Elsevier

Journal of Alloys and Compounds

Volume 587, 25 February 2014, Pages 800-806
Journal of Alloys and Compounds

Letter
States of hydrogen and deuterium in chemically charged high purity aluminum

https://doi.org/10.1016/j.jallcom.2013.10.229Get rights and content

Abstract

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.

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], [3], [4], [5], [6], [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], [9], [10]. During cooling and solidification, dissolved hydrogen exists in excess of the extremely low 10−4 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]. 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 micro-tomography has been used to observe hydrogen micro-pores and their shrinkage, annihilation, reinitiation, and growth behaviors during hot and cold plastic deformation and subsequent high-temperature 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], [15], [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 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 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]:2Al++++3H2O=Al2O3+6H+Al2O3+H2O=2AlO2-+2H+Al=Al++++3e-Al+2H2O=AlO2-+4H++3e-

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 γ-Al2O3⋅H2O or böhmite, crystallizing in the rhombohedral system, then the trihydrate Al2O3⋅3H2O 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], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [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.

Section snippets

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

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

Discussion

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

Conclusions

  • 1.

    In the hydrogenation of aluminum under high fugacity conditions (such as electrochemical and chemical charging), non-steady state diffusion produces concentration–distance profiles that cannot be calculated by assuming simple diffusion behavior. Moreover, hydrogen–vacancy interactions related to micro-structural 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

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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