Fusion Science and Technology ISSN: 1536-1055 (Print) 1943-7641 (Online) Journal homepage: http://www.tandfonline.com/loi/ufst20 Hydrogen Isotopes Permeability in Eurofer 97 Martensitic Steel A. Aiello, I. Ricapito, G. Benamati & R. Valentini To cite this article: A. Aiello, I. Ricapito, G. Benamati & R. Valentini (2002) Hydrogen Isotopes Permeability in Eurofer 97 Martensitic Steel, Fusion Science and Technology, 41:3P2, 872-876, DOI: 10.13182/FST41-872 To link to this article: http://dx.doi.org/10.13182/FST41-872 Published online: 10 Aug 2017. Submit your article to this journal Article views: 4 View related articles Citing articles: 3 View citing articles Full Terms & Conditions of access and use can be found at http://www.tandfonline.com/action/journalInformation?journalCode=ufst20 Download by: [University of Florida] Date: 28 October 2017, At: 07:47 HYDROGEN ISOTOPES PERMEABILITY IN EUROFER 97 MARTENSITIC STEEL R. Valentini University of Pisa Dept. of mechanical engineering Pisa, Italy Downloaded by [University of Florida] at 07:47 28 October 2017 A. Aiello, I. Ricapito, G. Benamati ENEA Fusion Division - CR Brasimone 40032 Camugnano Italy tel.+39.0534.8011 ABSTRACT In considering structural materials for fusion reactors a detailed understanding of the transport parameters and solubility of hydrogen and its isotopes is an important issue which deal with safety and blanket performance aspects. The experimental activities were focused on the determination of hydrogen/deuterium transport parameters through Eurofer 97 in the temperature range 423+723K using a time dependant permeation technique The hydrogen permeation and diffusivity at room temperature and density of trapping sites were also evaluated using Devanathan 's technique. Hydrogen / deuterium permeation experiments on Eurofer 97 showed a non-negligible decrease in permeability with respect to other fusion oriented martensitic steels, even if it remains about one order of magnitude higher compared with that of austenitic AISI 316L steel. I INTRODUCTION The martensitic steel Eurofer 97 represents the latest generation of martensitic steels for fusion applications and is presently the candidate material for the first wall and structural components of the demonstration fusion reactor DEMO. It belongs to the Reduced Activation Martensitic (RAM) steels of the 7-10% Cr family. A precise determination of the permeability, diffusivity and solubility of hydrogen isotopes through such materials is of crucial importance to evaluate tritium behaviour and inventory in the blanket modules. These martensitic steels have approximately one order of magnitude higher permeability than austenitic steels and a 872 complete characterisation of hydrogen isotope transport parameters is necessary to evaluate the tritium behaviour in the DEMO blanket. Moreover, the hydrogen isotope transport in martensitic steels at temperatures lower than 200-250 °C is generally strongly affected by trapping phenomena. An understanding of trapping mechanisms is necessary to understand the real operative conditions at which hydrogen embrittlement (HE) can take place. In order to gather data on all of these phenomena, permeation experiments have been performed on Eurofer 97 in the temperature range 413+123 K with the device "PERI" (gas phase technique) and at room temperature with Devanathan's electrochemical cell. The obtained results have been compared with those of other candidate materials proposed for the first wall and blanket of a fusion reactor, such as F82H steel, which has been analysed in the past'1'2]. The composition of Eurofer 97 together with that of F82H is reported in tab. 1. Eurofer 97 F82H C Si Mn P 0.11 0.04 0.48 <0.005 0.003 0.09 0.11 0.16 0.002 0.002 Cr" Eurofer 97 F82H ~ Eurofer 97 F82H Mo" NT S_ V W 835 <0.001 0U021 CL20" L08" 7.66 <0.01 0.02 0.16 2.00 Al Nb Cu Co" Ti ~ 0.002 0.006 0.006 0.009 0.002 <0.01 <0.01 0.01 0.001 <0.01 Tab.l: Composition of Eurofer 97 compared with that of F82H FUSION SCIENCE AND TECHNOLOGY VOL. 41 MAY 2002 872 Aiello et al. II EXPERIMENTAL SET UP II.I Hydrogen isotopes HYDROGEN ISOTOPES PERMEABILITY IN EUROFER 97 MARTENSITIC STEEL permeation at high Downloaded by [University of Florida] at 07:47 28 October 2017 temperature Eurofer 97, used for the hydrogen/deuterium permeation experiment underwent the following heat treatment by the manufacturer in order to produce a fully ferrite free martensitic phase: normalisation at 975°C for 15 minutes, cooling to RT in air, tempering at 740°C for 45 minutes, cooling to RT in air. The samples used in the permeation apparatus consisted of discs of 48 mm in diameter and about 1 mm in thickness. They were machined from a sheet supplied by Boehler Edelstahl GmbH (Germany). The permeation device "PERI" is described elsewhere'11 and a schematic view is reported in fig. 1. Fig. 1: Schematics of the permeation apparatus PERI for the gas-phase measurements. High pressure deuterium gas, with a nominal purity of 99.7 %, is taken from a cylinder, although hydrogen gas has also been used. The hydrogen had an impurity content less than 1 vppm of H 2 0 and 0 2 , and was produced by a hydrogen generator connected to a hydrogen purifier. The hydrogen/deuterium gas is admitted to the sample via a control valve which, together with a pressure transducer and a Baratron, enables the pressure to be set at any value between 100-1.5-105 Pa. The sample is heated by a resistance furnace. Gas permeates through the sample causing a rise in pressure in the outlet volume. For each run, an analysis of the inlet and permeated gas is carried out by a quadrupole mass spectrometer in order to check for possible contaminants. The National Instruments system (Labview) is used for data acquisition. II.II Hydrogen permeation at room temperature Hydrogen permeation experiments FUSION SCIENCE AND TECHNOLOGY VOL. 41 at temperature have been performed using the Devanathan's technique. A 1 mm thick, square shaped specimen of Eurofer 97 as received was inserted in a double sided electrolytic cell, with the specimen separating the two sides. The anodic cell was filled with a solution of NaOH 0.1 M, and the cathodic one with a solution of H 2 S0 4 0.1 M and thiourea as a hydrogen recombination inhibitor. The two cells were aerated using a flux of N2 to reduce the background noise. The cathodic cell contains the working electrode, represented by one side of the membrane, a platinum counter electrode and a calomel standard potential electrode. The exit side of the membrane was polarised at a potential of 200 mV, high enough to reduce protons to hydrogen atoms before their recombination. Hydrogen was produced using a cathodic density of current of 10 mA/cm2, low enough to prevent hydrogen embrittlement phenomena. Ill RESULTS AND DISCUSSION III.I High temperature H2/D2 permeation tests The experiments were performed in a temperature range of 473+723 K using a hydrogen or deuterium upstream pressure of about 75000 Pa. The results of permeability <P and effective diffusivity D for Eurofer 97 in comparison with F82H steel'2' are presented as Arrhenius plots in figures 2 and 3, respectively. looon- (1/K) Fig. 2: Permeability of hydrogen/deuterium through Eurofer 97 compared to F82H low MAY 2002 873 Aiello et al. HYDROGEN ISOTOPES PERMEABILITY IN EUROFER 97 MARTENSITIC STEEL The correlation factors obtained by the two fitting procedures and experimental curves were very high, and the parameters obtained with the two routines were in good agreement. Permeability, lattice diffusivity and Sieverts constant Ks i for deuterium in Eurofer 97 were calculated as: IHt/U e~~*^(m2 D, = 1.5-10'7 Ks l = 1.02-KT'e Downloaded by [University of Florida] at 07:47 28 October 2017 O = 1.53 -10~ 8 e" Fig. 3 Diffusivity of hydrogen/deuterium in Eurofer 97 compared to F82H The ratio <P(m/<&(D2) is about 1.6 in this temperature range, a little higher than that expected by the classical theoretical value of diffusion coefficient ratio -Jl = (mD / mH)V2. Variation of the steady state flux with deuterium pressure was determined at different temperatures. In the pressure range 103-105 Pa a near half-power pressure dependence of the permeation fluxes is observed. As a consequence, the whole transport mechanism is controlled by the diffusion through the bulk, at least under these operative conditions. The experimental data of permeated flux were fitted using linear and general fitting techniques in order to give precise values for diffusivity and permeability. In the case of linear fitting, permeability is calculated by linearisation of the pressure-time curve referred to the low pressure side in steady state flux regime, and diffusivity is determined by the time lag. In the case of general fitting the parameters obtained from the linear fitting are adjusted performing a least square best fit of the solution of the general diffusion equation (eq.l) in the whole experimental data set. In eq.l D, p and d represent respectively permeability, diffusivity, upstream pressure and specimen thickness. Q(t)= f „ 7 71 874 D d 6D Zr-T-exp t l n2 -S1) 23810 f RT 38280 RT m'1 mol mol m •s Pa (2) 2 Pa " 2 Significant hydrogen trapping was observed below 573K. In this region, the diffusion coefficient drops sharply below the values obtained by a direct extrapolation at lower temperatures. It is worth noting that in F82H the trapping phenomena were evident only at temperature below 523 K. In modelling diffusivity data to extract the density and mean energy of the trapping sites, it has been assumed that the hydrogen atoms in the material can move through ordinary lattice sites, characterised by a lattice diffusion energy Ed, or through trapping sites, in which a potential well deep enough produces stable bound of hydrogen atoms to specific trapping centres at a certain temperature. The mean interaction energy Et can be evaluated. Using Ni and N t to represent the density of lattice and trapping sites, the effective diffusivity D eff may be related to the lattice one Di as131: D,r = , AN \ + — > Ml. e RT (3) It is evident from eq.3 that the trapping effects become negligible at high temperatures where Defr approaches Di. Using the non linear fitting of diffusivity data (fig. 4) to extract the values of E, and N t it is assumed that the concentration of lattice sites is 5.2-1029 m"3 assuming six tetrahedral lattice sites per host atom (bcc iron) [1] . (D -D—T-t d ) FUSION SCIENCE AND TECHNOLOGY VOL. 41 MAY 2002 Aiello et al. HYDROGEN ISOTOPES PERMEABILITY IN EUROFER 97 MARTENSITIC STEEL Taking into account these data and considering the grain dimension of Eurofer 97 (ASTM 10) and its composition, the main trapping sites are probably the interfaces between martensitic laths together with carbide precipitates and dislocations. III.II Diffusivity and density of trapping sites were measured by experiments of hydrogen permeation at room temperature using Devanathan's technique. The relationship between the hydrogen permeated flux J and the anodic current in the electrochemical cell is: Downloaded by [University of Florida] at 07:47 28 October 2017 Fig. 4: Arrhenius plot of deuterium diffusivity in Eurofer 97 The obtained values, compared with those of F82H and Manet II[2], are presented in tab. 2. 3 N, (m" ) E, (J/mol) Ed (J/mol) MANETII 1.6 1025 48500 13210 F82H 1.6-1023 55938 Eurofer 97 1.04-1024 57898 13950 14474 Tab.2: trapping parameters in MANET II, F82H and Eurofer 97 martensitic steels The most interesting result is that the density of trapping sites in Eurofer 97 is about one order of magnitude higher than in F82H. This circumstance was confirmed by experiments of permeation at low temperature, as will be seen in the next section. Some published data for the trapping energy in various trapping sites in ferritic steels taken from literature'3"101 are reported in tab. 3. Type of trap Single vacancy Atomic traps Substitutional Cr atoms Substitutional Mo atoms Substitutional V atoms Substitutional Mn atoms Interstitial C atoms Substitutional Ni atoms Interstitial N atoms Grain boundaries Second phase particles A1N, ecarbide MnS Dislocation Trapping energy (kJ/mol) 50, 46.6, 78.3 Ref. 26.1 27 57 10.6 3.3 -11.6 12.5 59,32 5 5 5 5 6 5 6 7 65 72 20 to 30 8 9 10 J{t) = m - L (4) where F is the Faraday constant and L the thickness of the specimen. When the measured current reaches a steady state value corresponding to the hydrogen saturation of the specimen, the diffusivity can be evaluated using the time lag or breakthrough methods. In both cases the current is integrated versus time, determining the total amount of hydrogen permeated through the sample during the experiment. It can be seen that the cumulative amount of permeated gas asymptotically approaches a straight line (fig. 5). 3,4 Tab. 3: Trapping energies from literature 875 FUSION SCIENCE AND TECHNOLOGY Hydrogen permeation at room temperature VOL. 41 MAY 2002 Fig. 5: evolution of density current integrated over the time in Devanathan's experiment HYDROGEN ISOTOPES PERMEABILITY IN EUROFER 97 MARTENSITIC STEEL Aiello et al. This line intercepts the time axis at a value defined as: tL = L2 using the time lag method, or 6-Deff Downloaded by [University of Florida] at 07:47 28 October 2017 tB = L2 15.3 -Deff using the breakthrough method. In the first case the time is measured starting from the moment at which electric current is applied to the inlet side of the sample. In the second case the time is measured starting from the moment in which the hydrogen from the outlet side of the specimen appears. Independently of the method used to fit the experimental data (breakthrough or time lag), the effective diffusivity measured is in the range 6-10"12-s-4.9-10"um2/s, as can be seen in tab. 4. respect to F82H martensitic steel. For instance, at 350°C the permeability of deuterium in Eurofer 97 is about 2.5 times less than in F82H. Also the diffusivity is lower in Eurofer 97 and this difference is particularly marked at 250°C where the trapping phenomena are more evident in Eurofer 97 than in F82H. In addition, also if the interaction energy for the trapping sites is practically the same for the two steels, the density of traps in Eurofer 97 is about one order of magnitude higher than in F82H. This circumstance was also confirmed by permeation tests carried out at room temperature using Devanathan's technique. REFERENCES [1] [2] Parameter Steady state current (A/cm2) Dbreakthrough (m'/s) Dtime tag (m2/s) First permeation 14 Second permeation 10.4 6.00e-12 2.40e-11 3.30e-ll 4.90e-11 [3] [4] [5] [6] Tab.4: Diffusion coefficients determined using Devanathan's technique The second permeation run, carried out just after a degassing of the specimen at the end of the first permeation test, gives the highest values of diffusivity because the irreversible traps had already been filled during the first run. A concentration of trapped hydrogen of 0.48 wppm was measured, which corresponds to a density of irreversible traps of 2.2-1024 m"3. This value is in quite good agreement with the density of traps calculated from fitting the diffusivity data, measured at high temperature, with eq. 3. IV [7] [8] [9] [10] E. Serra, A. Perujo, G. Benamati, Journal of Nuclear Materials, 245, 108-114 (1997) E. Serra, G. Benamati, , Material Science and Technology, 14, 573-578 (1998) S.M. Myers, S.T. Picraux, R.E. Stroltz, J. Appl. Phys, 50, 5710 (1979) K.B. Kim, S. Pyun, Arch. Eisenh. 53, 397 (1982) A.I. Shirley, C.K. Hall, Scr. Metall., 17, 1003 (1983) J.J. Au, H.K. Birnbaum, Acta Metall., 26, 1105 (1978) J.P. Hirth, Metall. Trans., 11 A, 861 (1980) H.H. Podgurski, R.A. Oriani, Metall. Trans., 3, 2055 (1972) J. Chene, J.O. Garcia, C.P. De Oliveira et oth., J. Microsc. Spectrosc. Electron., 4, 37 (1979) C.A. Wert, in "Topics in Applied Physics, Hydrogen in metals", eds. G. Alefeld , 305 (1978) CONCLUSIONS Hydrogen / deuterium permeation experiments on Eurofer 97, carried out in the temperature range 473-723 K, showed a non-negligible decrease in permeability with 876 FUSION SCIENCE AND TECHNOLOGY VOL. 41 MAY 2002
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