ZIRLO Alloy Research Articles

Tailored high-temperature corrosion behavior of Cr coatings using high power impulse magnetron sputtering on ZIRLO alloys for accident-tolerant fuel application

To further enhance the oxidation resistance of Zirconium alloys during Loss of Coolant Accident (LOCA), protective Cr coatings were deposited in synchronized pulse bias mode through high power impulse magnetron sputtering (HiPIMS). The results showed the weight gain of the Cr coatings with synchronized pulse bias was reduced by approximately 61.7 % compared to that of the Cr coatings without synchronized pulse bias after 90 min of steam oxidation at 1200 °C. This was mainly attributed to the formed intense (200) preferred orientation of the Cr coatings with synchronized pulse bias during deposition, accelerating the recrystallization growth of the residual Cr layer in a high-temperature steam environment, thereby reducing the rapid diffusion pathways for oxygen. Moreover, the synchronized pulse bias induced Cr grains refinement, favoring the formation of a more compact Cr2O3 scale, and therefore efficiently inhibiting the diffusion of external oxygen to the underlying alloy.

High-temperature degradation behavior of PEO-coated ZIRLO alloy in N2 and N2+steam environments at 900 and 1000 °C

A compact oxide coating of ∼10 μm thick on ZIRLO alloy was fabricated by plasma electrolytic oxidation (PEO). The morphologies, compositions, phase constituents before and after exposures in N2 and N2+steam environments at 900 and 1000 °C were characterized. The PEO coating greatly enhanced the corrosion resistance of ZIRLO alloy in the N2+steam atmosphere at 900 and 1000 °C. The kinetic curves transformed from parabolic behavior for bare alloy and exponential behavior for PEO-coated alloy at pre-transition stage to linear at post-transition stage, due to the formation of ZrN and its re-oxidation.

Effects of steam concentration and flow rate on the high temperature oxidation of PEO-coated zirconium alloy at 1000 °C and 1200 °C

A compact oxide coating of 7 μm thick on ZIRLO zirconium alloy was fabricated by plasma electrolytic oxidation (PEO) in phosphate electrolyte. The effects of steam concentration and flow rate on the steam oxidation behavior of PEO-coated ZIRLO alloy at 1000 °C and 1200 °C were evaluated by thermogravimetric analysis (TGA). The morphologies, compositions, phase constituents before and after the steam oxidation were characterized. It is found that the critical steam concentration without steam starvation for PEO-coated ZIRLO alloy at 50 ml/min flow rate is 35 % at 1000 °C. Meanwhile, the critical flow rate at 50 % steam concentration is 35 ml/min at 1000 °C and 150 ml/min at 1200 °C. A dimensionless number (P = 39) is suggested corresponding to the critical steam concentration and flow rate. It means that the minimum mass of steam injection is about 39 per unit mass gain of sample, in order to avoid the steam starvation for Zr alloys after 3600 s steam exposure. The steam oxide layers consist of monoclinic zirconia (M-ZrO2) and a little tetragonal zirconia (T-ZrO2) phase at different temperatures, steam concentrations and flow rates. The hydrogen has not diffused into the steam oxide layer and ZIRLO substrate due to the hindrance of ZrO2 coating to the hydrogen diffusion.

Analyses of electrochemical behavior of plasma electrolytic oxidation film on Zirlo alloy in lithium borate buffer solution at 25–300 °C

The electrochemical behavior of plasma electrolytic oxidation (PEO) film on Zirlo alloy in 25–300 °C lithium borate buffer solution was evaluated by in-situ electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization. As the solution temperature rose to a critical value (150 °C), a new relaxation process related to the distinguishable maximum in phase angle vs. frequency curves was observed at medium frequency and became more pronounced with increasing temperature. This relaxation process almost disappeared after a short exposure time under 300 °C. The transition of the relaxation process was closely related to the impedance of PEO film. The new relaxation process was ascribed to the diffusion of corrosion product through the PEO film. In addition, the degradation of the protective properties of PEO film was observed with increasing temperature, which resulted from the enhanced ionic conductivity of doped zirconia under high temperature. Finally, the PEO film hardly changed in morphology and microstructure after in-situ EIS tests at 25–300 °C. • Electrochemical behavior of PEO film was evaluated by in-situ electrochemical impedance spectroscopy. • A new relaxation process was observed at medium frequency. • The new relaxation process was ascribed to the diffusion of corrosion product. • The conduction mechanism of corrosion species through PEO film was analyzed based on a mix-conduction model.

Grid-to-rod fretting wear study of SiC/SiC composite accident-tolerant fuel claddings using an autoclave fretting bench test

Grid-to-rod-fretting (GTRF) in pressurized water reactors (PWRs) is known to cause wear and surface damage on the fuel claddings, potentially leading to radioactive leakage. One of the accident-tolerant fuel (ATF) concepts is to use advanced cladding materials that could withstand higher temperatures. This study investigated the wear behavior of candidate silicon carbide (SiC)-based composite claddings with different levels of surface finish in fretting against a commercial ZIRLO alloy grid using a unique bench-scale autoclave GTRF rig. The experiments mimicked the environment in an industrial full-assembly PWR simulator. Fretting tests were conducted with a realistic load (∼0.5 N) in deionized water under a pressure of 20–23 bar at 204 °C for 100 h. While the SiC/SiC composite claddings showed significantly higher wear resistance than the commercial ZIRLO alloy cladding as expected, the smoother versions experienced surprisingly higher wear than the much softer counterface, ZIRLO grid. The wear mechanism of the SiC/SiC cladding was attributed to the SiC wear debris that was trapped at the fretting interface causing both 3-body and 2-body (embedded into the grid surface) abrasion of the cladding. Rougher SiC/SiC claddings had less material loss but caused more wear on the ZIRLO grid. Pre-oxidized ZIRLO grid showed better compatibility with the SiC/SiC cladding to protect both the cladding and grid as a result of reduced wear debris trapping.

Effects of Li, B and H elements on corrosion property of oxide films on ZIRLO alloy in 300 °C/14 MPa lithium borate buffer solutions

Bare and plasma electrolytic oxidation (PEO)-coated ZIRLO alloys were exposed in 300 °C/14 MPa lithium borate solutions, then the distributions of Li, B and H elements in the oxide films were measured by glow discharge optical emission spectroscopy (GDOES). The PEO film can protect the ZIRLO alloy from the corrosion attack of Li＋. Addition of H3BO3 into the LiOH solution suppressed the diffusion of Li element into oxide film to mitigate the LiOH corrosion for ZIRLO alloy, while LiOH could also inhibit the entrance of B element. The enrichment of H near the oxide film/Zr interface was detected.

Effect of Zn(CH3COO)2 Addition on Corrosion of ZIRLO Alloy in Simulated PWR Primary Loop Medium with LiOH and H3BO3

Effect of Zn(CH3COO)2 Addition on Corrosion of ZIRLO Alloy in Simulated PWR Primary Loop Medium with LiOH and H3BO3

Analyses of hydrogen release on zirlo alloy anode during plasma electrolytic oxidation

Plasma electrolytic oxidation (PEO) is a complex process, accompanied by spark discharge and intensive gas evolution around metal anode. In this paper, the gas released from Zirlo alloy electrode during PEO process in KF solution and phosphate solution was collected and its composition was analyzed by gas chromatograph (GC). The possibility of H2 evolution from Zirlo alloy anode was evaluated and its formation mechanism was discussed. It was found that the gas released from discharging anode contained H2 and O2. The H2 up to 81.6 vol.% was detected under strong plasma discharge in phosphate solution. Furthermore, a little CO was also detected while the glycerol was added into electrolyte. Hence, a large amount of H2 could be evolved on Zirlo alloy anode during PEO process, which was different from the traditional electrochemical theory and common knowledge of PEO technology for O2 evolution on anode. Relatively less H2 was released in KF solution due to weak plasma discharge. The H2 concentration evolved during PEO process was related to the intensity of plasma discharge. It was believed that the H2 and O2 mainly resulted from the direct thermal decomposition of water vapour in plasma discharge zone, though the electrolyte was also decomposed to form H2, O2 and CO.

Tribological properties of microarc oxidation coatings on Zirlo alloy

ABSTRACTThe ceramic coatings were prepared on Zirlo alloy using Microarc Oxidation (MAO) technology. Their tribological properties were evaluated by a ball-on-disc friction and wear tester in dry sliding against GCr15 steel ball, and their wear mechanism was also discussed. Compared to bare Zirlo alloy, the MAO coatings had good wear resistance. Their lowest wear rate was about 4.59 × 10−6 mm3 N−1 m−1, which was only about 1/60 of bare Zirlo alloy. In addition, the polishing treatment of MAO coatings could reduce the wear rate and improve their wear resistance. Composition and micrograph of worn surface indicated that the wear mechanism of MAO coatings was mainly the adhesive wear while the bare Zirlo alloy showed mainly the abrasive wear.

Evaluation of SMUT Properties according to Nb Content in the Pickling Process of Nuclear Fuel Cladding Tube

Currently, the Korean nuclear industry uses ZIRLO as material for nuclear fuel cladding(zirconium alloy). KEPCO Nuclear Fuel is in the process of developing a HANA alloy to enable domestic production of cladding. Cladding manufacture involves multistage heat treatments and pickling processes, the latter of which is vital for the removal of defects and impurities on the cladding surface. SMUT that forms on the cladding surface during such pickling process is a source of surface defects during heat treatment and post-treatment processes if not removed. This study analyzes ZIRLO, HANA-4, and HANA-6 alloy claddings to extensively study the SEM/EDS, XRD, and particle size characteristics of SMUT, which are second phase particles that are formed on the cladding surface during pickling processes. Using the analysis results, this study observes SMUT formation characteristics according to Nb concentration in Zr alloys during the washing process following the pickling process. In addition, this study observes SMUT removal characteristics on cladding surfaces according to concentrations of nitric acid and hydrofluoric acid in the acid solution.

Effects of hydride precipitation on the mechanical property of cold worked zirconium alloys in fully recrystallized condition

Abstract The effects of hydrogen precipitation on the mechanical properties of Zircaloy–4 and Zirlo alloys were examined with uniaxial tensile tests at room temperature and at 400 °C and accompanying microstructural changes in the Zircaloy–4 and Zirlo alloy specimens were discussed. The elastic moduli of Zircaloy–4 and Zirlo alloys decreased with increasing hydrogen concentrations. Yield strengths of both materials tended to decrease gradually. The reductions of yield stress seems to be caused by the dissipation of yield point phenomena shown in stress-strain curves. Ultimate tensile strengths (UTS) of Zircaloy–4 and Zirlo slightly increased at low hydrogen contents, and then decreased when the concentrations exceeded 500 and 700 wppm, respectively. Uniform elongations were stable until 600 wppm and drops to 0% around 1400 wppm at room temperature.

Effect of Al2O3 coating on fretting wear performance of Zr alloy

Fretting wear caused by flow-induced vibration (FIV) is one of the predominant tribological failure modes in nuclear power plants and industry. This paper presents an experimental study of the effect on fretting performance of alumina (Al2O3) coatings produced by atomic layer deposition (ALD) on Zirlo (a Zr alloy), a common nuclear fuel cladding alloy. A coating with uniform thickness of about 750 nm was deposited onto a Zirlo flat. Because the typical thickness of fuel cladding is 1 mm, an ultra-thin coating is required. The hardness and elastic modulus of the ALD coating were measured by nano-indentation techniques. With a pin-on-flat contact configuration in low-amplitude reciprocating contact, fretting performance was measured for uncoated Zirlo, Al2O3-coated Zirlo, and Al2O3-coated Zirlo followed by heat treatment. Results showed that the as-deposited Al2O3 coating provided slight improvement in fretting wear performance. However, the heat-treated coating showed two orders of magnitude reduction in fretting wear of the Zr alloy. Wear in the uncoated material involves some local plastic deformation and debris accumulation at the contact interface. In the Al2O3-coated material, wear occurred by transfer of transparent Al2O3 oxide into the steel pin counterface. Localized removal of coating from the Zirlo alloy flat resulted in a “shark whale” pattern in a scanning electron microscopy image. There was minimal debris accumulation with the heat-treated sample.

Pre-transition Corrosion Behavior of SZA-4 and ZIRLO Alloys in Dissolved Oxygen Aqueous Condition at 360 ℃

Pre-transition Corrosion Behavior of SZA-4 and ZIRLO Alloys in Dissolved Oxygen Aqueous Condition at 360 ℃

Effect of Processing Techniques on the Mechanical Property of New Zirconium Alloy Strip

In this paper, a typical new zirconium alloy (Zr-Sn-Nb system) was used as the research object. The zirconium alloy strip was prepared from a series of production processes from smelting. By optimizing the process of cold processing with the change of the deformation amount and the intermediate heat treatment, the mechanical properties of the zirconium alloy were studied by different processing. The results are as follows: (1) The mechanical properties of the new zirconium alloy have reached the mechanical properties of Zirlo alloy with the optimization of the alloy composition, and (2) The zirconium alloy can refine the grain strength by the control of deformation degree and annealing temperature during cold deformation. The larger of the cold deformation, the smaller the average of the grains and the higher the yield strength of the material. The yield strength of the material was decreased by cold rolling and annealing more than once; (3) The larger the cold rolling variable of the zirconium alloy in cold deformation, it is beneficial to the second phase to be fine, dispersed and uniformly distributed, thus improving the strength of the material.

Zeta potential of microarc oxidation film on zirlo alloy in different aqueous solutions

Zeta potentials of the bare Zirlo alloy and its microarc oxidation (MAO) film in KCl, NaCl, H3BO3 and LiOH solutions were measured. Their corrosion behavior in different solutions were evaluated by electrochemical impedance spectroscopy (EIS). The correlation between zeta potential and corrosion behavior was also analyzed. The MAO treatment shifted the iso-electric point (IEP) to a higher pH value in 1500 ppm B3+ and 2.3 ppm Li+ mixed aqueous solution for simulating normal water chemistry in nuclear reactors. Besides, MAO film with higher zeta potential could reduce the adsorption of lithium ions, which improved the corrosion resistance of Zirlo alloy.

The effects of anion deposition and negative pulse on the behaviours of plasma electrolytic oxidation (PEO)—A systematic study of the PEO of a Zirlo alloy in aluminate electrolytes

Plasma electrolytic oxidation (PEO) of a Zirlo alloy has been carried out under pulsed bipolar and pulsed unipolar regimes in aluminate electrolytes with the aim to promote a better understanding of the coating formation mechanisms and the role of negative pulse. Real time imaging, optical emission spectroscopy (OES), gas collection, scanning electron microscopy (SEM) assisted with energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD) have been used to characterize the PEO processes and the resultant coatings. The results show that the anion deposition process has significantly affected the composition, microstructure of the coatings and the plasma discharge behaviour. For the coatings formed with less incorporation from the electrolyte anions, the predominant discharge type is the energetic penetrating discharges, usually forming the pancake structure and big internal pores. However, the formation of coatings by the fast deposition of anion species with the assistance of “soft sparking” or “sintering arcs” occurs when PEO was carried out in concentrated electrolytes. The occurrence of such discharges is correlated with the fact that less energy is required for the anion deposition process. The hydrogen evolution associated with the negative pulse during PEO can promoted the transportation of electrolyte species to the electrode surface, favoring the formation of homogenous coatings with the incorporation of more electrolyte species and a modified discharge types.

Formation and fine-structures of nano-precipitates in ZIRLO

Transmission electron microscopy and energy dispersive X-ray spectroscopy have been used for crystallographic and compositional characterization of nano-precipitates distributed within the α-Zr matrix of the ZIRLO alloy. Intermetallic nano-particles with the C14 Laves-phase structure were found to be the dominant precipitate phase typically larger than 60 nm in sizes, which is assigned to the Zr(Fe,Nb)2 formula with Fe and Nb being mixed on the second sublattice sites in the bracket. Stacking faults were also observed in the ternary Laves phase, together with dislocations presenting on prism planes of the matrix solid solution. Besides, some fine spherical β-Nb particles about 30 nm in size were also found to precipitate within the α-Zr grains, with the α/β interface being crystallographically coherent.

원자력산업에서 지르코늄 스크랩 재활용을 위한 세정기술에 관한 연구

본 연구에서는 지르코늄 피복관 제조공정에서 발생되는 스크랩을 원전급(nuclear grade)으로 재활용하기 위해 스크랩 표면에 부착되어 있는 오염물 제거조건을 최적화하였다. 주 오염물은 피복관 제조시 필거링 공정에서 사용하고 있는 수용성 냉각윤활제 잔류물로서 튜브 표면에 압착 및 탄화된 것으로 가정된다. 스크랩 발생 빈도가 높은 <TEX>${\phi}9.50mm$</TEX>, zirlo 합금 튜브를 피 세정 대상물로 선정하여 세정 후 피 세정물 표면에 잔존하고 있는 오염물의 특성분석과 피 세정물의 표면 성분분석으로 세정성을 평가하였다. 세정제별 세정능력을 평가하기 위하여 수산화나트륨(sodium hydroxide) 계열 2종과 수산화칼륨(potassium hydroxide) 계열 3종을 선정하여 비교하였다. 또한 온도 및 초음파 강도에 따른 세정 효과 분석을 위해 상온, <TEX>$40^{\circ}C$</TEX>, <TEX>$60^{\circ}C$</TEX>에서 각각 세정한 결과, 세정온도 및 초음파 강도가 높을수록 세정효과도 높은 것으로 나타났다. 육안검사 결과 수산화나트륨 계열은 초음파 강도와 무관하게 모두 양호한 것으로 나타났으나 수산화칼륨 계열은 초음파 강도 120 W 이상에서 피 세정물의 표면상태가 양호한 것으로 나타났다. 중량측정법에 의한 세정효과 분석결과 수산화나트륨 계열은 세정효율이 97.6% (<TEX>$60^{\circ}C$</TEX>, 120 W)까지 나타났으나 수산화칼륨 계열은 피 세정물의 표면상태 불량으로 중량측정 방법을 적용하는 것이 부적합한 것으로 나타났다. 피 세정물의 표면 오염물 분석 결과 C, O, Ca, Zr 성분이 검출되었으며 그 중 C, O의 성분이 대부분을 차지하였음을 알 수 있었다. 피 세정물의 세정 정도에 따라 C, O 구성 비율의 변화가 큰 것으로 나타났으며 세정이 잘될수록 C의 구성비율이 감소되며 상대적으로 O의 구성 비율이 증가되었다. 본 연구 결과를 바탕으로 산업현장에 적용하기 위하여 세정공정을 알카리세정, 수세, 건조의 3단계로 구분하고 각 단계별로 세정변수를 조정함으로써 세정효과의 극대화를 기대할 수 있다. In this study, we optimized the removal condition of contaminants attached on the scrap surface to recycle the scrap generated from the Zr alloy tube manufacturing process back to the nuclear grade. The main contaminant is remnant of watersoluble cooling lubricant that is used in the pilgering manufacture during the tube production, and it is assumed to be compressed and carbonized on the surface of tube. Zirlo alloy tube of <TEX>${\phi}9.50mm$</TEX>, which has high occurrence frequency of scrap, was selected as the object to be cleaned, and cleaning abilities of reagents were evaluated by measuring the characteristics of contaminants remained and by analyzing the surface of the tube after cleaning process. For evaluation of each cleaning agent, we selected two types of sodium hydroxide series and three types of potassium hydroxide series. Furthermore, to confirm dependence on tempe-rature and ultrasonic intensities, cleaning at the room temperature, <TEX>$40^{\circ}C$</TEX>, and <TEX>$60^{\circ}C$</TEX> was conducted, and results showed that higher the cleaning temperature and higher the ultrasonic intensity, better the cleaning effect. As a result of the bare-eye inspection, while the use of sodium hydroxide provided satisfactory condition on the tube surface, the use of potassium hydroxide series provided satisfactory condition on the tube surface only when the ultrasonic intensity was over 120 W. In the cleaning effect analysis using the gravimetric method, cleaning efficiency of sodium hydroxide series was as high as 97.6% (<TEX>$60^{\circ}C$</TEX>, 120 W), but since the tube surface condition was poor after the use of potassium hydroxide, the gravimetric method was not appropriate. In the analytical result of surface contaminants on the tube surface, C, O, Ca, and Zr were detected, and mainly C and O dominated the proportion of contaminants. It was also found that the degree of cleaning on the tube affected the componential ratio of C and O; if the degree of cleaning is high, or if cleaning is well-conducted, the proportion of C is decreased, and the proportion of O is increased. Based on these results, optimal cleaning for application in the industry can be expected by categorizing cleaning process into three steps of Alkali cleaning, Rinsing, and Drying and by adjusting cleaning parameters in each step.

Hydrogen Diffusivity and Hydride Formation in Rich-Zirconium Alloys Used in Nuclear Reactors

Hydrogen gas permeation tests were performed on two Nb-modified Zr alloys used in the nuclear industry. The influence of the microstructure on hydrogen diffusivity in each alloy is discussed and a mechanism of hydride formation is presented. The hydrogen binding energy for different trap sites was calculated in the M5 alloy as well as hydrogen diffusivity value in the Zirlo alloy at 300oC, D = 2.5 x 10-13 m2/s.

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.