and morphologies, suggesting that micrometallurgical differences between batches may influence oxide structure irrespective of formation conditions and final thickness. Electron microscopy showed relatively few cracks and pores in the outer oxide films formed on belt-ground surfaces, except for the thick white oxide patches formed in 400°C steam. Much higher pore densities were seen at the oxide-metal interface than at the surface, in agreement with the higher volume fraction of t-ZrO 2 expected at the oxide-metal interface. Relative pore densities at the oxide-metal interface correlated well with the relative oxidation rates during the final oxidation period.

Reply to comment on “the ductile-brittle transition of a zirconium alloy due to hydrogen”

Bai estimates that the local strain rate at the notch tip in our notch tensile tests is at least equals to 2.4x10 -3 /s. Then, he compares our results with recent results by Bai et a1.[3-5] in which the specimens were tested at 10~/s or 10-3/s. In this work it was found that for the hydrided specimens, those tested at 10-4/s exhibited an usual ductilebrittle transition behavior at room temperature, while those tested at 10-3/S were brittle with elongations less than 2 %, independent of the testing temperatures and the hydrogen contents. Therefore, Bai comments that the different hydrogen contents for ductile-brittle transition may be also attributed to the strain rate effect beside with the notch effect. We can partly agree with this point of view. It is well acknowledged that the local strain rate at a notch tip can be magnified to a large extent and the attendant increase of strain rate can induce brittle behavior of metals. However, from our results on smooth hydrided specimens tensile tested at 5x103/s, as shown in Figure 1, no ductilebrittle transition in reduction of area was found up to a hydrogen content of 340 ppm. Since the local strain rate at the notch tip, as estimated by Bai, is close to the strain rate used in tensile tests for the smooth specimens, it is clear that strain rate per se is not significant for the early occurrence of ductile-brittle transition. In fact, Gill et al.[6] performed notch tensile tests on hydrided zirconium and found an overall lowering of elongation values for notched specimens. This was attributed to the effects of triaxiality in the stress system introduced by notch. The triaxiality can reduce the magnitude of shear stress which are responsible for plastic deformation. Thus, the notched specimens exhibit brittle behavior at a lower hydrogen content. Bai's findings of brittleness of the specimens tested at 103/s were obtained from specimens containing at least 500 ppm of hydrogen. Since we did not perform any tests on specimens with hydrogen content higher than 350 ppm, no direct comparison can be made with Bai's results. The other point mentioned by Bai is the effect of hydriding temperature on the mechanical properties of hydrided specimens and the hydrides themselves. The hydriding temperature in our study was purposely chosen to avoid extraneous changes in alloy microstructure. Since the stress relief temperature of zirconium alloys is 480595 °C [7], hydriding at a temperature above 500°C may produce changes in alloy microstructure and thereby change the mechanical properties of zirconium alloys. Therefore, it is not surprising that the ductile-brittle transition on Zircaloy-4 specimens hydrided at 700°C occurs at a different hydrogen content than those hydrided at 400°C. However, for the specimens hydrided at temperatures lower than the stress relief temperature, such as 400°C or 350°C, it is not expected that any extraneous changes in alloy microstructure from the effect of temperature will occur; as a result, the mechanical properties of the zirconium matrix, regardless hydrided at 350°C or 400°C, will

Effect of hydrogen contents on the mechanical properties of Zircaloy-4

Zircaloy-4 plate specimens were gaseously hydrided up to 340 ppm H and then tested at 25, 100, 200 and 300°C. Notched tensile specimens were chosen to better understand the ductile-brittle transition associated with hydrogen content. The results showed that, at 25°C, a ductile-brittle transition occurred on the notched specimens of zircaloy-4 with hydrogen contents between 30 ppm and 140 ppm. The ductile-brittle transition became less distinct as temperature increased to 100°C and above. This was attributed to the improved ductility of the zirconium matrix by examining the temperature dependence of fracture surface and hydride morphology. Load serrations were observed on the hydrided, notched specimens tested at 200°C and above, which was related to dynamic strain aging. The stress-strain curves exhibited yield points on the specimens without hydriding. The yield points were suppressed on the hydrided specimens, which was explained by the mechanism that twinning-assisted slip may be hindered by hydrides.