Corrosion Resistance Of Zirconium Alloys Research Articles

First-principles study on effects of alloying elements Sn and Nb on phase stability of corrosion oxide films of zirconium alloys

Water-side oxidative corrosion of zirconium alloy is a key problem in the design of nuclear fuel rods cladding materials in pressurised water reactors (PWRs), and its corrosion resistance is one of the main factors limiting service life. At present, Zr-Sn-Nb system alloys are still the main development direction of advanced zirconium alloys. Sn and Nb can exhibit a variety of valence states in the oxide film of the cladding and significantly affect the stability of ZrO<sub>2</sub>. However, the influence mechanism of Sn and Nb on the fraction of <i>t</i>-ZrO<sub>2</sub> and the <i>t</i>-<i>m</i> phase transition is unclear. In this work, the lattice properties, formation enthalpies, and oxygen vacancy formation energy of ZrO<sub>2</sub> under the doping conditions of Sn and Nb with different valence states are calculated based on the first-principles, and the influence mechanism of Sn and Nb on the stability of ZrO<sub>2</sub> is revealed at an atomic scale. The results show that there is a significant difference between the effects of Sn and Nb, as well as between low-valent and high-valent elements. Sn<sup>2+</sup> and Nb<sup>3+</sup> cause lattice swelling to be significantly distorted , Nb<sup>5+</sup> causes lattice to shrink, which contributes to reducing the stresses within the film, and Sn<sup>4+</sup> leads the lattice to slightly swell. The low-valent elements all make ZrO<sub>2</sub> less stable and are unfavourable for the stability of <i>t</i>-ZrO<sub>2</sub> relative to <i>m</i>-ZrO<sub>2</sub>. The high-valent Nb<sup>5+</sup>and Sn<sup>4+</sup> promote the relative stability of <i>t</i>-ZrO<sub>2</sub>, thus inhibiting the <i>t</i>-<i>m</i> phase transition, with Nb<sup>5+</sup> having a significant effect and Sn<sup>4+</sup> having a weak effect. The relative stability of <i>t</i>-ZrO<sub>2</sub> increases with pressure rising in a range of 0–3.5 GPa. Compared with high-valent elements, the low-valent elements are favourable for introduing oxygen vacancies into <i>t</i>-ZrO<sub>2</sub>, thus stabilising the interfacial <i>t</i>-ZrO<sub>2</sub> and enhancing the corrosion resistance of the cladding. By investigating the electronic structure, it is found that the oxygen vacancy formation energy is positively correlated with the magnitude of charge transfer (or degree of electron localisation) between the alloying element ion and the oxygen vacancy. These results contribute to optimizing the composition and designing the structure for corrosion resistance of zirconium alloys.

Effect of oxygen content in 400 ℃ super-heated steam on the corrosion resistance of Zr-xSn-0.35Fe-0.15Cr-0.15 Nb alloys

Zr-xSn-0.35Fe-0.15Cr-0.15 Nb (x = 1.0, 1.2, wt%) alloys were corroded in deaeration (DE) and 1000 μg/L oxygen (DO) super-heated steam at 400 °C/10.3 MPa to explore the effect of DO content and Sn content on the corrosion resistance of zirconium alloys. Results show that reducing the Sn content from 1.2% to 1.0% inhibits the precipitation of Nb as second phase particles and improves the corrosion resistance of the alloys in DE and DO environments; DO accelerates the corrosion of 1.0Sn and 1.2Sn alloys, however, when Sn content increases from 1.0% to 1.2%, the effect of DO on enhanced corrosion weakens to some extent.

Deoxidation Process of Oxidized Zirconium Alloy

When zirconium alloy is corroded, an oxide film is formed on the surface, which hinders the ion transfer during the corrosion process. Therefore, the analysis of the oxide film is an important part of the research on the corrosion resistance of zirconium alloys. In this paper, two kinds of Zr-Sn-Nb alloys were corroded in 400 °C/10.3 MPa pure steam and 500 °C/10.3 MPa pure steam in autoclave to obtain samples with oxide thickness of 14 um and 18 um respectively. Then they were annealed at 800 °C at a pressure of 10-4 Pa for 18 h. XRD and WDS studies were used to analyze the structure and oxygen content of the oxide film after annealing. The results indicate that the oxide films of alloys change from zirconium dioxide to zirconium after annealing. The oxygen diffuses into the substrate and its content decreases continuously with increasing diffusion distance. Combined with the SEM analysis of cross-section samples, it is found that the annealed samples are composed of several layers. An oxygen-saturated zirconium layer, a transitional layer with micro-cracks, an oxygen-dissolved α-Zr layer and a β-Zr layer are identified. Based on these results, the mechanism of the ion transfer in the oxide film during annealing is analyzed deeply. It is proposed that space charges in the oxide film have a major impact on deoxidation kinetics. This study provides a new research method for the corrosion mechanism of zirconium alloys.

Effect of dissolved oxygen on corrosion behavior of Zr–0.85Sn–0.16Nb–0.37Fe–0.18Cr alloy in 500 °C and 10.3 MPa super-heated steam

Abstract To better understand the role of dissolved oxygen (DO) in affecting corrosion behavior of zirconium alloys, the Zr–0.85Sn–0.16Nb–0.37Fe–0.18Cr (wt.%) alloy was corroded in super-heated steam at 500 °C and 10.3 MPa under 1×10−6 DO and deaeration conditions. The microstructure of the alloy and oxide films was investigated by SEM, TEM, EDS and EBSD. Results show that the corrosion is aggravated under 1×10−6 DO. Compared with the deaeration condition, the oxide film is looser, and has more micro-cracks and more uneven inner surface under DO condition. For the oxide film forming under deaeration condition, the selected area diffraction (SAED) spots of planes (002)m, and (101)t are strong, while those of the (001)m and are weak. However, for the oxide film forming under DO condition, the SAED spots of planes (111)m, (200)m and (101)t are strong, while those of the (100)m and (110)m are weak. The higher DO content in super-heated steam accelerates the growth of oxide films, thus decreasing the corrosion resistance of zirconium alloys.

Calculation of Oxygen Diffusion Coefficients in Oxide Films Formed on Low-Temperature Annealed Zr Alloys and Their Related Corrosion Behavior

The growth of oxide film, which results from the inward oxygen diffusion from a corrosive environment, is a critical consideration for the corrosion resistance of zirconium alloys. This work calculates the oxygen diffusion coefficients in the oxide films formed on zirconium alloys annealed at 400~500 °C and investigates the related corrosion behavior. The annealed samples have a close size for the second-phase particles but a distinctive hardness, indicating the difference in substrate conditions. The weight gain of all samples highly follows parabolic laws. The weight gain of the sample annealed at 400 °C has the fastest increase rate at the very beginning of the corrosion test, but its oxide film has the slowest growth rate as the corrosion proceeds. By contrast, the sample annealed at 500 °C shows the lowest weight gain but the highest corrosion rate constant. Such a corrosion behavior is attributed to the amount of defects existing in the oxide film formed on the annealed samples; fewer defects would provide a lower fraction of short-circuit diffusion in total diffusion, resulting in a lower diffusion coefficient of oxygen in the oxide film, thereby producing better corrosion resistance. This is consistent with the calculated diffusion coefficients of oxygen in the oxide films: 3.252 × 10−11 cm2/s, 3.464 × 10−11 cm2/s and 3.740 × 10−11 cm2/s for the samples annealed at 400 °C, 450 °C, and 500 °C, respectively.

Microstructure of Zircaloy-4 alloy during β phase quenching and determination of critical quenching diameter of its rods

The corrosion resistance of zirconium alloys is closely related to the size of precipitates. β-phase quenching is the key process for controlling the precipitate size of zirconium alloy. In this study, the microstructure and the precipitates size of nuclear graded Zircaloy-4 (hereinafter called as Zr-4) alloy treated by the quenching in different cooling mediums were studied, and the critical quenching diameter of the Zr-4 alloy rod was determined by finite element method. The results showed that, the cooling rate of the β-phase quenching had small effect on the grain size of the alloy, however the size of the needle α grains increased with a decrease of cooling rate. The thickness of α grains quenched in water, quenched in oil, quenched at room temperature and quenched at 440 °C were about 1 μm, 5 μm, 9 μm, and 15 μm, respectively. When the cooling rate was high (water-quenching and oil-quenching), the precipitates formed only along the grain boundary, and with the length of less than 100 nm. When the cooling rate was low (air quenching), precipitates formed not only at the grain boundaries but also inside the grains, and the precipitates length was about 150–200 nm. When the Zr-4 alloy rod was quenched in water or oil, the maximum diameter of the specimen with suitable precipitate size (about 100–200 nm) was 140 and 97 mm, respectively.

Preparation, structure, and properties of high-entropy alloy multilayer coatings for nuclear fuel cladding: A case study of AlCrMoNbZr/(AlCrMoNbZr)N

AlCrMoNbZr/(AlCrMoNbZr)N multilayer coatings with equal individual layer thicknesses varying from 5 to 50 nm were deposited by using magnetron co-sputtering technology on N36 substrates (Zr1Sn1Nb-0.3Fe (wt.%)), to enhance the corrosion resistance of zirconium alloys in the event of a loss-of-coolant accident (LOCA). Comprehensive characterization by using X-ray diffraction and transmission electron microscopy revealed a multilayer structure consisting of a face-centred cubic (FCC) (AlCrMoNbZr)N layer and an amorphous AlCrMoNbZr layer. The structure and properties of the AlCrMoNbZr/(AlCrMoNbZr)N multilayer coatings with different individual layer thicknesses were studied, and the structure of all multilayer coatings contained coexisting amorphous and FCC phases. In addition, the multilayer coatings exhibited good interfacial bonding strength and good hydrophobicity. Corrosion tests were performed in static pure water at a temperature of 360 °C and a pressure of 18.7 MPa for 30 days. The results showed that the AlCrMoNbZr 50 nm/(AlCrMoNbZr)N 50 nm multilayer coating had superior anti-corrosion properties (4.4 mg/dm2 weight gain) to those of the AlCrMoNbZr 5 nm/(AlCrMoNbZr)N 5 nm and AlCrMoNbZr 10 nm/(AlCrMoNbZr)N 10 nm multilayer coatings. The AlCrMoNbZr/(AlCrMoNbZr)N multilayer structures inhibited Al migration and suppressed boehmite phase formation more effectively than single-layer AlCrMoNbZr or (AlCrMoNbZr)N.

Degradation and structure evolution in corrosive LiOH solution of microarc oxidation coated Zircaloy-4 alloy in silicate and phosphate electrolytes

To improve the corrosion resistance of zirconium alloys, two separate coatings with distinct structure were prepared on Zircaloy-4 alloy by microarc oxidation in alkaline phosphate and silicate electrolytes, respectively. The coating formed in silicate solution has more dense and homogeneous structure than the other, thus it shows better corrosion resistance. According to the electrochemical impedance spectroscopy of the two samples immersed in LiOH solution for different time periods, the coating formed in phosphate solution is able to protect the substrate alloy from corrosion for 30days, while the coating formed in silicate can provide sufficient protection to the substrate up to 50days.

Oxidation behavior of Zr9S2 precipitates in Zr-0.8Sn-1.0Nb-0.3Fe-0.1Cr-xS alloys

Effect of trace sulphur (16–540μg/g, S) on the corrosion resistance of zirconium alloys and oxidation behavior of Zr9S2 precipitates were investigated. Results show that S dissolved in α-Zr is among 63 ∼170μg/g. Layered Zr9S2 particles precipitate when S addition reaches 170μg/g. Zr9S2 precipitates oxidize easily and contribute to humps in the oxide films. Oxidation of Zr9S2 precipitates causes regular shaped cavities with nanometer size grains at the inner surfaces of oxide films. The presence of cauliflower-shaped humps at the inner surfaces of oxide films is also discussed.

The Influence of Different Contents of Bi Addition on the Corrosion Behavior of Various Zirconium-Based Alloys

Zr-4(Zr-1.5Sn-0.2Fe-0.1Cr,wt%), S5(Zr-0.8Sn-0.34Nb-0.39Fe-0.1Cr), T5(Zr-0.7Sn-1.07Nb-0.32Fe-0.08Cr) and Zr-1Nb were adopted to prepare Bi-containing zirconium alloys for systematically investigating the effect of Bi addition on the corrosion resistance of zirconium alloys. The specimens were corroded in superheated steam at 400℃/10.3 MPa, and in lithiated water with 0.01 M LiOH or in deionized water at 360℃/18.6 MPa by autoclave testing. Results show that the corrosion resistance increases with the increasing of Bi content dissolved in α-Zr. But the presence of Bi-con- taining second phase particles (SPPs) is unfavorable for the enhancement of corrosion resistance. This indicates that the Bi dissolved in α-Zr matrix plays an important role in improving the corrosion resistance, while the precipitation of the Bi-containing SPPs does harm to the corrosion resistance.

Effects of alloyed Si on the autoclave corrosion performance and periodic corrosion kinetics in Zr–Sn–Nb–Fe–O alloys

This paper investigates the effect of alloyed Si on the autoclave corrosion performance and corrosion mechanisms of Zr–Sn–Nb–Fe–O alloys. Quantitative analyses are performed on the microstructure of alloys and their oxide behavior. The Si-containing alloy possesses improved corrosion resistance. The reason is that a trace of Si enhances the strength of alloy and thus slows the evolution of undulated interface of oxide/metal which is associated with the formation of cracks in the oxide. The generation of cracks is discussed in further detail in relation to corrosion kinetics. This work advances the understanding of improving the corrosion resistance of zirconium alloys.

Vacuum-arc chromium-based coatings for protection of zirconium alloys from the high-temperature oxidation in air

Multilayer Cr–Zr/Cr/Cr–N coatings for protection of zirconium alloys from the high-temperature oxidation in air have been obtained by the vacuum-arc evaporation technique with application of filters for plasma cleaning from macroparticles. The effect of the coatings on the corrosion resistance of zirconium alloys at test temperatures between 660 and 1100 °C for 3600 s has been investigated. The thickness, structure, phase composition, mechanical properties of the coatings and oxide layers before and after oxidation tests were examined by scanning electron microscopy, X-ray diffraction analysis and nanoindentation technique.It is shown that the hard multilayer coating effectively protects zirconium from the oxidation in air for 1 h at test temperatures. As a result of the oxidation in the coating the CrO and Cr2O3 oxides are formed which reduce the oxygen penetration through the coating. At maximum test temperature of 1100 °C the oxide layer thickness in the coating is about 5 μm. The tube shape remains unchanged independent of alloy type. It has been found that uncoated zirconium oxidizes rapidly throughout the temperature range under study. At 1100 °C a porous monoclinic ZrO2 oxide layer of ≥120 μm is formed that leads to the deformation of the samples, cracking and spalling of the oxide layer.

STUDY ON THE CORROSION RESISTANCE OF Zr-lNb-0.7Sn-0.03Fe-xGe ALLOY IN LITHIATED WATER AT 360 °C

Zirconium alloys have low thermal neutron absorption cross-section,good corrosion resistance and adequate mechanical properties.They have been successfully developed as fuel cladding materials in pressurized water reactors.It's well known that the corrosion resistance of Zr-Sn-Nb alloys is significantly superior to that of Zircaloy-4 alloy when corroded in lithiated water.The corrosion resistance of zirconium alloys is controlled by their chemical compositions,characteristics of second phase particles(SPPs) and microstructure evolution of the oxide in them.The corrosion tests of Zr- 1Nb-0.7Sn-0.03Fe-xGe(x=0,0.05,0.1,0.2,mass fraction,%) alloys were investigated by means of an autoclave test in lithiated water with 0.01 mol/L LiOH at 360℃.under a pressure of 18.6 MPa. The microstructures of the alloys and oxide films on the corroded specimens were examed by using TEM and SEM.The sample for the oxide microstructure observation was prepared by a HELIOS- 600I focused ion beam.The results reveal that the corrosion resistance of Zr-1Nb-0.7Sn-0.03Fe-xGe (x=0.05,0.1,0.2) alloys was remarkably superior to that of Zr-1Nb-0.7Sn-0.03Fe alloy.The corrosion resistance of Zr-1Nb-0.7Sn-0.03Fe alloys is markedly improved by the addition of(0.05%- -0.2%)Ge. In addition to Zr-Nb-Fe-Cr SPPs with the tetragonal crystal structure(tet) and /3-Nb SPPs with the bcc crystal structure,the Zr-Nb-Fe-Cr-Ge SPPs with the tet structure and Zr_3Ge SPPs with the tet structure were detected out in Zr-1Nb-0.7Sn-0.03Fe-.rGe alloys.The oxidation of SPPs was found to be slower than that of Q-Zr matrix.There exist a few micro-cracks and more ZrO_2 columnar grains in the oxide film formed on Zr-1Nb-0.7Sn-0.03Fe-0.1Ge alloys corroded for 190 d.However, more micro-cracks and ZrO_2 equiaxed grains appear in the oxide film formed on Zr-1Nb-0.7Sn-0.03Fe alloys corroded for 130 d.Because the P.B.ratio of Ge is smaller than those of Zr,Nb,Fe and Cr,it is likely that the volume expansion of the oxide on Zr-1Nb-0.7Sn-0.03Fe-xGe(x=0.05,0.1,0.2) alloys is smaller than that on Zr-1Nb-0.7Sn-0.03Fe alloy,and the compressive stress can be reduced and the micro-cracks can be effectively decreased in the oxide on Zr-1Nb-0.7Sn-0.03Fe-xGe(x=0.05,0.1, 0.2) alloys.The addition of Ge can not only delay the developing process of the defects in oxide films to form micro-pores and micro-cracks,but also retard the microstructural evolution from columnar grains to equiaxed grains.Therefore,it is concluded that the addition of Ge can improve the corrosion resistance of alloy.

Optimization of N18 Zirconium Alloy for Fuel Cladding of Water Reactors

In order to optimize the microstructure and composition of N18 zirconium alloy (Zr-1Sn-0.35Nb-0.35Fe-0.1Cr, in mass fraction, %), which was developed in China in 1990s, the effect of microstructure and composition variation on the corrosion resistance of the N18 alloy has been investigated. The autoclave corrosion tests were carried out in super heated steam at 400 C/10.3 MPa, in deionized water or lithiated water with 0.01 mol/L LiOH at 360 C/18.6 MPa. The exposure time lasted for 300-550 days according to the test temperature. The results show that the microstructure with a fine and uniform distribution of second phase particles (SPPs), and the decrease of Sn content from 1% (in mass fraction, the same as follows) to 0.8% are of benefit to improving the corrosion resistance; It is detrimental to the corrosion resistance if no Cr addition. The addition of Nb content with upper limit (0.35%) is beneficial to improving the corrosion resistance. The addition of Cu less than 0.1% shows no remarkable influence upon the corrosion resistance for N18 alloy. Comparing the corrosion resistance of the optimized N18 with other commercial zirconium alloys, such as Zircaloy-4, ZIRLO, E635 and E110, the former shows superior corrosion resistance in all autoclave testing conditions mentioned above. Although the data of the corrosion resistance as fuel cladding for high burn-up has not been obtained yet, it is believed that the optimized N18 alloy is promising for the candidate of fuel cladding materials as high burn-up fuel assemblies. Based on the theory that the microstructural evolution of oxide layer during corrosion process will affect the corrosion resistance of zirconium alloys, the improvement of corrosion resistance of the N18 alloy by obtaining the microstructure with nano-size and uniform distribution of SPPs, and by decreasing the content of Sn and maintaining the content of Cr is discussed.

CORROSION BEHAVIOR OF Zr(Fex, Cr1-x)2 ALLOYS IN 400℃ SUPERHEATED STEAM

To study the corrosion behavior of second phase particles in zirconium alloys, Zr(Fex, Cr1−x)2 (x=1, 2/3, 1/3) metallic compounds which have the same composition as the second phase particles in Zr-4 alloy were prepared by vacuum non-consumable arc melting. XRD and energy filtered TEM were employed for analyzing the corrosion products, the element distribution and grain morphology after corrosion tests of Zr(Fex ,C r1−x)2 metallic compounds powder at 400 and 10.3 MPa superheated steam with different exposure times. The results show that Cr has a very strong effect on the corrosion resistance of Zr(Fex ,C r1−x)2 metallic compounds, increasing Cr content can improve the corrosion resistance. When Zr(Fex ,C r1−x)2 oxidation starts, zirconium oxide is formed while elements Fe and Cr are expelled from the zirconium oxide due to their low solid solubility in the oxide. α-Fe(Cr) and γ-Fe(Cr) are formed and then oxidized to the stable corrosion product (Fe, Cr)3O4. The different corrosion behaviors of metallic compounds will affect the microstructure evolution of zirconium oxide layer differently during the corrosion process, and hence affect the corrosion resistance of zirconium alloys.

Effect of Water Chemistry and Composition on Microstructural Evolution of Oxide on Zr Alloys

Abstract The microstructure of oxide films formed on Zircaloy-4 and Alloy No. 3, which has a composition similar to ZIRLO™, was investigated by high resolution transmission and scanning electron microscopy, and by scanning probe microscopy after corrosion tests performed at 360°C/18.6 MPa in deionized water or lithiated water with 0.01 M LiOH. The microstructural evolution of the oxide films was analyzed by comparing the microstructure at different depths in the oxide layer. The defects, consisting of vacancies and interstitials, such as points, lines, planes, and volumes, were produced during the oxide growth. Monoclinic, tetragonal, cubic, and amorphous phases were detected and their coherent relationships were identified. The characteristic of oxide with such microstructure had an internal cause, and the temperature and time were the external causes that induced the microstructural evolution during the corrosion process. The diffusion, annihilation, and condensation of vacancies and interstitials under the action of stress, temperature, and time caused stress relaxation and phase transformation. It was observed, in the middle of the oxide layer, that the vacancies absorbed by grain boundaries formed pores to weaken the bonding strength between grains. Pores formed under compressive stress lined up along the direction parallel to the compressive stress. Thus, cracks developed from the pores were parallel to the oxide/metal interface. Li+ and OH− incorporated in oxide films were adsorbed on the wall of pores or entered into vacancies to reduce the surface free energy of the zirconium oxide during exposure in lithiated water. As a result, the diffusion of vacancies and the formation of pores were enhanced, inducing the degradation of the corrosion resistance. The relationship between the corrosion resistance of zirconium alloys and the microstructural evolution of oxide films affected by water chemistry and composition is also discussed.

Tetragonal phase stability in ZrO 2 film formed on zirconium alloys and its effects on corrosion resistance

A thermodynamic model is developed to understand the origin of variation in the microstructure of ZrO 2 film formed on zirconium alloys and its effects on corrosion resistance. The correlation among the tetragonal phase fraction, the stress (macroscopic and internal one), the ZrO 2 grain size and the microstructural change of oxide film is formulized, and then analyzed. The results show that many complicated factors simultaneously govern the microstructure of oxide film. The tetragonal phase content near the oxide/metal interface, the macroscopic compressive stress near the interface, the decline gradient of macroscopic compressive stress and the internal stress induced by the transformation from the tetragonal to the monoclinic phase have very important influences on the transition from columnar grains to equiaxed grains, the crack formation and the degradation of oxidation resistance. The presence of intermetallic precipitates in oxide film may effectively relax the internal stress caused by transformation strain, stabilize the columnar-grain structure and reduce the probability of crack formation. How to reduce the transformation stress in the oxide film is a key to improve the corrosion resistance of zirconium alloys.

Effects of local stress on the stability of tetragonal phase in ZrO 2 film

An in-depth understanding of the phase stability in ZrO 2 film is very important for improving the corrosion resistance of zirconium alloys. ZrO 2 film formed on Zr–2.5Nb alloys was characterized by grazing incidence X-ray diffraction. It is shown that in comparison to the monoclinic phase crystallites, the tetragonal phase crystallites undergo larger compressive strain, indicating that there are some regions of local stress concentration in ZrO 2 film. These local stresses have an important influence on the phase stability in ZrO 2 film. In this paper, the correlation among the macroscopic compressive stress, the grain size and the local stress was analyzed qualitatively from a thermodynamic viewpoint, showing that the tetragonal phase may be partly stabilized by the local stresses. The local stresses mainly come from three sources: the local transformation strain produced by the tetragonal → monoclinic phase transformation, the oxidization of intermetallic precipitates and the different matching degrees between the ZrO 2 and Zr-based crystallites.

Potentialities of Mössbauer Spectroscopy for Studying Zirconium Alloys and Their Oxide Films

A brief review and an analysis of results of a study of iron- and tin-bearing zirconium alloys and their oxide films by the method of Mossbauer spectroscopy (MS) are presented. The potentialities of MS for studying the phase composition of zirconium alloys are described and the changes in the states of iron and tin atoms are presented as a function of additional alloying and thermomechanical treatment. The conditions of formation of Zr2Fe and Zr3Fe intermetallic compounds and chromium- and niobium-bearing compounds are considered. It is shown that some intermetallic compounds transform into other compounds at room temperature. Metallic iron and tin are shown to be present in oxide films of zirconium alloys, and their concentration is shown to affect the corrosion resistance of zirconium alloys.

Crystallization and degradation of zirconium oxide in various pH solutions

A series of modeling experiments was conducted to understand crystallization and degradation behavior of zirconium oxide occurring during corrosion of zirconium alloys. The crystallization of zirconia from amorphous Zr(OH) 4 was investigated through its hydrothermal treatment in various metal hydroxide solutions with different pH at 250°C for 6 h in mini-autoclaves. The degradation behavior of the tetragonal zirconia stabilized by 3 mol% Y 2O 3 was investigated at low temperatures of 90–200°C in water and LiOH solution with 3.5 and 350 ppm Li, respectively. It is demonstrated that pH is a main factor governing the crystallization of zirconia, leading to 35% tetragonal and 65% monoclinic zirconia at pH 7–10 and 100% monoclinic zirconia at pH > 12. Therefore, the change of the corrosion resistance of zirconium alloys with pH at the beginning of corrosion is suggested to be attributed to the crystallization behavior of the zirconia varying with pH. On the other hand, the tetragonal zirconia degraded due to the tetragonal–monoclinic transformation, which was accelerated only in LiOH solution, not in LiNO 3 or KOH. On the basis of this finding, the accelerated corrosion of zirconium alloys in LiOH solutions is discussed.