States of hydrogen and deuterium in chemically

Journal of Alloys and Compounds 587 (2014) 800–806

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Letter

States of hydrogen and deuterium in chemically charged high purity aluminum P. Rozenak ⇑ Hydrogen Energy Batteries, Ltd., POB 195, Omer 84965, Israel

a r t i c l e

i n f o

Article history: Received 10 September 2013 Received in revised form 30 October 2013 Accepted 30 October 2013 Available online 9 November 2013 Keywords: Hydrogen Aluminum Deuterium Vacancies Chemical charging Voids

a b s t r a c t In the present study, the deuterium distribution through the specimen thickness during charging and aging at various temperatures was characterized using the secondary ion mass spectrometry (SIMS) method. The advantages of this method are that the actual concentration–depth profiles are obtained from hydrogen (H) introduced during the growth, storage and melting environments (in single crystal or polycrystalline) of aluminum. Separately, the deuterium (D) concentration–depth profiles introduced from chemical processes or interactions with water during the experimental test processes, obtained by charging was characterized. Analysis of the pores, voids and bubbles related to hydrogen and deuterium was performed by small and wide X-ray scattering (SAXS and WAXS) and by high magnification transmission and scanning electron microscopy (TEM and SEM). Large (some micrometers in diameter) to very small (nanometers in diameter) sizes in the distribution and variability in the density of the voids/bubbles were found. The internal and surface bubbles and dense distributions of voids near dislocations related to the hydrogen distribution were studied. Moreover, the hydrogen–vacancy interactions related to microstructure changes must be taken into account in the process of characterizing the state of hydrogen in aluminum. An interpretation of the two dimensional anisotropic iso-intensities obtained by SAXS for octants voids was coded to obtain iso-intensity in the [1 1 0] plane in reciprocal space. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction The only direct attempt to determine the rate of diffusion of hydrogen through solid aluminum concluded that aluminum is impermeable to hydrogen at temperatures up to 823 K (550 °C). However, very slow rates of diffusion would have been detected [1]. Uncertainty also exists in the results of hydrogen diffusion measurements in aluminum, which show range scatter. Reported values of D0 and the diffusion enthalpies vary in the large scales [2–7]. Hydrogen solubility against the temperature is considerably greater in the liquid than in the solid state at in 1 atm. of hydrogen pressure (Fig. 1) [8–10]. During cooling and solidification, dissolved hydrogen exists in excess of the extremely low 104 wppm. At room temperatures, solid solubility may precipitate in the molecular form, resulting in the formation of primary and/or secondary voids. Primary or interdendritic porosity forms when the hydrogen content is sufficiently high such that hydrogen is rejected at the solidification front, resulting in supercritical saturation and void/pore formations. Secondary porosity occurs when the dissolved hydrogen content is low, and void formation occurs at a characteristically subcritical hydrogen concentration [11]. ⇑ Tel.: +972 52 3302942; fax: +972 77 2102822. E-mail address: [email protected] 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.10.229

Percentage porosity has been studied by X-ray micro-tomography to observe hydrogen micro-pores during high temperature exposure in high purity Al–Mg alloys with different hydrogen contents and in 99.999% pure aluminum [12,13]. Synchrotron X-ray microtomography has been used to observe hydrogen micro-pores and their shrinkage, annihilation, reinitiation, and growth behaviors during hot and cold plastic deformation and subsequent hightemperature exposure in high purity Al–Mg alloys. The final high temperature exposure was applied to verify whether complete healing of micro-pores is achieved after plastic deformation. It has been clarified that, although micro-pores tend to gradually shrink and close, a variety of geometrically variable behaviors are observed upon closer inspection of a single specimen at high temperatures. Chemical and electrochemical charging techniques result in hydrogen concentrations on the order of 600–1000 appm, suggesting that they correspond to very high hydrogen fugacity methods [14–16]. Typical values of the surface hydrogen concentration are very high because cathodic charging is equivalent to gaseous charging at extremely high hydrogen fugacity (1015 atm) [15]. Low diffusivity of hydrogen in aluminum at room temperature (hydrogen weakly diffuses interstitially in the aluminum lattice) [1], coupled with its high fugacity, is responsible for the high surface concentrations achieved during charging and also for the very high hydrogen concentration gradients beneath the surface. The

P. Rozenak / Journal of Alloys and Compounds 587 (2014) 800–806

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they involve, apart from the chemical reagents, an electric reagent (negative electrons) which acts at the interface between a metal (or another phase with metallic conduction) and the solution of the electrolyte. The solubility of aluminum and its oxides can be controlled by [18]: þþþ

þ 3H2 O ¼ Al2 O3 þ 6Hþ

2Al

Al2 O3 þ H2 O ¼ 2AlO2 þ 2Hþ þþþ

Al ¼ Al

þ 3e

SIMS technique has been used to characterize the hydrogen and deuterium distribution in chemically charged aluminum in order to obtain concentration–depth profiles [17]. The advantages of this method are that the actual concentration–depth profiles are obtained from hydrogen (introduced during the growth of single crystal or polycrystalline aluminum and the storage environment) in melting of aluminum and, separately, the deuterium concentration–depth profiles (introduced by chemical processes or interactions with water, when deuterium cannot be found free in nature) during chemical charging. These include micro-structural changes, such as defects such as vacancies, voids (pores), bubbles, dislocations, micro-cracks, and surface oxides that form during environmental growth and chemical charging [17]. It is clear that hydrogen mobility is affected by some structural defects, such as dislocations, voids, impurities, compounds, particles, interfaces, and grain boundaries. A reduction in the degree of purity of aluminum increases the types and numbers of defects and increases the hydrogen diffusion coefficient (Fig. 2). In all our experiments, hydrogen and deuterium were introduced into the aluminum from aqueous solutions [17], using electrochemical etching or charging and chemical charging methods. Electrochemical reactions differ from chemical reactions in that

Fig. 2. Diffusion coefficients of hydrogen in different purities of aluminum versus temperature [6].

ð2Þ ð3Þ

Al þ 2H2 O ¼ AlO2 þ 4Hþ þ 3e Fig. 1. Graph of the solubility (PPM) of hydrogen against the temperature at 1 atm. of hydrogen pressure in aluminum.

ð1Þ

ð4Þ

Aluminum decomposes water with the addition of a sufficiently acidic solution, leading to the evolution of hydrogen, dissolving as trivalent Al+++ ions and leaving the electrons on the metal. In the presence of a sufficiently alkaline solution (NaOH), aluminum decomposes water with the evolution of hydrogen, dissolving as aluminate ions (AlO2 ) and leaving the electrons on the electrode. Aluminum oxide or alumina (Al2O3) occurs in various forms on aluminum metal. The physical and chemical properties of alumina depend, to a large extent, on the temperature range, time, chemical and other conditions during its preparation. When an alkali is added to an acidic solution, an aluminate precipitate is obtained. This is the hydroxide gel, corresponding to the composition Al(OH)3, which is amphoteric in nature. This aluminum hydroxide gel is not stable; however, it crystallizes over time to give first the monohydrate c-Al2O3H2O or böhmite, crystallizing in the rhombohedral system, then the trihydrate Al2O33H2O or bayerite, crystallizing in the monoclinic system. This development of aluminum hydroxide is known as ‘‘aging’’. In acidic or alkaline solutions, the aluminum will be attacked as soon as the oxide film is eliminated. This dissolution is slower in acidic solutions than in alkaline solutions [18]. Many studies have been published related to aluminum hydride formation [19–35]. In a previous study [35], high purity aluminum (Al) samples containing different levels of hydrogen were used as the base metal for anodization. Experimental observations showed that differences in the hydrogen content affected the amount of energy consumed. The Al samples led to the formation of an anodic aluminum oxide (AAO) film in a sulfuric acid solution, which produced crystallized böhmite. This study proposes a unique tool for understanding certain special anodic behaviors of pure Al, where in the branching or merging of pore channels and the partial crystallization of the AAO film can be ascertained by looking at irregularities in the voltage–time curves [35]. Hung et al. published microscopic observations of voids in an anodic oxide film by TEM [36]. In the present study, the deuterium distribution through the specimen thickness during charging and aging at various temperatures was characterized using the secondary ion mass spectrometry (SIMS) method. A number of deuterium (D) and hydrogen (H) depth profiles were obtained for each sample. The advantages of this method are that the actual concentration–depth profiles are obtained from the hydrogen (H) environment during the melting of aluminum and, separately, the deuterium (D) concentration– depth profiles (introduced from chemical processes or interactions with water, as deuterium cannot be found free in nature) are obtained though experimental tests by chemical charging. Internal voids/pores, bubbles and hemispherical surface blisters of aluminum hydro-oxides with symmetrical bend contours contrasts in the sample and the dense distributions of voids near tangled dislocation reactions related to hydrogen (deuterium) distributions were studied.

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2. Experimental procedure Pure aluminum (99.999%) specimens were used in this study. For some experiments, specimens were cut from a single crystal ingot grown in the [1 1 0] direction in a vacuum by the Bridgeman method. The only other detectable elements were Si, Fe, and Cu, with corresponding concentrations of 1, 2.2, and 1 wppm. The ingot was cut into pieces by spark erosion followed by careful mechanical polishing to minimize damage to the surface. Care was taken to avoid the introduction of hydrogen during polishing; thus, contact with water was avoided at this stage. Some of the polycrystalline high purity samples with a grain size of a few millimeters were used for comparison. Specimens used for secondary ion mass spectrometry were 2 mm thick, flat, and with mirror-like surfaces. Analyses of the deuterium (D) distribution were carried out using the SIMS method with a IMS 3f ion microprobe camera with the sample cooled to 140 K. A 17-keV Cs+ primary beam was used for depth profiling of the sample beneath the surface at the MRS laboratory at the University of Illinois, at Urbana-Champaign, USA. For the Cs+ primary ion, the depth sensitivity of the analysis was about 5 nm. The primary ion beam was raftered over an area of 250 lm 250 lm while the secondary ions were analyzed from the central 10 lm 10 lm, in order to eliminate spurious ions from the crater edges and to obtain the best lateral resolution. The data were normalized using 27Al by counts to minimize instrumental effects. The depth scale was determined by measuring the sputtered craters using a stylus profilometer. The depth sensitivity measurements were in the range of 107 cm. Deuterium was introduced into the SIMS samples by chemical charging, using 0.1 N NaOD (D2O) solutions and by electrochemical charging, using 50 mA/cm2 current densities in 1 N D2SO4 (D2O) solution containing 0.25 g/l of NaAsO2 as a D recombination ‘‘poison’’ at room temperature and at higher temperatures for various periods of time. After chemical charging, the samples were kept at 77 K (196 °C) (in liquid nitrogen) to avoid D loss and redistribution before being transferred into the SIMS instrument for analysis. The concentrations of hydrogen introduced during electrochemical and chemical charging were monitored by gas extraction analysis and a LECO instrument. Al single and polycrystalline aluminum were used in the measurements according to the Laue method. Mo white radiation was used. The WAXS enables studies of stress, strain, texture, and void or bubble size and morphology. SAXS is a characterization technique for studying defects in the nanometer range. Typically small-angle X-ray scattering measurements were carried out at the Center For Small Angle X-Ray Scattering Research (CSASR) at the Oak Ridge National Laboratory, USA, using a parallel to the [1 1 0] direction 1 mm2 in diameter beam of Cu Ka radiation in a 10 m CCD camera fitted with an area detector and double-axis tilting stages. The area detector had a 20 cm 20 cm active area with a resolution of 6.25 104 radians at a sample to detector distance of 5 m. To fit the instrumental resolution to particle sizes of about 10 Å, the source to sample distance was shortened to 1.6 m and the sample to detector distance set at 1.2 m. The white beam generated by the source can also be Cu Ka radiation tuned with the use of monochromators. Another benefit is that the small wavelengths induce small Bragg angles of the diffracted beam and thus create the opportunity to collect several diffractions on an area detector. Fast area CCD-detectors was used and high brilliance X-rays enable time-resolved studies of, for instance, the state of hydrogen in aluminum (Fig. 3). For WAXS experiments, Mo white radiation X-ray are transmitted through or back reflected the about 70 lm sample and the scattered X-ray are detected by image plates (IP) have less spatial distortion. A subset of TEM samples were prepared from the surface of aluminum that was electrochemically charged for 24 h. By using a single jet polishing technique, the depth of the examined layer was controlled. To prevent hydrogen entry from the back side while the front side was being thinned, the back side was temporarily painted with non-conducting lacquer. The other 70 lm samples were prepared with complete ‘‘holes’’, i.e. in a 1:4 nitric acid and methanol mixture at 30 V and 257 K (16 °C), by single jet polishing, which was essentially crack- and dislocation-free. Hydrogen pick-up during polishing would be very high in compared with that introduced by chemical charging. The characterization of the defects that formed after electrochemical charging was done using low accelerating voltage

(100 keV) TEM. As the purpose was to minimize the irradiation effects, 30–45 s were taken to obtain the optimal conditions for high-resolution pictures under the microscope. In the present study, the hydrogen and the deuterium distributions through the specimen thickness during charging and aging at various temperatures was characterized using the SIMS method. A number (three to five) D depth profiles were obtained for each sample. Lattice parameter measurements were made immediately after charging using step scans of the X-ray peaks obtained with Cu Ka radiation. The lattice parameters were calculated from lines at various Bragg angles, h, to increase the precision of measurement of Da/a0; which was estimated to be 104. Length change measurements were made during hydrogen charging, using a linear variable differential transformer (LVDT). Length change measurements were made along the length of the sheet specimens, while the hydrogen entered through the lateral surface only. The estimated precision of the Dl/l0 or Dw/w0 (thickness change) measurements 5 107 for the LVDT measurements were found.

3. Experimental results Typical results are shown in Fig. 4 for the ion distributions of H1, and D2 in a single crystal of Al electrochemically charged for 2 h at room temperature. Ions of hydrogen, H1, result from the interaction of hydrogen (or water vapor from moisture in the air) during growth by the Bridgeman method and from water vapor (which can be found on the aluminum sample during sample storage) during the SIMS measurements. Hydrogen exhibited a maximum value at the sample surface of aluminum and then decreased linearly to a depth of 0.1 lm. Hydrogen can be formed from moisture in the air [37] and diffuse into the specimen during ion irradiation of high-purity aluminum [38]. To separate the effects before electrochemical charging and the experimental procedure, we used D2SO4, NaOD, and heavy water (D2O) for the chemical solutions. Electrochemical charging of the single crystal of aluminum for 2 h at room temperature led to the formation of a high concentration of deuterium on the surface of the aluminum that decreased linearly to a depth of 1.4 lm, and then decreased further in the low deuterium penetration zones. Complete results are shown in Fig. 4 in Ref. [17], for distribution of 16O, 27Al, 1 H and 2D in single crystal of Al electrochemically charged for 2 h at room temperature. Ions of oxygen 16O (upper curve) show a maximum value on the surface, decreasing linearly to a depth of about 0.1 mm with negative high slope value. The slope value then decreases slightly to a depth of about 0.6 lm under the specimen’s surface. From that point, the values of oxygen concentration maintain at the same level. Ions of 27Al exhibit maximum values on the sample surface and then decrease exponentially to a depth of about 0.08 lm. Only small changes in the aluminum concentration were observed in the deeper layers. All these behaviors show that the aluminum hydroxide in the thickness of 0.08–0.1 lm was form before the deuterium tests of electrochemical and chemical specimens charging. On the surface aluminum layer, regions of alumina hydroxide Al(OH)3 was formed, about 0.08 lm thick, as an anodic oxide film on the aluminum obtained from thin surface

Fig. 3. Schematic illustration of small angle X-ray scattering (SAXS) and wide angle X-ray scattering (WAXS) experiments.


Fig. 4. SIMS profiles of –H1 and –D2 versus depth in single crystal of aluminum electrochemically charged during 2 h at room temperature.

absorption of hydrogen and increased levels of aluminum and oxygen. High hydrogen fugacity (fH2 1015 atm) occurs on the sample surface during electrochemical and chemical charging [17]. In the previous study, nearly the same surface deuterium concentrations (Fig. 6(a)–(c) red lines Ref. [17]) were obtained in the single-crystal samples after electrochemical charging (15 min, 1 h and 2 h) at room temperature. The deuterium concentration dropped linearly to a depth of about 0.08 lm under the surface samples, and then exhibited an exponential distribution depending on the duration of electrochemical charging (Fig. 6(a), Ref. [17]). Electrochemical charging for 15 min produced deuterium profiles whose depths varied by about 0.5 lm. The rate of movement of the profiles inward from the surface is a function of the charging time in Fig. 6(b) and (c) in Ref. [17]. A concentration dip near the surface and gentle rise toward the interior were obtained in chemically charged single crystal aluminum specimens [15]. The concentration dip near the surface can be explained by the existence of traps that are located in these regions, such as the formation of anodic oxide films [29]. Aging for 15 min, 1 h and 2 h at 723 K, 573 K and 423 K, respectively, in the same sample after charging resulted in significant deuterium profile changes (Fig. 6(a) blue line in Ref. [17]). The

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profiles dropped between 10 and 20% on the surfaces of various specimens after aging heat treatments. In all samples, Al(OD)3 was stable during aging in a 0.08 lm-thick surface layer. The deuterium concentrations decreased in intensity and slightly increased in depth (to about 1.5 lm) of penetration in the specimens electrochemically charged for 2 h and specimens aged for 1 h at 423 K (150 °C) (Fig. 6(c) blue line in Ref. [17]). The solubility of hydrogen in aluminum in equilibrium, at 1 atm and 933 K (660 °C) (Fig. 1), is 2.5 101 ppm wt. The solubility in a solid solution of aluminum at 25 °C (298 K) is very low (1 103 ppm wt). Chemical and electrochemical charging methods introduce very high hydrogen fugacity at the specimen surface [14], which is the reason for microstructural changes in the aluminum. Development 3-D tracking of hydrogen micropores during the aluminum production process has been done [12,13,39]. The wide angle X-ray scattering results was published in the same journal before in Ref. [15]. The small angle X-ray scattering results done by the Cu Ka radiation was oriented along the [1 1 0] direction. Two-directional iso-intensity Iq versus qx,y,, when qx,y = 4p sin h/k, h is the Bragg angle, and k is the wavelength, was obtained from a single crystal aluminum reference. Fig. 5(a) shows the as-received specimens that scattered in iso-intensity with spherical circular symmetry. Fig. 5(b) was obtained from a single crystal of aluminum after it was electrochemically charged for 24 h, and exhibits anisotropic streak formation in the directions of [1 1 1]and [1 1 1], [1 1 0], and [0 0 1]. The contour lines correspond to count rates of 800 c/s (near center) to 7 c/s background. Fig. 5(d) shows prefer orientation iso-intensities in the direction of [1 1 1] obtained from an electro-polished single crystal of aluminum. Chemical charging of a single crystal of aluminum for 24 h resulted in anisotropic streaks formation in the directions of [1 1 1] and [1 1 1] only (Fig. 5(e)). In addition, the opposite intensities were obtained in the [1 1 1] and [1 –1 1] directions for lower h Bragg angles, shown in Fig. 5(e), indicating anisotropic behavior. The shape of the defects during the early stages of annealing after electrochemical charging can be obtained from analysis of the SAXS experiments. Annealing at 473 K (150 °C) after 2 h of electrochemical and chemical charging (Fig. 5(c) and (f))

Fig. 5. Two-dimensional SAXS iso-intensity patterns obtained from a single crystal of aluminum oriented along the [1 1 0] direction: (a) as received; (b) electrochemically charged for 24 h; (c) electrochemically charged for 24 h and aged at 473 K (150 °C) for 2 h; (d) electrochemically polished for 1 h; (e) chemically charged for 24 h; (f) chemically charged for 24 h and aged at 473 K (150 °C) for 2 h.

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24 h electrochemical charging [16]. An equilibrium stress state forms around the bubbles in the aluminum matrix. These levels of stress result in plastic deformations in regions near the bubbles.

4. Discussion

Fig. 6. TEM micrograph showing the formation of a bubble (indicated by the arrow) recognized by the bend contours contrast in the crater. Dislocation formation near the bubbles was tangled by voids in a single crystal of aluminum.

showed a decrease in the intensity and a complete disappearance of anisotropic scattering when the voids were changed to a circular isotropic concentric shell geometry. Voids grew to larger dimensions than SAXS could detect due to the instrumental limits, as well. The significance of the phenomenon is obtaining a low quantity of large dimension sphere-like defects. The quantity of the vacancies increased during the annealing at 473 K, as hydrogen vacancies interacted to form spherical clusters. Electro-polishing resulted, to anisotropic scattering in iso-intensities, the appearance of scattering in the [1 1 1] direction only, in single aluminum specimens (Figs. 5(d)). The reason for that may be in the aluminum ion interactions with a water solution in the chemical reactions and the effect of defects formation can be detected on the aluminum electrode, too. X-ray techniques (Laue, SAXS, WAXS, TEM, and SEM) all showed the formation of defects (vacancies, voids/pores, surface bubbles/ blisters, volume bubbles, and micro-cracks [16]) in electrochemically and chemically hydrogen-charged aluminum samples. The results suggest that the lattice expansion is much less in Al than in other fcc metals and intrinsic defect formation can take place in these kinds of materials. TEM techniques revealed the existence of interactions between surface bubbles (Fig. 6) between voids and dislocations. The aluminum with pressurized hydrogen formed a hydrogen–vacancy complex near the surface, which diffused into the volume and then clustered to form internal H2 bubbles. A large size distribution (from nanometers (voids with whiter contrasts from electron transparency)) to a few micrometers in diameter (bubbles, pointed by the arrow (Fig. 6)) and variability in the density of surface bubbles were found [15]. Between the densely packed surface bubbles, the formation of dense dislocations was seen. Internal bubbles with symmetrical bend contours contrast (contour contrast lines by tilting of the sample under the microscope are copied the topographical structure of the sample like: hills or valleys) and a dense distribution of dislocations were observed (Fig. 6). The tangled dislocation formation-related surface close to internal bubbles and small voids (30–50 nm in diameter, indicated by arrows in Fig. 6) can be formed from high H2 pressure inside the bubbles, 0.3–1000 lm in diameter, i.e. approximately found to be ph = 1.46 104 Pa. We obtained maximum stresses in the case r = a on the bubble surfaces, the compressive rr = 9.7 103 Pa and tensile stress rt = 1.2 104 Pa in the surface of the bubble after

In previous studies [15,14], X-ray (Laue), SAXS, and electron microscopy techniques such as TEM and SEM revealed the existence of surface bubbles from interactions between aluminum hydroxide and hydrogen, forming hydrogen–vacancy complexes at the surface. These complexes diffuse into the volume and then cluster to form internal H2 bubbles in the aluminum. These experiments revealed the existence of a significant distribution of hydrogen voids and bubbles on and under the surface, produced during electrochemical charging. A large variation in the size and distribution, from large (some a micrometer in diameter) to very small (nanometers in diameter) and variability in the density of the surface and internal bubbles was obtained. In the case of densely packed surface and internal bubbles, the formation of dense dislocations could be seen, as in Fig. 6. The formation of micro-cracks in high purity aluminum during electrochemical charging by hydrogen has been studied [16]. The experiments reveal that, in aluminum samples, a wide distribution of hydrogen hemispherical bubbles on the surface (blisters [40]) and below the surface voids in the volume was produced during electrochemical charging. This phenomenon can lead to the formation of micro-cracks in the absence of externally applied stress [16]. Examination of electrochemically charged samples by TEM showed micro-cracks with a typically ductile mode of fracture. Highly plastically deformed volumes ahead of the crack tip indicated the appearance of extremely high dislocation density zones. The anisotropic reflection intensity obtained in Fig. 5(c) is shown by the formation of exocentric scattering around the diffractions points or the formation of plastic deformations [16,40]. The depth of these defects was controlled by electrochemical charging and depended on the charging time. Hydrogen only poorly entered the aluminum lattice interstitially in a weak manner. Hydrogen diffusion occurs by forming defects as the result of high hydrogen fugacity at the surface during electrochemical charging. Either a small contraction or zero change in the lattice parameter resulted when high hydrogen concentrations were introduced into the aluminum matrix. In an Al–H solid solution, the vacancy concentrations become many orders of magnitude larger than those generated in the H-free lattice at temperatures below 350 K (77 °C) [41]. This is consistent with the formation of hydrogen–vacancy complexes at the surface. In the work of Watson et al. [42], on the study of cathodic charging effects on the mechanical properties of aluminum, it was found that cathodic charging produced a severely hardened surface region. The hardness in this region became quickly saturated, and further charging increased the depth of this region. After electrochemical charging for 24 h, the hydrogen determination was found to be a concentration of 1000 appm in aluminum at 300 K (27 °C) [14]. The concentrations of hydrogen introduced during electrochemical and chemical charging were monitored by gas extraction analysis. In this work, this was characterized by the actual distribution of deuterium (hydrogen) in electrochemically and chemically charged aluminum. This amount of hydrogen can enhance the formation of vacancies, voids/pores and bubbles at room temperature. The effect of charging time is to increase the depth of deuterium distribution under the surface of the aluminum [16]. The deuterium depth is controlled by the electrochemical charging conditions and the period of charging. The effective diffusivity of deuterium (hydrogen) depends on the interstitial conditions and traps (vacancies, voids, bubbles, and dislocations), the


defect distribution, and hydroxide bubble formation on the surface of the charged aluminum. Simmons and Balluffi [43] demonstrated a type of experiment that appears capable of giving direct information about the nature of the predominant defects in aluminum. These experiments measured in Ref. [17], differences between the fractional lattice parameter change, Da/a0, as measured by X-ray diffraction, and the linear dilatation of the specimen, Dl/l0 (in length) or Dw/w0 (in thickness), since defects are generated in the crystal containing a constant number of atoms. Briefly, Da/a0 differs from Dl/l0 since the X-rays measure only the average dilatation of the inter-atomic spacing, this occurs when point imperfections are generated, whereas the macroscopic length measures both this dilatation and the dimensional changes due to the creation or destruction of defects that are generated at the source in the crystal. The difference then gives a direct measure of the numbers of atomic sites created or destroyed. Surface formation of aluminum oxide is not taken into account since it has only a minor effect. For example, if vacancies are generated, new sites are created and (Dl/l0Da/a0) is positive. If interstitials are generated, atomic sites are destroyed and (Dl/l0Da/a0) is negative. Lattice thermal expansion does not contribute to the difference in a cubic crystal (fcc), because it makes an equal contribution to both Dl/l0 and Da/a0. In the case when the hydrogen interactions are on surface regions of the specimens, (Dl/l0Da/a0) can be written as (Dw/w0Da/a0) = (4.9 105 – (4.22 104)) = 4.71 104 for aluminum electrochemically charged with hydrogen for 24 h and as (Dw/w0 Da/a0) = (1.5 105 – (3.38 104)) = 3.53 104 for aluminum chemically charged with hydrogen for 24 h. These results can be accounted for by the formation of a lattice vacancy at the surface accompanying the introduction of an H solute, followed by diffusion of the Hvacancy complex into the volume [14]. In this mechanism, the vacancies are formed at the external surface and the additional lattice sites formed are introduced at the external surface, rather than at internal vacancy sources. In special individual situations where there are H/mono-vacancy defects, the volume change is calculated whereby Dv is the volume change, vAl is the free aluminum lattice volume, DvH is the volume change by hydrogen atoms in the aluminum lattice, Dvv is the volume change by vacancies in the aluminum lattice and CH and Cv are the concentrations of hydrogen and vacancies, respectively. Dvv/vAl = 0.49 is the magnitude of the lattice relaxation due to a vacancy in the aluminum. The Cv/CH values were calculated from Eq. (5) in [14]. While there was considerable scatter in the results, the observed low values of lattice change can be accounted for by values of Cv/CH between about 0.25 and 4, with the highest values of the vacancies being introduced under severe electrochemical charging conditions. As will be seen, there is evidence that the H-vacancy complexes do not remain as single defects, but rather cluster into voids and bubble defects in the aluminum matrix. Clustering would result in an increase in the observed values of Da/a0 and Dw/w0 on the order of 5 105, which could be observed in this study and hence in the values of Cv/CH. As was mentioned earlier paragraph there is direct experimental evidence of hydrogen bound to a vacancy in the aluminum lattice. A process of vacancy production and their trapping by the hydrogen atoms during surface reactions can be describe schematically in Fig. 7. In this case a vacancy is left behind an aluminum atom diffusing outward through a surface oxide film. A trapped by a hydrogen atom may disappear when the hydrogen–vacancy pair diffuses and arrives at a vacancy sink such as a dislocation. The remaining ‘‘equilibrium’’ concentration of vacancies trapped with hydrogen will be many orders of magnitude higher than the equilibrium concentrations of vacancies in the crystal free of atomic hydrogen [14].

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Fig. 7. Schematic view of hydrogen atom-lattice vacancy pair formation at an aluminum surface.

The measured values of thickness Dw/w0 after electrochemical and chemical charging were positive, i.e. there was an extension of the lateral specimen surface like in Ref. [44]. Small-angle scattering is a tool that could be value both as a rapid easy and accurate analytical tool to determine the properties of voids distributions in aluminum, Refs. [45–47]. It is well established that at large jRg form factor, the small-angle scattering from an isolated particle is primarily determined by the structure of the particle’s surface. Our work, in Fig. 5(b) and (e), shows a typical result for the [1 1 0] plane that are in agreement with results obtained for radiation defects in single crystal of Al, for small angle X-ray scattering, small angle neutron scattering and transmission electron microscopy (SAXS and SANS and TEM). A small angle neutron scattering (SANS) experiments should show several advantages over SAXS. No double Bragg scattering is possible. kn > 2dmax were dmax is the maximum lattice plane distance. No double Bragg 0 scattering is possible. For aluminum, this correspond to kn > 4.66 Å A. The neutron beam in Fig. 7 in Ref. [47] is perpendicular to the page and parallel to [1 1 0], averaged over truncated octahedral voids (Fig. 8); the size distribution of the defects in Fig. 5(b) and (e) is very like to the reference samples. For randomly oriented particles, the average form factor h|F(j)|2i behaves asymptotically as j4 (Porod’s law). It can be seen that for |jRg| < 4, the form factor is spherically symmetric, but above this value, it is highly asymmetric, with rods of high intensity extending in the [0 0 1], [1 1 0] and [1 1 1], [ 1 1 1] and their negative directions. These observations are in complete agreement with X-ray small angle scattering of Fig. 5(b) and (e) obtained in this study. Hydrogen in the aluminum solid solution at the concentration introduced in the present experiments is in a thermodynamically unstable state and should be eventually precipitate into gaseous

Fig. 8. One octant of a truncated octahedral void. The formula gives the volume of p the void when (5) t = b/a; (6) V(B,t) = 2/3(1 3t3)B3; the surface area (7) p p p S(B,t) = 2 3[1 + ( 3–3)t2]B2 and the radius of gyration (8) Rg(B,t) = 3/20(1– 10t3 + 15t4 8t5/t 3t3)1/2B.

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voids and bubbles. Vacancies clusters on {1 1 1}, {1 1 0} and {1 0 0} planes, which were found in SAXS experiments could be either a metastable phase containing atomic hydrogen or a hydrogen truncated octahedral void or hydrogen spherical bubbles. The former possibility seems to more likely since hydrogen hemispherical bubbles [40] on the surface, would provide perfect sink for migrating hydrogen atoms, resulting in a low bulk hydrogen concentration and bubbles lying close to the surface. 5. Conclusions 1. In the hydrogenation of aluminum under high fugacity conditions (such as electrochemical and chemical charging), nonsteady state diffusion produces concentration–distance profiles that cannot be calculated by assuming simple diffusion behavior. Moreover, hydrogen–vacancy interactions related to microstructural changes (defect formation) must be taken into account in the process of characterizing the state of hydrogen in aluminum. 2. Hydrogen entered the aluminum lattice interstitially in a weak manner. Hydrogen enters an aluminum matrix, accompanied by vacancies formed at the surface, under high fugacity conditions. Interactions occur between aluminum hydroxide and hydrogen, forming hydrogen–vacancy complexes at the surface, which diffuse into the volume and then cluster to form internal H2 bubbles in the aluminum. 3. The SIMS technique was used to characterize the hydrogen/ deuterium distribution in chemically charged aluminum. It was found that, during hydrogenation of Al, Al(OH)3 formed during electrochemical and chemical charging. The advantages of this method lie in obtaining actual concentration–depth profiles, including micro-structural changes such as defects (vacancies, voids, bubbles, micro-cracks, dislocations, and surface oxides) that form during electrochemical and chemical reactions in aluminum with aqueous solutions. 4. Information regarding defects was obtained by using X-ray small-angle techniques, indicating that anisotropic intensity distribution is consistent with scattering from a truncated octahedral symmetry. Annealing at 473 K (200 °C) after electrochemical and chemical charging for 2 h, led to a decrease in the intensity of anisotropic scattering and a change into circular concentric isotropic spheres. The interpretation of this behavior is in obtaining lower quantity and higher dimension pores, with a more spherical geometry. 5. TEM techniques revealed the existence of surface bubbles from interactions between aluminum hydroxide and hydrogen and form a hydrogen–vacancy complex at the surface, this diffuses into the volume and then clusters to form internal H2 bubbles. 6. The rate of movement of the deuterium profile inward from the aluminum surface is controlled by electrochemical and chemical charging and is a function of the charging time. The deuterium concentration decreased in intensity and slightly increased in the depth of penetration in electrochemically charged aluminum aged at various temperatures. The quantity of vacancies increased during annealing since the hydrogen vacancies interacted to form spherical clusters during aging. 7. An interpretation of the two dimensional asymmetric iso-intensities obtained by SAXS and the computation of |Ftx|2 amplitudes for all octants was coded obtain iso-intensity in the [1 1 0] plane in reciprocal space. A truncated octahedral symmetry was obtained from hydrogen (deuterium) filled voids in electrochemically and chemically charged single crystal and polycrystalline aluminum.

Acknowledgment The author would like to thanks the Oak Ridge National Laboratory at Tennessee, USA, for permitting us to use the SAXS equipment.

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