Cracks In Oxide Film Research Articles

Microstructure analysis of Xe20+ irradiation and postirradiation corrosion of Zr-4 and Zr–1Nb alloys

The microstructure evolution of 5 MeV Xe20+ irradiation and the corrosion behavior of postirradiation in Zr-4 and Zr–1Nb alloys have been investigated. Positron results showed that vacancy defects, such as vacancies and vacancy clusters, are produced in Zr-4 and Zr–1Nb alloys after Xe20+ irradiation, and less vacancy defects are formed at 360 °C irradiation than at room temperature irradiation, resulting in lower S parameters due to recombination of defects. The nanoindentation results found that vacancy defects produced by irradiation lead to the hardening of both Zr-4 and Zr–1Nb alloys. Corrosion weight gain tests showed that the damaged layer induced by irradiation accelerates the oxidation rate during the corrosion process. Compared to the Zr-4 alloy, the damaged layer of Zr–1Nb alloy contains more vacancy defects after room temperature irradiation, which correspond to higher hardness increase and faster corrosion weight gain, resulting in the formation of cracks in oxide films as is observed by SEM. At the early stage of the corrosion, the postirradiation corrosion resistance of Zr-4 alloy may be better than that of Zr–1Nb alloy.

3D focused ion beam sectioning of zirconium oxides in Zircaloy-4 for the characterisation of cracking

Sequential 2D sectioning and imaging using a dual-beam focused ion beam and scanning electron microscope has been used to characterise the interconnectivity of cracks in oxide films on Zircaloy-4. Evidence of cracks perpendicular to the metal/oxide interface have been observed in multiple oxides of different thicknesses. Such vertical features are considered critical in the provision of a percolation path between the environment and innermost band of lateral cracks (closest to oxide/environment interface), in between the lateral bands of cracks and between the final band of lateral cracks and the metal/oxide interface. Given the limited occurrence of cracks perpendicular to the metal/oxide interface, localised tensile surface strains have been considered as a potential source of initiation of these cracks.

Study on Corrosion Resistance of N36 Zirconium Alloy in LiOH Aqueous Solution

Zr-Sn-Nb-Fe alloys are one of the important directions for continuous improvement of zirconium alloys for high burn-up fuel assemblies. The corrosion resistance of Zr-Sn-Nb-Fe alloys is closely related to the alloying element and water chemical condition. To better understand the effect of Sn on corrosion resistance of Zr-Sn-Nb-Fe alloy, the normal N36 (Zr-1Sn-1Nb-0.3Fe) and low-tin N36 (Zr-0.8Sn-1Nb-0.3Fe) alloy sheets were prepared and tested in static autoclave in both of 0.01 mol/L LiOH and 0.03 mol/L LiOH aqueous solution at 360°C and 18.6 MPa. The characteristics of the microstructure and oxide film of alloys were analyzed by TEM and SEM respectively. It was shown that that the corrosion transition of the normal N36 appears earlier and the weight gain is higher than the low-tin N36 in two corrosive mediums. The cracks paralleling to the interface of oxide/metal are formed in the fracture surface of the oxide film and the micrographs at the oxide film/substrate interface appear uneven morphology. With the increasing of corrosion gain, there are more parallel cracks in oxide film and the uneven morphology at the oxide film/substrate interface is more obvious.

The influence of second phase particles on the crack formation in oxide films formed on zirconium alloys

The oxidation behavior of second phase particles (SPPs) and the formation of cracks in oxide films on Zr-0.93Sn-0.82Nb-0.17Fe-0.019S alloy in lithiated water with 0.01 M LiOH were investigated. Results show that direction of crack propagation in the oxide films coincides with stringer direction of SPPs, indicating a close relationship between the crack formation and SPPs distribution. β-Nb particles along the cracks are oxidized faster as compared to those separating the crack. Therefore, the formation of cracks along the stringer can accelerate oxidation of SPPs, which in turn can encourage the formation of cracks in oxide films.

A superior corrosion behavior of Zircaloy-4 in lithiated water at 360 °C/18.6 MPa by β-quenching

To further understand the mechanism on the heat-treatment effect on corrosion behavior of Zircaloy-4, β-quenched and recrystallized specimens are corroded in an autoclave with 0.01 M LiOH aqueous solution at 360 °C/18.6 MPa. Results show that the β-quenched specimen possesses excellent corrosion resistance, the weight gain of which is consistently comparable to that of ZIRLO and N18 alloys during 529 days exposure, while the recrystallized specimen exhibits accelerated corrosion rate after 100 days exposure. The increase of the supersaturated solid solution contents of Fe and Cr in α-Zr matrix is responsible for the improvement of corrosion resistance by β-quenching treatment. The fracture surface and inner surface morphology of oxide films indicates that β-quenching treatment significantly slow down the development of pores to cracks in oxide films and reduces the uneven growing tendency of the oxide, which is related to the corrosion behavior.

Application of a Straining Electrode to High-Temperature Corrosion

A tensile stress test combined with an electrochemical polarization technique employing a molten salt as electrolyte, namely, a high-temperature straining electrode (HTSE) system, has been developed for study on the spalling and cracking characteristics of oxide films on metals and alloys. A 25 mol % -25 mol % -50 mol % melt was used at 773 K as the molten electrolyte and type 304 stainless steel (SS) as the specimen in this study. The SS was first polarized in a passive potential region in the molten sulfate mixture, and the anodic current during deformation at a constant strain rate was monitored. From both results of the current response and scanning electron microscope observation, it was confirmed that the current induced by spalling and cracking of oxide films was successfully monitored using the developed HTSE system. This system was also applicable for mechanistic study on anodic dissolution and subsequent passivation in the molten sulfates that occur immediately after the mechanical breakdown of oxide films. © 2004 The Electrochemical Society. All rights reserved.

Critical Strain for Cracking of Oxide Films on Zircaloy

Critical Strain for Cracking of Oxide Films on Zircaloy

Critical strain for cracking of oxide films on zircaloy.

(1984). Critical Strain for Cracking of Oxide Films on Zircaloy. Journal of Nuclear Science and Technology: Vol. 21, No. 3, pp. 243-244.

A porosimeter for determining the sizes of flaws in zirconia or other insulating films “In Situ”

Because other methods had proved inadequate for determining the sizes of pores and cracks in oxide films on zirconium alloys a mercury porosimeter was specially developed for this purpose. By plotting the resistive component of the impedance of the oxide film as a function of pressure (up to 10 5 psi) for a specimen immersed in mercury it was possible to determine the size distribution of flaws in the oxide. Evidence is presented to show that the defects detected in the oxide film are real and were not created by the conditions in the porosimeter. It is concluded that the smallest flaws identifiable in the porosimeter are below the resolution of a conventional two-stage electron microscope replica technique.

The Microtopography of Oxide Films on Niobium

An electron microscopic study of the oxide films formed on niobium permits the apparently anomalous oxidation rate behavior of niobium to be explained in terms of the formation of small, blister‐like cracks in the oxide film. An oxidation model, based on the idea that an oxidation process which is maintained by interstitial‐anion or anion‐vacancy diffusion leads to the generation of stresses in the oxide film, is proposed to account for these results. The observation of similar blister‐like cracks in oxide films on tantalum is also reported.