Hydride Rim Formation in E110 Zirconium Alloy During Gas-Phase Hydrogenation

Article Hydride Rim Formation in E110 Zirconium Alloy during Gas-Phase Hydrogenation

Viktor Kudiiarov * , Ivan Sakvin , Maxim Syrtanov, Inga Slesarenko and Andrey Lider National Research Tomsk Polytechnic University, Tomsk 634050, Russia; [email protected] (I.S.); [email protected] (M.S.); [email protected] (I.S.); [email protected] (A.L.) * Correspondence: [email protected]; Tel.: +7-(913)870-0989

Received: 31 December 2019; Accepted: 10 February 2020; Published: 12 February 2020

Abstract: The work is devoted to the study of the laws of the formation of a hydride rim in E110 zirconium alloy claddings during gas-phase hydrogenation. The problem of hydrogen penetration and accumulation and the subsequent formation of hydrides in the volume of zirconium cladding tubes of water-cooled power reactors remain relevant. The formation of brittle hydrides in a zirconium matrix firstly, leads to a significant change in the mechanical properties, and secondly, can cause the destruction of the claddings by the mechanism of delayed hydride cracking. The degree of the hydride’s effect on the mechanical properties of zirconium cladding is mainly determined by the features of the hydride’s distribution and orientation. The problem of hydride rim formation in zirconium alloys with niobium is quite new and poorly studied. Therefore, the study of hydride rim formation in Russian zirconium alloy is important and necessary for predicting the behavior of claddings during the formation of the hydride rim.

Keywords: hydride rim; E110 zirconium alloy; cladding; gas-phase hydrogenation

1. Introduction Zirconium alloys are widely used in nuclear reactors [1–5], as zirconium alloys possess a low thermal neutron capture cross-section; they are characterized by corrosion resistance, good strength properties, and resistance to radiation damage. Fuel cladding, grid assembly, and fuel channels are produced from zirconium alloys in reactors. The E110 (Zr1%Nb) alloy is used in Russia’s reactors to produce fuel cladding, while Zircaloy-2 (Zr1.5%Sn0.12%Fe0.1%Cr0.05%Ni0.13%O) and Zircaloy-4 (Zr1.55%Sn0.22%Fe0.12%Cr0.12%O0.015%C0.01%Si) alloys have been used internationally for these purposes since the middle of the previous century. Among one of the main requirements of zirconium fuel cladding is low hydrogen absorption capacity, since, under certain conditions, absorbed hydrogen causes embrittlement and consequent destruction in compliance with the mechanism of delayed hydride cracking, which may result in a loss of sealing [6–10]. Hydrogen solubility in zirconium alloys at room temperature does not exceed 1 10 5 wt.% while at an operational temperature (~350 C). This value is in the range of 2 10 2 wt.%. × − ◦ × − With the exceedance of the hydrogen solubility limit in zirconium alloys, the formation of hydride phases takes place; the latter causes the greatest embrittlement, as hydrides possess significantly lower plasticity in comparison with zirconium. Furthermore, hydrides in the zirconium matrix can serve as the locations for crack initiation, followed by a subsequent crack opening [11–13] and the formation of through-the-thickness damage. The effect of hydrogen on the mechanical properties of zirconium alloys is mainly determined by its amount, as well as by the hydride’s orientation and localization. Hydrides located in the bulk of zirconium cladding have a minimal effect on the mechanical properties of zirconium cladding. However, frequently, fuel cladding hydrogenation occurs nonuniformly, and local clusters of hydrides

Metals 2020, 10, 247; doi:10.3390/met10020247 www.mdpi.com/journal/metals Metals 2020, 10, 247 2 of 10 take place on the outer surface of the cladding. The formation of hydrides on the outer surface of the cladding leads to the degradation and destruction of the mechanical properties by a delayed hydride cracking mechanism [7–9]. During the operation in water-cooled power reactors and pressurized water reactors (PWR) in zirconium fuel claddings, the gradient of hydrogen concentration through the cladding wall thickness and hydride initiation is formed, accompanied by hydride rim formation. Formation of the hydride layer with the thickness of 50–100 µm takes place on the outer surface of the fuel cladding; the hydride layer thickness depends on the degree of hydrogenation [14–19]. The degree of the zirconium alloy’s hydrogenation significantly depends on the reactor’s operational state or the parameters of hydrogenation (in the laboratory settings). One of the most commonly used methods for zirconium alloy’s saturation with hydrogen is the gas-phase hydrogenation method. This method’s key parameter, which effects the hydrogen’s interaction behavior with the materials, is the temperature. The other parameters’ values remain unchanged; the hydrogenation temperature value is the determinant in hydride rim formation. The works [18,19] demonstrated that during hydrogenation—under the temperature lower than determined—the threshold temperature in the formation of the hydride rim in fuel cladding from Zircaloy-2 and Zircaloy-4 alloys takes place. During hydrogenation at the temperatures higher than the threshold temperature, the formation of a hydride rim does not take place. In this respect, there is a necessity to determine the threshold temperature of gas-phase hydrogenation that enables the formation of the hydride rim in E110 alloy fuel claddings developed for the Russian water-cooled nuclear reactors. Another factor that affects the degree of the zirconium alloy’s hydrogenation is their surface state—namely, oxide film [20,21]. Zirconium alloy surfaces, coated with homogeneous thin oxide film, absorb hydrogen weakly, even at high temperatures [22]. On the one hand, the presence of such oxide film plays a positive role in the operating conditions, as it reduces hydrogen permeability. On the other hand, there is a necessity to prepare the experimental samples of zirconium alloys with different hydrogen concentrations and distributions for prospective research purposes (e.g., to conduct mechanical testing). In experimental samples, preparation of the oxide film will hinder hydrogenation and make it impossible in a number of cases where the temperature volume is limited (e.g., when preliminary thermal treatment of the material takes place). Thus, it becomes relevant to investigate the methods of increasing hydrogen permeability in zirconium alloys. The technology of ion-beam cleaning of the alloy’s surface with subsequent nickel coating can be applied to increase hydrogen permeability of zirconium alloys. Ion-beam cleaning allows the removal of oxide film, while nickel coating performed immediately after cleaning prevents rapid oxide film formation. Furthermore, nickel is an alloying element that facilitates hydrogen absorption due to the suppression of hydrogen atomic recombination in molecules [23,24]. Zirconium alloy surface cleaning will lead to an increase of the hydrogen absorption rate; in turn, the latter will cause changes in the values of the threshold temperature of hydride rim formation. Due to this, the goal of the present research is to establish behavior and determine the conditions for hydride rim formation in the E110 alloy cladding tubes during gas-phase hydrogenation. The formation of the hydride rim in the Russian zirconium alloy E110 has not been previously studied. The relevance of the work is primarily due to the need to develop a technique that will allow, under controlled conditions, the preparation of samples of E110 zirconium alloy with a hydride rim for further mechanical tests.

2. Materials and Research Methods The test material is the cladding tubes Ø 9.5 8.33 mm (batch # 5080-12-02/4-4) made × from an E110 (Zr1%Nb) zirconium alloy based on sponge and produced in compliance with TC 001.411-2009 (cold-rolled seamless tubes for fuel claddings for PWR reactors); the cladding tubes were provided by A.A. Bochvar High-technology Scientiﬁc Research Institute of Inorganic Materials in the delivery condition. Metals 2020, 10, 247 3 of 10

The studies of microstructure were performed using an Olympus GX 51 optical microscope (Tokyo, Japan). The studies of microstructure were performed with a light field test and demonstrated the uniformly distributed inclusions of the second phases in the alloy matrix, without clusters, and having an oval shape. The studies performed in the polarized light demonstrated that, when subjected to finish annealing, all the tubes possess a recrystallized structure with an equiaxed grain shape. The average grain size is within the range of 2.8–3.7 µm. The minimum grain size is 0.6–1.4 µm, while the maximum grain size, taking into consideration their conglomeration due to the close proximity of neighboring grains’ polarization, comprises 8.1–10.2 µm. This part of the research was performed on the large-scale research facility IRT-T research reactor (Tomsk, Russia). Gas-phase hydrogenation was performed on an automated complex gas reaction controller (Advanced Materials Research, Pittsburgh, PA, USA). The hydrogenation temperature varied from 300 ◦C to 550 ◦C, the hydrogen pressure was constant and comprised 2 atm., the heating rate comprised 6 ◦C/min, the cooling rate was ~2 ◦C/min, and the hydrogenation time varied from several minutes to several hours. Hydrogen concentration measurements were performed on the hydrogen analyzer RHEN602 (LECO, St. Joseph, MI, USA). The studies performed using all the aforementioned equipment were conducted on the premises of Tomsk Polytechnic University. Nanohardness tests were carried out using the Agilent Technologies G200 (Santa Clara, CA, USA) using the Instrumented Testing method (instrumental determination of hardness by indenter indentation), which involves measuring the indenter penetration depth during loading and unloading. The set load during the tests was 8 mN. Under this load, it is possible to measure the mechanical characteristics of hydrides up to 5 µm in size without fear of incorrect results. The Berkovich diamond pyramid with an angle at the apex of 65.27◦ was used as an indenter. The indentation hardness Hit (nanohardness) was determined by the Oliver–Farr method as the ratio of the maximum indentation load to the projection area of the indenter contact with the material, with the exception of deflection at the edge of the print.

3. Results and Discussion

3.1. The Effect of the Gas-Phase Hydrogenation Temperature on the Value of the Threshold Temperature of Hydride Rim Formation Below, Figure1 presents the results of the metallographic examination using the optical microscope performed for the hydrogenated samples of the E110 alloy cladding tubes at the temperatures 350 ◦C, 380 ◦C, and 420 ◦C, with the pressure in the chamber 2 atm. up to a concentration of 0.1 wt.%. It has been established that the hydrogenation temperature effects hydride rim formation. Rim formation takes place at the temperature 320 ◦C, while, at the temperature 380 ◦C, the hydride rim is practically not observed, and at the temperature 420 ◦C, the hydride rim is not present. This is explained by the following: hydride rim formation takes place in the cases where the hydrogen solubility in the thin layer of zirconium alloy during hydrogenation is exceeded and the hydrogen absorption rate is higher than the hydrogen diffusion rate in relation to the observed thin layer. After hydrides have been formed in the thin layer, hydrogen diffusion penetrates through this layer further to the alloy volume, where hydrides are also being formed. Metals 2020, 10, 247 4 of 10 Metals 2020, 10, x FOR PEER REVIEW 4 of 10

Figure 1. 1. E110E110 zirconium zirconium alloy alloy transverse transverse metallographic metallographic section section after after hydrogenation at the

temperatures 350 ◦ °CC( (a),), 380 380 ◦ °CC( (bb),), andand 420 420 ◦ °CC( (cc),), andand the the pressure pressure 2 2 atm. atm. up to hydrogen concentration 0.1 wt.%.wt.%.

Table1 1represents represents the the results results of the of X-raythe X di-rayﬀraction diffraction analysis analysis of the E110 of the zirconium E110 zirconium alloy cladding alloy tubecladding samples tube aftersamples hydrogenation after hydrogenation under diﬀ undererent temperatures.different temperatures.

Table 1.1. The results of X-rayX-ray didiffractionﬀraction analysisanalysis ofof thethe samples.samples.

HydrogenationHydrogenation Phase PhaseContent Content,, Observed PhasesPhases Lattice ParametersLattice Parameters TemperatureTemperature VolumeVolume.%.% Zirconium hydridehydride 55 55a = 4.7776 a = 4.7776 350 ◦C350 °C Zirconium 45 45a = 3.2299; ac= = 3.2299;5.1380 c = 5.1380 Zirconium hydridehydride 19 19a = 4.7656 a = 4.7656 380 ◦C380 °C Zirconium 81 81a = 3.2294; ac= = 3.2294;5.1398 c = 5.1398 Zirconium hydridehydride 5 5a = 4.7632 a = 4.7632 420 ◦C420 °C Zirconium 95 95a = 3.2243; ac= = 3.2243;5.1401 c = 5.1401

The results results of of the the X-ray X-ray diﬀ diffractionraction analysis analysis reveal reveal that, with that, a with hydrogenation a hydrogenation temperature temperature increase, thereincrease is a, decreasethere is a in decrease the hydride in the phase hydride volume phase in zirconium volume alloy, in zirconium which is alloy in good, which agreement is in good with theagreement results ofwith the the metallographic results of the examination. metallographic A hydrogenationexamination. A temperature hydrogenation increase temperature leads to increase hydride formationleads to hydride in the formation material bulk in the and, material correspondingly, bulk and, correspondingly, the inclusion volume the inclusion of the hydrides volume inof thethe near-surfacehydrides in the layers near decreases.-surface layers decreases.

3.2. The Effect Effect of Hydrogen Concentration on the Hydride Rim Thickness Below,Below, FigureFigure2 presents 2 presents the resultsthe results of the metallographic of the metallographic examination examination using the optical using microscope the optical performedmicroscope for performed the hydrogenated for the sampleshydrogenated of the E110 samples alloy of cladding the E110 tubes alloy at the cladding temperature tubes 350 at the◦C, thetemperature pressure in350 the °C, chamber the pressure 2 atm., in andthe chamber up to concentrations 2 atm., and up 0.1–0.25–0.4 to concentrations wt.%. 0.1–0.25–0.4 wt.%. It has has been been established established that that hydride hydride rim rim thickness thickness depends depends on on hydrogen hydrogen concentration. concentration. An increaseincrease in concentration from 0.1 wt.%wt.% to 0.4 wt.%wt.% leads to an increase in thethe hydridehydride rim thickness from 9090 µµmm toto 180180µ µm.m. TableTable2 2represents represents the the results results of of the the X-ray X-ray di diffractionffraction analysis analysis of of the the samples samples of theof the E110 E110 zirconium zirconium alloy alloy cladding cladding tubes tubes after after hydrogenation hydrogenation to ditoff differenterent concentrations. concentrations.

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FigureFigure 2. E110 2. zirconiumE110 zirconium alloy transverse alloy transverse metallographic metallographic section section after hydrogenation after hydrogenation at the temperature at the temperature 350 °C and the pressure 2 atm. up to concentrations 0.1 (a) – 0.25 (b) – 0.4 (c) wt.%. 350 ◦C and the pressure 2 atm. up to concentrations 0.1 (a) – 0.25 (b) – 0.4 (c) wt.%. Table 2. The results of X-ray diffraction analysis of the samples. Table 2. The results of X-ray diffraction analysis of the samples. Hydrogen Concentration Observed Phases Phase Content, Volume.% Lattice Parameters Hydrogen Zirconium hydride Phase55 Content, a = 4.7776 0.1 wt.% Observed Phases Lattice Parameters Concentration Zirconium Volume.%45 a = 3.2299; c = 5.1380 ZirconiumZirconium hydride hydride 27 55 a = a4.7668= 4.7776 0.1 wt.%0.25 wt.% ZirconiumZirconium 73 45 a = a3.2294;= 3.2299; c = 5.1398 c = 5.1380 Zirconium hydride 83 a = 4.7768 0.4 wt.% Zirconium hydride 27 a = 4.7668 0.25 wt.% Zirconium 17 a = 3.2300; c = 5.1450 Zirconium 73 a = 3.2294; c = 5.1398 Zirconium hydride 83 a = 4.7768 0.4The wt.% results of the X-ray diffraction analysis reveal that, with a hydrogen concentration increase at the same hydrogenation temperatureZirconium, the increase of the hydride 17 phase volume a =in3.2300; the zirconium c = 5.1450 alloy takes place, which is in good agreement with the results of the metallographic examination. The Thehydrogen results concentration of the X-ray increase diffractions at analysisthe same reveal temperature that, with of hydrogenation a hydrogen concentration, which leads to increase a at thehydride same hydrogenationrim thickness increase temperature, [25]. the increase of the hydride phase volume in the zirconium alloy takesThus, place, gas- whichphase hydrogenationis in good agreement of the E110 with zirconium the results alloy of cladding the metallographic tubes in the examination. delivery condition under the constant pressure of 2 atm. within the temperature range (400–550) °C and The hydrogen concentration increases at the same temperature of hydrogenation, which leads to a subsequent slow cooling leads to the formation of hydrides formed in the material bulk. hydride rim thickness increase [25]. Hydrogenation at the temperatures that are lower than the threshold temperature (400 ± 20) °C is Thus,accompanied gas-phase by hydride hydrogenation rim formation of the in E110 the cladding zirconium tubes; alloy the cladding hydride tubesrim th inickness the delivery is within condition the underrange the (90 constant–180) µm pressure, with hydrogen of 2 atm. concent withinrations the within temperature the range range (0.1–0.4 (400–550) wt.%). ◦C and subsequent slow cooling leads to the formation of hydrides formed in the material bulk. Hydrogenation at the temperatures3.3. The Effect that of are the lowerGas-Phase than Hydrogenation the threshold Temperature temperature on the (400 Value of20) the CThresho is accompaniedld Temperature by of hydride ± ◦ rim formationHydride Rim in Formation the cladding in E110 tubes; Zirconium the hydride Alloy Subjected rim thickness to Ion-Beam is within Cleaning the and range Nickel (90–180) Coating µm, with hydrogenBelow concentrations, Figure 3 withinpresents the the range results (0.1–0.4 of the wt.%). metallographic examination using the optical microscope performed for the hydrogenated samples of the E110 alloy cladding tubes subjected to 3.3. Theion- Ebeamffect of cleaning the Gas-Phase and nickel Hydrogenation coating (the Temperature method of on ion the-beam Value cleaning of the Threshold and nickel Temperature coating is of Hydridedescribed Rim Formation in [23]) at in the E110 temperatures Zirconium 350 Alloy °C, 380 Subjected °C, 420 to °C, Ion-Beam 500 °C, and Cleaning 550 °C and. The Nickel pressure Coating in the Below,chamber Figure is 2 atm.3 presents and up theto the results concentration of the metallographic of 0.1 wt.%. examination using the optical microscope performed for the hydrogenated samples of the E110 alloy cladding tubes subjected to ion-beam cleaning and nickel coating (the method of ion-beam cleaning and nickel coating is described in [23]) at the temperatures 350 ◦C, 380 ◦C, 420 ◦C, 500 ◦C, and 550 ◦C. The pressure in the chamber is 2 atm. and up to the concentration of 0.1 wt.%. Metals 2020, 10, 247 6 of 10

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FigureFigure 3. E110 3. E110 zirconium zirconium alloy alloy transverse transverse metallographic section section subjected subjected to ion to- ion-beambeam cleaning cleaning and and nickel coating after hydrogenation at the temperatures 350 °C (a), 400 °C (b), 450 °C (c), 500 °C (d), nickel coating after hydrogenation at the temperatures 350 ◦C(a), 400 ◦C(b), 450 ◦C(c), 500 ◦C(d), and and 550 °C (e), and the pressure 2 atm. up to hydrogen concentration 0.1 wt.%. 550 ◦C(e), and the pressure 2 atm. up to hydrogen concentration 0.1 wt.%. It has been established that hydride rim formation in the E110 zirconium alloy subjected to ion- It has been established that hydride rim formation in the E110 zirconium alloy subjected to beam cleaning and nickel coating takes place at the temperatures 350 °C (a), 400 °C (b), and 450 °C ion-beam(c). At cleaning the temperature and nickel 500 coating°C, the hydride takes place rim atis practically the temperatures not observed 350 ◦,C and (a), at 400 the◦ Ctemperature (b), and 450 ◦C (c). At550 the °C, temperaturethe gradient is 500 not ◦present.C, the hydride rim is practically not observed, and at the temperature 550 ◦C, theThus, gradient argon ion is not-beam present. cleaning (under the voltage 2000 V, the power 1000 W, the current 0.5 A, andThus, the argon pressure ion-beam 6 × 10 cleaning−2 Pa during (under 5 min the) voltage and subsequent 2000 V, the nickel power coating 1000 using W, the the current magnetron 0.5 A, and the pressuresputtering 6 method10 2 Pa (under during the 5voltage min) and500 V, subsequent the power 2000 nickel W, coating the current using 3 A the, and magnetron the pressure sputtering 1 × × − method10−1 (underPa) with the the voltage thickness 500 ~1 V, µm the on power the E110 2000 zirconium W, the current alloy cladding 3 A, and tubes the pressureleads to an 1 increase10 1 Pa) in with × − the thicknessthe threshold ~1 µ m temperature on the E110 value zirconium of hydride alloy rim cladding formation tubes in ~100 leads °C to, anwhich increase is related in the to threshold the significant increase of the hydrogen absorption rate. It is worth noting that the observed features of temperature value of hydride rim formation in ~100 ◦C, which is related to the signiﬁcant increase of the hydride distribution behavior throughout the zirconium cladding volume correlate to the data the hydrogen absorption rate. It is worth noting that the observed features of the hydride distribution available from the results of the analysis of hydride distribution in the cladding after the operation behavior[26]. throughout the zirconium cladding volume correlate to the data available from the results of the analysis of hydride distribution in the cladding after the operation [26]. 3.4. Studies of the Properties of the Hydride Rim Formed during Gas-Phase Hydrogenation in E110 3.4. StudiesZirconium of theAlloy Properties of the Hydride Rim Formed during Gas-Phase Hydrogenation in E110 Zirconium Alloy Below, Figure 4 presents the results of the studies of element distribution throughout the thicknessBelow, Figure of the4 presentsE110 zirconium the results alloy of with the the studies formed of element hydride distributionrim of the thickness throughout 110 µm. the thicknessThe of theelement E110 zirconium signal strength alloy iswith presented the formed in the hydride relative rim unit of values the thickness, since, to 110 beµ ablem. The to determine element signal strengthhydrogen is presented concentration in the relative using this unit method values,, it since, is necessary to be able to to prepare determine the reference hydrogen samples concentration of usinghydrogen this method, and itoxygen is necessary in zirconium to prepare alloys the. However, reference samples based onof the hydrogen correlation and of oxygen the signals in zirconium of alloys.different However, elements based, it onis possible the correlation to judge ofthe the correlation signals ofof ditheseﬀerent elements elements,’ quantit it isies possible in the samples. to judge the correlation of these elements’ quantities in the samples. Metals 2020, 10, x FOR PEER REVIEW 7 of 10

MetalsAs2020 the, 10 ,studies 247 have revealed, there is a layer up to ~110 µm with high hydrogen concentration7 of 10 in the sample. At that, with the depth growth, the zirconium signal strength increases, which evidences the decrease of hydrogen concentration and the presence of the gradient of hydrogen distributionAs the studies right in have the revealed,hydride layer. there isThis a layer can upbe toexplained ~110 µm by with the high following: hydrogen hydride concentration formation in thetakes sample. place immediately At that, with after the depth hydrogen growth, penetration the zirconium in a thin signal layer strength of zirconium. increases, Further which penetration evidences Metals 2020, 10, x FOR PEER REVIEW 7 of 10 theof hydrogen decrease of leads hydrogen to an increase concentration in the and thickness the presence of the of hydride the gradient layer directly of hydrogen in the distribution process of righthydrogenation. in the hydrideAs Thisthe studies layer.situatio have Thisn revealed,is canobserved be there explained isat a alayer high up by hydrogento the ~110 following: µm with penetration high hydride hydrogen rate formation concentration and a low takes diffusion place immediatelyrate. In ourin the after experiment, sample. hydrogen At that, this penetration with was the achieved depth in a growth thin by layer, the cleaning of zirconium zirconium. the signal surface, Further strength high penetration increases pressure, which of, hydrogenand low leads to anevidences increase the in decrease the thickness of hydrogen of the concentration hydride layer and directlythe presence in the of the process gradient of hydrogenation.of hydrogen This temperature.distribution Thus, ifright the in rate the hydrideof the layer.outflow This ofcan hydrogen be explained from by the a following:thin surface hydride layer formation of zirconiu m is situationgreater than istakes observed the place rate immediately atof athe high penetration after hydrogen hydrogen of penetration penetrationhydrogen in rateinto a thin andthe layer amaterial,low of zirconium. diff usionthen Further the rate. hydride penetration In our rim experiment, will not thisform was during achievedof hydrogen hydrogenation. by cleaningleads to an the In increase our surface, experiment, in the high thickness pressure, this of the was and hydride achieved low layer temperature. bydirectly increasing in Thus, the process the if the temperature of rate of the outflow(Figuresof 1chydrogenation. hydrogen and 3e). In from Thisthea situatiocase thin of surfacen isthe observed formation layer at ofa highzirconium of thhydrogene hydride is penetration greater rim directly than rate theand duringa rate low ofdiffusion thehydrogenation, penetration of hydrogenrate. into In our the experiment, material, thenthis was the achieved hydride by rim cleaning will notthe surface, form during high pressure hydrogenation., and low In our the hydrogentemperature. distribution Thus, ifgradient the rate of over the outflow the depth of hydrogen of the layerfrom a is thin observed surface layer due of to zirconiu the lowerm is diffusion experiment,rate of hydrogengreater this than wasin zirconiumthe achieved rate of the hydride by penetration increasing compared of hydrogen the temperature to intopure the zirconium. material, (Figures then 1thec andhydride3e). rim In will the not case of the formationform of the during hydride hydrogenation. rim directly In our during experiment, hydrogenation, this was achieved the hydrogen by increasing distribution the temperature gradient over the depth of(Figures the layer 1c and is 3e). observed In the case due of the to formation the lower of ditheff usionhydride rate rim directly of hydrogen during hydrogenation, in zirconium hydride comparedthe to purehydrogen zirconium. distribution gradient over the depth of the layer is observed due to the lower diffusion rate of hydrogen in zirconium hydride compared to pure zirconium.

FigureFigure 4.4. DistributionFigure 4. Distribution of elements of elements through through the the thicknessthickness of ofE110 E110E110 zirconium zirconiumzirconium alloy with alloyalloy the withwith formed thethe formedformed hydride rim of the thickness 110 µm at the temperature 350 °C. hydridehydride rimrim ofof thethe thicknessthickness 110110 µµmm atat thethe temperaturetemperature 350350◦ °C.C. This fact has been confirmed by the studies of the sample’s nanohardness distribution in the ThisThis factfacttransverse hashas beenbeenmetallographic conﬁrmedconfirmed section. byby The thethe measurement studiesstudies ofof thetheresults sample’ssample for the’ ssample nanohardnessnanohardness’s microhardness distributiondistribution before inin thethe transversetransverseand metallographicmetallographic after hydrogenation section.section. are presented The measurement in Figure 5. Theresults hydride for the layerthe sample’ssample is characterized’s microhardnessmicrohardness by high beforebefore hardness in comparison to the material volume. andand after after hydrogenation hydrogenation are are presented presented in Figure in Figure5. The 5. hydride The hydride layer is layer characterized is characterized by high hardness by high inhardness comparison in comparison to the material to the volume. material volume.

a) b)

Figure 5. The measurement results of E110 zirconium alloy microhardness before (a) and after (b) hydrogenation, with the hydride rim formed with the thickness of 110 µm.

FigureFigure 5.5. The measurement results of E110 zirconium alloy microhardness before (a) andand afterafter ((b)) hydrogenation,hydrogenation, withwith thethe hydridehydride rimrim formedformed withwith thethe thicknessthickness ofof110 110µ µmm..

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The volumevolume ofof thethe zirconium zirconium alloy alloy is is characterized characterized by by the the uniform uniform distribution distribution of hardnessof hardness in the in thickness.the thickness. After After hydrogenation hydrogenatio at then at temperature the temperature lower than lower the than threshold the threshold temperature, temperature, the hardness the signiﬁcantlyhardness significantly increases upincreases to the depthup to the of 100 depthµm, of which 100 µm, can which be explained can be byexplained hydride by rim hydride formation. rim Theformation. results of The the results studies of of the the studies sample’s of nanohardness the sample’s nanohardnessdistribution in distributionthe transverse in metallographic the transverse sectionmetallographic are presented section in are Figure presented6. in Figure 6.

Figure 6.6. TheThe measurementmeasurement results results of of nanohardness nanohardness of of E110 E110 zirconium zirconium alloy alloy with with hydride hydride rim 110rim µ110m. µm. As the studies have revealed, hardness in the hydride rim decreases from the outer layer into the materialAs the volume, studies which have evidences revealed, the hardness presence in of the the hydride hydrogen rim concentration decreases from gradient the outer in the layer hydride into rim,the material which, in volume turn, confirms, which theevidences suggested the mechanism presence of of the hydride hydrogen rim formation. concentration In the gradient hydride rim,in the in regionshydride close rim,to which, the surface, in turn, hydrogen confirms concentration the suggested is very mechanism high, and of the hydride formation rim of formation. epsilon hydrides In the takeshydride place. rim, By in approachingregions close towards to the surface the bulk,, hydrogen hydrogen concentration concentration is decreases, very high and, and at the some formation depths, itof is epsilon not enough hydrides for epsilon takes place. hydride By approaching formation. Hence, towards the the delta bulk, hydrides hydrogen form. concentration The observed decreases variation, inand hardness at some isdepth dues to, it this is not phase enough change: for epsilon i.e., close hydride to the formation. surface, we Hence, are measuring the delta thehydrides hardness form. of epsilonThe observed hydrides, variation but close in to hardness the bulk, is we due are to measuring this phase the change hardness: i.e., of close delta to hydrides. the surface, we are measuringHydride the rim hardness formation of epsilon takes hydrides, place in the but cases close whento the hydrogenbulk, we are solubility measuring is exceeded the hardness in the of thindelta layer hydrides. of zirconium alloy during hydrogenation and when the hydrogen absorption rate exceeds the hydrogenHydride rim diff usionformation rate oftakes the place thin layerin the under cases study.when hydrogen After hydride solubility formation is exceeded in the in thin the layer, thin hydrogenlayer of zirconium diffusion alloy occurs during through hydrogenation this layer further and when into the the alloy hydrogen volume, absorption where hydride rate exceeds formation the alsohydrogen takes place. diffusion rate of the thin layer under study. After hydride formation in the thin layer, hydrogen diffusion occurs through this layer further into the alloy volume, where hydride formation 4. Summary also takes place. The present research is focused on the studies of the behavior of hydride rim formation in the E110 zirconium4. Summary alloy cladding tubes during gas-phase hydrogenation. The problem of hydrogen penetration and accumulationThe present research and subsequent is focused hydride on the formation studies of in the volumebehavior of of zirconium hydride rim cladding formation tubes in of the PWR-typeE110 zirconium reactors alloy remains cladding relevant. tubes The during formation gas-phase of fragile hydrogenat hydridesion. in the The zirconium problem matrixof hydrogen leads, firstly,penetration to significant and accumulation changes in mechanical and subsequent properties, hydride and secondly, formation can in cause the volume through-the-thickness of zirconium damagecladding of tubes cladding of the in PW complianceR-type reactors with the remains mechanism relevant. ofdelayed The formation hydride of cracking. fragile hydrides The degree in the of thezirconium hydrides’ matrix negative leads, eff firstly,ect is for to thesig mostnificant part changes determined in mechanical by the specifics properties, of their and distribution secondly, andcan orientation.cause through The-the problem-thickness of hydride damage rim of formation cladding is in relevantly compliance new withand insu theffi mechanismciently studied. of delayed Due to this,hydride the studiescracking. of theThe behavior degree of of the hydride hydrides rim’ formation negative effect in Russian is for zirconiumthe most part alloy determined are of importance by the andspecifics necessary of their to bedistribution able to prognose and orientation. the cladding The tube problem behavior of hydride during hydriderim formation rim formation. is relevantly new Theand resultsinsufficiently obtained studied. during Due this to research this, the allow studies us to of make the behavior the following of hydride conclusions: rim formation in Russian zirconium alloy are of importance and necessary to be able to prognose the cladding tube behavior during hydride rim formation. The results obtained during this research allow us to make the following conclusions:

Metals 2020, 10, 247 9 of 10

1. It has been established that, during gas-phase hydrogenation of the E110 zirconium alloy, the cladding tubes in the delivery condition under the constant pressure of 2 atm., within the temperature range (400–550) ◦C, up to hydrogen concentrations (0.1–1) wt.%, and subsequent slow cooling is where the formation of hydrides in the bulk of the material takes place. 2. It has been demonstrated that in the E110 zirconium alloy cladding tubes, the formation of hydride rims of different thicknesses is achieved due to hydrogenation at the temperatures lower than the threshold temperature (400 20) C up to different hydrogen concentrations. The observed ± ◦ specifics of hydride distribution in the volume of zirconium cladding coincide with the data available as a result of the analysis of hydride distribution in the cladding after the operation. 3. It has been experimentally proven that the hydride rim formed in the E110 zirconium alloy cladding tubes is characterized by the nonuniform distribution of hardness and hydrogen concentration through the thickness. 4. It has been experimentally proven that E110 zirconium alloy surface modification, using the method of argon ion-beam cleaning (under the voltage 2000 V, the power 1000 W, the current 0.5 A, and the pressure 6 10 2 Pa during 5 min) and subsequent nickel coating using the method ×· − of magnetron sputtering (under the voltage 500 V, the power 2000 W, the current 3 A, and the pressure 1 10 1 Pa) with the thickness of ~1 µm on the E110 zirconium alloy cladding tubes, ×· − leads to the increase of the threshold temperature value of hydride rim formation in 100 ◦C, which is related to the significant increase of the hydrogen absorption rate. Thus, the conducted series of studies and the developed methods allow a detailed investigation into the processes of hydrogen embrittlement and the delayed hydride cracking of zirconium alloys to enable the validation of the project criteria and ensure the operating capacity of pressured water reactors’ cladding tubes.

Author Contributions: V.K. made the organization of the workflow and preparation of the article, M.S. performed XRD tests, I.S. (Ivan Sakvin) conducted experiments on hydrogenation, I.S. (Inga Slesarenko) and A.L. made an analysis of the results. All authors have read and agreed to the published version of the manuscript. Funding: The research was funded by the Governmental program “Science”, research project No. FSWW-2020-0017. Acknowledgments: This work was carried out within the framework of the Competitiveness Enhancement Program of National Research Tomsk Polytechnic University. Conflicts of Interest: The authors declare no conflict of interest.

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