Highly sensitive secondary ion mass spectrometric analysis of time variation of hydrogen spatial distribution in austenitic stainless steel at room temperature in vacuum

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Hydrogen contained in austenitic stainless steel is classified as diffusible or nondiffusible. The hydrogen distribution in austenitic stainless steel changes with time owing to hydrogen diffusion at room temperature, and such changes in hydrogen
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  Highly sensitive secondary ion mass spectrometricanalysis of time variation of hydrogen spatialdistribution in austenitic stainless steel at roomtemperature in vacuum Tohru Awane a,b, *, Yoshihiro Fukushima a,c , Takashi Matsuo e ,Yukitaka Murakami a,d , Shiro Miwa  f  a Research Center for Hydrogen Industrial Use and Storage, National Institute of Advanced Industrial, Science andTechnology (HYDROGENIUS), 744 Moto oka, Nishi-ku, Fukuoka 819-0395, Japan b Kobe Material Testing Laboratory Co., Ltd., 47-13 Niijima, Harima-cho, Kako-gun, Hyogo 675-0155, Japan c Department of Mechanical Engineering Science, Graduate School of Engineering, Kyushu University, 744 Moto oka,Nishi-ku, Fukuoka 819-0395, Japan d International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University, 744 Moto oka,Nishi-ku, Fukuoka 819-0395, Japan e Hydrogen Energy Test and Research Center (HyTReC), 915-1 Tomi, Itoshima, Fukuoka 819-1133, Japan f  Cameca Division, AMETEK Co., Ltd., 1-1-30 Shibadaimon, Minato-ku, Tokyo 105-0012, Japan a r t i c l e i n f o Article history: Received 22 April 2013Received in revised form11 October 2013Accepted 20 October 2013Available online 22 November 2013 Keywords: HydrogenSecondary ion mass spectrometryTime variationAustenitic stainless steelHydrogen embrittlement a b s t r a c t Hydrogen contained in austenitic stainless steel is classified as diffusible or nondiffusible.The hydrogen distribution in austenitic stainless steel changes with time owing tohydrogen diffusion at room temperature, and such changes in hydrogen distribution causethe mechanical properties of the steel to change as well. It is therefore important toanalyze the time variation of the hydrogen distribution in austenitic stainless steel at roomtemperature to elucidate the effects of hydrogen on the steel’s mechanical properties. Inthis study, we used secondary ion mass spectrometry (SIMS), a highly sensitive detectionmethod, to analyze the time variation of the distribution of hydrogen charged into 316Laustenitic stainless steel. SIMS depth profiles of hydrogen that were acquired at the threemeasurement times were analyzed, and the results were compared among the measure-ment times.  1 H  intensities and distribution of the intensities changed with time due todiffusion of hydrogen in the hydrogen-charged 316L steel sample at room temperature.Moreover, the time variation of the hydrogen concentration distribution of the hydrogen-charged 316L sample was calculated using a one-dimensional model based on Fick’s sec-ond law. The time variations of the measured hydrogen intensities and of the calculatedvalues are compared.Copyright  ª  2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rightsreserved.*  Corresponding author . Present address: Research Center for Hydrogen Industrial Use and Storage, Kyushu University (HYDROGENIUS),744 Moto oka, Nishi-ku, Fukuoka 819-0395, Japan. Tel.:  þ 81 92 802 3924; fax:  þ 81 92 802 3894.E-mail addresses: awntr@kne.biglobe.ne.jp, awane.toru.438@m.kyushu-u.ac.jp (T. Awane).  Available online at www.sciencedirect.com ScienceDirect  journal homepage: www.elsevier.com/locate/he international journal of hydrogen energy 39 (2014) 1164 e 1172 0360-3199/$  e  see front matter Copyright  ª  2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.ijhydene.2013.10.116  1. Introduction Austenitic stainless steel is a metallic material used for fuel-cell vehicles or hydrogen infrastructure. When the materialis exposed to hydrogen gas, hydrogen atoms enter the mate-rial. The hydrogen atoms in the material can degrade its me-chanical properties by means of “hydrogen embrittlement,” aphenomenon that has been studied extensively [1 e 7,22 e 28].Murakami et al. analyzed hydrogen contained in ahydrogen-charged 316L austenitic stainless steel using ther-mal desorption spectroscopy (TDS), and revealed that thehydrogen-charged stainless steel contained both diffusibleand nondiffusible hydrogen [3]. Nondiffusible hydrogen ishydrogenthatentersthesteelatthetimeofmanufacture,andispresenteveninuncharged(withnohydrogencharging)316Laustenitic stainless steel, with very little diffusion at roomtemperature. When uncharged 316L stainless steel is exposedto hydrogen gas, hydrogen entering the steel diffuses in thesteel at room temperature, so that the concentration distribu-tion of the diffusible hydrogen changes over time. Changes inthe concentration distribution of the diffusible hydrogen arelikelytocausechangesinthemechanicalpropertiesofthe316Laustenitic stainlesssteel [1 e 5,22 e 28]. It is thereforeimportantto analyze the time variation of hydrogen distribution in 316Lausteniticstainlesssteeltounderstandtheeffectsofhydrogenon the steel’s mechanical properties.Recently, we have developed a highly sensitive detectionmethod for hydrogen using secondary ion mass spectrometry(SIMS), and used this method to observe the distribution of hydrogen charged into 316L austenitic stainless steel [15].When hydrogen in a sample is measured with SIMS, not onlynet hydrogen (H N ) in the sample but also background-srcinated hydrogen (H BG ) is simultaneously detected. TheHBG srcinates from moisture (H 2 O), hydrocarbons (C x H Y  ), ororganic materials (C x H Y  O Z ) existing in the SIMS chamber oron the sample surface [8]. This H BG  precludes an accuratemeasurement of H N  because H BG  and H N  cannot be distin-guished in a SIMS profile. Carbon, oxygen, and nitrogen inresidual gas in a SIMS chamber or on an inner surface of thechamber are also likely to be background sources when car-bon, oxygen, and nitrogen contained in a sample areanalyzed with SIMS. There have been several studies of background sources and effective methods to improvedetection limits in SIMS analysis of hydrogen, carbon, oxy-gen, and nitrogen in semiconductor materials [9 e 14]. Toaccurately analyze hydrogen charged into 316L stainless steelsamples, further improvements to these methods are neededbecause even uncharged 316L stainless steel containshydrogen that enters the steel at the time of manufacture,and because the amount of H BG  emitted changes with timeduring SIMS measurements. However, in past studies, grossintensity of hydrogen (total intensity of H N  and H BG ) has beenmerely considered in SIMS analysis of hydrogen in metallicmaterial [19 e 21]. The original highly sensitive detectionmethod developed by the authors consists of a procedure inwhich a silicon wafer is sputtered by a SIMS primary ionbeam near an analyzed area to reduce H BG  in SIMS mea-surement of hydrogen, and a method to determine the in-tensities of H BG  in measurements of a hydrogen-chargedsample by estimating the time variation of hydrogen in-tensities in measurements of an uncharged sample [15]. Inthe present study, the time variation of the distribution of hydrogen charged into 316L austenitic stainless steel at roomtemperature in vacuum is revealed by our SIMS method. Inaddition, the time variation of the hydrogen concentrationdistribution in the hydrogen-charged 316L sample is calcu-lated using a one-dimensional model on the basis of Fick’ssecond law. The time variations of the measured hydrogenintensities and of the calculated values are compared. 2. Experimental procedure 2.1. Samples [15] The 316L austenitic stainless steel (iron-base, 0.01 wt% C,0.53wt%Si,0.77wt%Mn,0.023wt%P,0.001wt%S,12.13wt%Ni,17.16wt%Cr,and2.86wt%Mo)wasusedforthisstudy.Fig.1(a)shows two rod-shaped samples manufactured from the 316Lsteelwithadiameterof5mmandlengthof40mm.Onesample(H-PRECHARGE-0) was treated by ultrasonic washing withacetoneandthenexposedtohydrogengasat10MPaand250  Cfor 192 h with the hydrogen exposure facility HYDROGENIUS[17]. After hydrogen exposure, the bulk hydrogen concentra-tionwas22.4massppm.Anothersample(NON-CHARGE-0)wasprepared to estimate H BG  in SIMS measurements of H- Fig. 1  e  Dimensions of the 316L austenitic stainless steel samples [15]. international journal of hydrogen energy 39 (2014) 1164 e 1172  1165  PRECHARGE-0. NON-CHARGE-0 was not exposed to hydrogenand had a bulk hydrogen concentration of 1.7 mass ppm. Boththe bulk hydrogen concentrations in H-PRECHARGE-0 andNON-CHARGE-0 were analyzed by TDS. A disk of approxi-mately 3 mm thickness was cut from both samples for SIMSmeasurements. The disks were named H-PRECHARGE-01 andNON-CHARGE-01, respectively. After the cutting, cross-sections of H-PRECHARGE-01 and NON-CHARGE-01 weresmoothlypolishedfor44minwithdiamondlappingfilms.Afterpolishing, H-PRECHARGE-01 and NON-CHARGE-01 werewashed in acetone using ultrasonic washing for 1 min Fig. 1(b)and (c) shows the shapes and the dimensions of H-PRE-CHARGE-01 and NON-CHARGE-01 after polishing. 2.2. SIMS data acquisition from H-PRECHARGE-01 andnon-charge-01 SIMS analyses in this research were performed with an IMS-7f mass spectrometer (Cameca, France). The IMS-7f is a SIMSinstrument equipped with a cesium ion source and a double-focus sector-type mass analyzer. The measured element inthe depth profiles with the SIMS was  1 H  . In measurementsof the time variations of   1 H  intensities of H-PRECHARGE-01,the finish time of the first set of SIMS measurements of H-PRECHARGE-01 was defined as 0 h. Then, H-PRECHARGE-01data were acquired 27.2 h after 0 h (which was the finish timeof the second set of SIMS measurements) and at 484.3 h after0 h (which was the finish time of the third set of SIMS mea-surements). Measurements of NON-CHARGE-01 were ob-tained before and after measurements of H-PRECHARGE-01 toinvestigate the time variation of H BG  in the measurements forH-PRECHARGE-01. H-PRECHARGE-01 measurements werecarried out at measurement points of different distancesfrom a cross-section edge to estimate the spatial distributionof hydrogen in H-PRECHARGE-01. The time required to obtainone measurement point for H-PRECHARGE-01 was 1934 s,and for NON-CHARGE-01 it was 664 s. Cs þ was used as theprimary ion in SIMS measurements. The acceleration voltageof the primary ion beam was 15 kV, which corresponded tothe difference in voltage between a Cs ion ( þ 10 kV) and thesample voltage (  5 kV). The intensity of the primary ionbeam was constantly maintained at 60 nA while measuring all samples. H-PRECHARGE-01 and NON-CHARGE-01 werescanned by the primary ion beam in a range of 50- m m-square,and then an area of 5.6  m m diameter was analyzed. In-tensities of   1 H  were measured with an electron multiplier(EM). 2.3. Temperature of the SIMS sample stage In this study, H-PRECHARGE-01 and NON-CHARGE-01 wereset in the SIMS chamber vacuum, and then the time variationof the hydrogen distribution at room temperature wasanalyzed with SIMS. The temperature inside the laboratorywhere the SIMS is installed is constantly maintained atapproximately 21   C by an air conditioner. The temperatureof the sample stage in the SIMS chamber becomes constantat approximately 26   C due to the heat emitted from a vac-uum gauge. 2.4. Correction method for sensitivities for hydrogenchanging with time When SIMS measurements for an element that stably anduniformly exists in a sample are conducted over time, detec-ted intensities of the element may vary even though thesample is analyzed under identical measurement conditions.Differencesinthedetectedintensitiesaremainlyattributedtovariation in the sensitivity of the EM used as the detector forsecondary ions. In this research, to correct differences of detection sensitivities for  1 H  among the 0, 27.2, and 484.3 hmeasurements, the following method was applied. A siliconwafer implanted with hydrogen (H e Si) [15] was measuredwith a depth profile method before the measurements of H-PRECHARGE-01 and NON-CHARGE-01 at each measurementtime. The implanted hydrogen atoms (H IMP ) exist at a certaindepth in the H e Si. Since the H IMP  hardly shifts in the silicon atroom temperature, the hydrogen concentration in the H IMP layer is always constant [29]. In this study, a peak intensity inthe depth profile of   1 H  emitted from the H IMP  layer at 0 h wasused as a reference to correct detection sensitivities for  1 H  at27.2 and 484.3 h. Correction coefficients for 27.2 and 484.3 hwereobtainedbydividingthepeakintensityat0hby thepeakintensities at 27.2 and 484.3 h, respectively. The correctioncoefficient was 1.48 for 27.2 h and 1.19 for 484.3 h. 2.5. Techniques for H BG  reduction and H BG  correction inSIMS measurements [15] To reduce H BG  in the measurements of H-PRECHARGE-01 andNON-CHARGE-01, the silicon sputtering method was con-ducted before acquiring SIMS measurements of hydrogencontained in H-PRECHARGE-01 and NON-CHARGE-01 at eachmeasurement time. In the silicon sputtering method, a wafermade from highly pure silicon is sputtered by a primary ionbeam near an analyzed area. Since sources of the H BG , such asmoisture or hydrocarbon existing on a sample surface or inthe SIMS chamber, are covered with the sputtered silicon, H BG emission from the source is inhibited. H e Si was used for thesilicon sputtering method. The sputtering time of silicon was54,137 s at 0 h, 7974 s at 27.2 h, and 15,368 s at 484.3 h. More-over, a cold trap using liquid nitrogen was used to reduce theH BG  during all measurements.The H BG  in SIMS measurements of H-PRECHARGE-01 wereacquired by the following procedure. The measurements forNON-CHARGE-01 before and after the measurements for H-PRECHARGE-01 were conducted to investigate time-variationof the H BG  in the measurements for H-PRECHARGE-01. Theaverage value of the  1 H  intensities for NON-CHARGE-01,which were acquired before the measurements of H-PRE-CHARGE-01, was calculated ( I 1 ). The average value of the  1 H  intensities for the NON-CHARGE-01, which were acquiredafter the measurements of H-PRECHARGE-01, was alsocalculated ( I 2 ). It is assumed that the time-variation of the H BG in the measurements of H-PRECHARGE-01 varied along thepower approximation curve between the  I 1  and the  I 2  on theplot of time versus secondary ion intensity. The intensities of H BG  in the measurements of H-PRECHARGE-01 were deter-minedby insertingthefinishtimesofthemeasurementsofH-PRECHARGE-01 into the approximation curve. international journal of hydrogen energy 39 (2014) 1164 e 1172 1166  3. Results and discussion 3.1. Depth profile of   1 H  of H-PRECHARGE-01 and NON-CHARGE-01 with SIMS Fig.2(a) e (c)showsthedepthprofilesof  1 H  thatwereacquiredfrom H-PRECHARGE-01 measurement points using SIMS(Fig. 2(a): at 0 h, Fig. 2(b): at 27.2 h, Fig. 2(c): at 484.3 h). The intensities of   1 H  detected from the near-surface region werecomparatively high mainlydue to hydrocarbons and moistureattached to the cross-section surface at any measurementtime. The profiles increased approximately linearly from thedepths of the minimal intensities to depths of approximately10  m m for all measurement times. The intensities were almostconstant at regions deeper than approximately 10  m m at 0 h(Fig. 2(a)). We deduced that the hydrogen contained in theregion from the surface to approximately 10  m m deep diffuseddue to frictional heat generated by cutting and polishing H-PRECHARGE-01, so that most hydrogen in that region escapedfrom the cross-section. The depth profiles of   1 H  other thantheH-B7at 27.2h (Fig.2(b))showtrendssimilarto thoseat0 h:the  1 H  intensities in the depth profiles at 27.2 h becamealmost constant at regions deeper than approximately 10  m m.The depth profiles of   1 H  at 484.3 h (Fig. 2(c)) increasedapproximately linearly from the depths of the minimal in-tensitiestothebottomofthesputtercrater.Thedepthprofilesof   1 H  at 484.3 h show a concentration gradient of hydrogenthat occurred with the progression of hydrogen diffusion withtime in the measured depth range (0 to approximately 25  m m)of H-PRECHARGE-01.Fig. 2(d) e (f) shows the depth profiles of   1 H  that were ac-quired from NON-CHARGE-01 measurement points using SIMS(Fig.2(d):at0h,Fig.2(e):at27.2h,Fig.2(f):at484.3h).The intensities of   1 H  became almost constant at a depth of approximately 3  m m for all measurement times. The averageintensity of   1 H  from 3 to 6  m m deep in NON-CHARGE-01 (0 h:26.8 e 74.5cps,27.2h:19.1 e 35.2cps,484.3h:30.0 e 55.8cps)waslower than the minimal intensity of   1 H  acquired from H-PRECHARGE-01 (0 h: 462.2 e 1140.1 cps, 27.2 h: 207.4 e 430.2 cps,484.3 h: 57.8 e 230.1 cps) at each measurement time. Thus it isfound that H BG  could effectively be sufficiently reduced toestimate the significant difference of the intensities of   1 H  between H-PRECHARGE-01 and NON-CHARGE-01 at anymeasurement time. 3.2. Determination of the intensities of H N  in H- precharge-01 Table 1(a) e (c) shows the average intensities of   1 H  from 10 to16  m m deep, and the pressures in the vacuum system justbefore the measurements for H-PRECHARGE-01 together withthe averages (Ave), standard deviations (SD), and variationcoefficients (VC  ¼  SD/Ave). Table 1(d) e (f) shows the averageintensitiesof  1 H  from3to6 m mdeep,andthepressuresinthevacuum system just before the measurements for NON-CHARGE-01 together with the Ave, SD, and VC values. The 1 H  intensities at 27.2 and 484.3 h were corrected by multi-plying the correction coefficients for the hydrogen sensitiv-ities, which were 1.48 and 1.19, respectively. The chambervacuum temperature remained almost constant through all Fig. 2 e Depth profiles of   1 H L acquired from H-PRECHARGE-01 and NON-CHARGE-01 using SIMS. (a) H-PRECHARGE-01 at 0 h[15]. (b) H-PRECHARGE-01 at 27.2 h (c) H-PRECHARGE-01 at 484.3 h (d) NON-CHARGE-01 at 0 h [15]. (e) NON-CHARGE-01 at  27.2 h (f) NON-CHARGE-01 at 484.3 h. international journal of hydrogen energy 39 (2014) 1164 e 1172  1167  the measurements of H-PRECHARGE-01 and NON-CHARGE-01as shown in Table 1(a) e (f) Therefore, we concluded that thechange in H BG  during the measurements of H-PRECHARGE-01and NON-CHARGE-01 was not caused by moisture or hydro-carbons in the chamber vacuum. We estimated the distribu-tions of H N  in H-PRECHARGE-01 using the average intensitiesof   1 H  from 10 to 16  m m deep in Table 1(a) e (c) However, sincethe averageintensities in Table 1(a) e (c) included intensities of H BG  as well as the intensities of H N , it is necessary to subtractthe intensities of H BG  from the average intensities (the grossintensity of   1 H  ) for determinations of H N . The intensities of  Table 1 e Average intensities of   1 H L and pressures in the vacuum system just prior to SIMS measurements of H-PRECHARGE-01 and NON-CHARGE-01, together withaverages (Ave), standard deviations (SD), and variationcoefficients (VC [ SD/Ave). (a) H-PRECHARGE-01 data at 0 h, together with distances from the cross-section edge[15]. (b) H-PRECHARGE-01 data at 27.2 h, together withdistances from the cross-section edge. (c) H-PRECHARGE-01dataat484.3h,togetherwithdistancesfromthecross-sectionedge.(d)NON-CHARGE-01dataat0h[15].(e)NON-CHARGE-01 data at 27.2 h (f) NON-CHARGE-01 data at 484.3 h. The  1 H L intensities at 27.2 and at 484.3 h werecorrectedbymultiplyingthecorrectioncoefficientsforthehydrogen sensitivities, which were 1.48 and 1.19,respectively. (a)Distance (mm) 10 e 16  m m deep Chamber vacuum(    10  10 Torr) 1 H  (cps) H-A1 0.07 1128 2.6H-A2 0.54 1160 2.6H-A3 1.01 2342 2.8H-A4 1.40 4314 2.8H-A5 1.67 2879 3.1H-A6 2.46 3275 2.8Ave 2516 2.78SD 1135.4 0.167VC 0.451 0.060 (b)Distance (mm) 10 e 16  m m deep Chamber vacuum(    10  10 Torr) 1 H  (cps)    1.48 H-B1 0.14 678 2.7H-B2 0.42 644 2.5H-B3 0.81 821 2.5H-B4 1.34 959 2.6H-B5 1.73 900 2.5H-B6 1.58 2350 2.4H-B7 2.00 876 2.4Ave 1033 2.51SD 548.4 0.099VC 0.531 0.039 (c)Distance(mm) 10 e 16  m m deep Chamber vacuum(    10  10 Torr) 1 H  (cps)    1.19 H-C1 1.46 591 2.9H-C2 0.16 224 2.9H-C3 0.73 239 3.1H-C4 1.52 711 2.9H-C5 1.31 199 3.0H-C6 1.82 829 3.0Ave 466 2.97SD 254.7 0.075VC 0.547 0.025 (d)3 e 6  m m deep Chamber vacuum(    10  10 Torr) 1 H  (cps) U-A1 66 2.6U-A2 75 2.5U-A3 44 2.6 Table 1 (  continued  ) (d)3 e 6  m m deep Chamber vacuum(    10  10 Torr) 1 H  (cps) Ave. 62 ( I d1 ) 2.57SD 12.8 0.047Variation coefficient 0.207 0.018U-B1 27 3.0U-B2 28 3.0U-B3 30 2.8U-B4 44 2.8Ave. 32 ( I d2 ) 2.90SD 6.8 0.100Variation coefficient 0.212 0.034 (e)3 e 6  m m deep Chamber vacuum(    10  10 Torr) 1 H  (cps)    1.48 U-C1 52 2.4U-C2 39 2.4U-C3 28 2.6Ave. 40 ( I e1 ) 2.47SD 9.7 0.094Variation coefficient 0.246 0.038U-D1 32 2.4U-D2 28 2.3U-D3 33 2.5Ave. 31 ( I e2 ) 2.40SD 2.1 0.082Variation coefficient 0.067 0.034 (f)3 e 6  m m deep Chamber vacuum(    10  10 Torr) 1 H  (cps)    1.19 U-E1 66 2.8U-E2 45 2.9U-E3 42 3.0Ave. 51 ( I f1 ) 2.90SD 10.7 0.082Variation coefficient 0.209 0.028U-F1 58 2.7U-F2 40 2.8U-F3 36 3.0Ave. 44 ( I f2 ) 2.83SD 9.6 0.125Variation coefficient 0.215 0.044 international journal of hydrogen energy 39 (2014) 1164 e 1172 1168
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