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Rapid Diffusion and Nano-Segregation of Hydrogen in Magnesium Alloys from Exposure to Water Michael Patrick Brady, Anton V. Ievlev, Mostafa Fayek, Donovan N Leonard, Matthew Frith, Harry M. Meyer, Anibal J. Ramirez-Cuesta, Luke L. Daemen, Yongqiang Cheng, Wei Guo, Jonathan Poplawsky, Olga S. Ovchinnikova, Jeffrey Thomson, Lawrence M. Anovitz, Gernot Rother, Dongwon Shin, Guang-Ling Song, and Bruce Davis ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b10750 • Publication Date (Web): 10 Oct 2017 Downloaded from http://pubs.acs.org on October 13, 2017
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ACS Applied Materials & Interfaces
Rapid Diffusion and Nano-Segregation of Hydrogen in Magnesium Alloys from Exposure to Water Michael P. Brady1,*, Anton V. Ievlev1, Mostafa Fayek2, Donovan N. Leonard1, Matthew Frith1, Harry M. Meyer III1, Anibal J. Ramirez-Cuesta1, Luke L. Daemen1, Yongqiang Cheng1, Wei Guo1, Jonathan Poplawsky1, Olga S. Ovchinnikova1, Jeffrey Thomson1, Lawrence M. Anovitz1, Gernot Rother1, Dongwon Shin1, Guang-Ling Song1,†, 3Bruce Davis 1
Oak Ridge National Laboratory (ORNL), Oak Ridge TN USA 37831
2
University of Manitoba (UM), Department of Geological Sciences, Winnipeg, MB R3T 2N2
Canada 3
Magnesium Elektron North America (MENA), Madison, IL USA 62060
KEYWORDS: magnesium, corrosion, hydrogen, stress corrosion cracking (SCC), hydrogen storage
ABSTRACT: Hydrogen gas is formed when Mg corrodes in water; however, the manner and extent to which the hydrogen may also enter the Mg metal is poorly understood.
Such
knowledge is critical as stress corrosion cracking (SCC)/embrittlement phenomena limit many otherwise promising structural and functional uses of Mg. Here we report via D2O/D isotopic tracer and H2O exposures with characterization by secondary ion mass spectrometry (SIMS),
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inelastic neutron scattering (INS) vibrational spectrometry, electron microscopy, and atom probe tomography (APT) techniques direct evidence that hydrogen rapidly penetrated tens of microns into Mg metal after only 4 h exposure to water at room temperature. Further, technologically important microalloying additions of < 1 wt.% Zr and Nd used to improve the manufacturability and mechanical properties of Mg significantly increased the extent of hydrogen ingress, whereas Al additions in the 2-3 wt.% range did not. Segregation of hydrogen species was observed at regions of high Mg/Zr/Nd nanoprecipitate density and at Mg(Zr) metastable solid solution microstructural features. We also report evidence that this ingressed hydrogen was unexpectedly present in the alloy as nanoconfined, molecular H2.
These new insights provide a basis for
strategies to design Mg alloys to resist SCC in aqueous environments, as well as potentially impact functional uses such as hydrogen storage where increased hydrogen uptake is desired.
1. INTRODUCTION Magnesium is of great interest for applications ranging from lightweight, fuel-efficient vehicles, to bioresorbable medical implants, components in fuel cells and batteries, and hydrogen storage materials1-11. A major challenge is to control the high corrosivity of Mg alloys under aqueous conditions1-7,10,11. Magnesium is susceptible to multiple forms of corrosion, including localized corrosion, galvanic corrosion, and stress corrosion cracking (SCC)1-5,10,11. The aqueous corrosion of Mg involves both metal dissolution and oxide-hydroxide film growth phenomena, accompanied by often extensive evolution of hydrogen gas1-7,10,11. However, the extent and manner to which hydrogen may also enter the Mg metal during corrosion are poorly understood15
. Such understanding is critical to achieving a more complete picture of the technologically
important corrosion reaction of Mg with water, as well as to provide a fundamental basis to
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develop strategies to mitigate detrimental phenomena such as SCC that can limit the use of Mg alloys.
We recently pursued a corrosion study of wrought commercial Mg alloys AZ31B (Mg-3Al-1Zn0.25Mn weight percent, wt.%) and ZE10A (Mg-1.5Zn-0.25Zr-< 0.5Nd wt.%) relative to cast pure Mg using isotopic tracer waters (D2O and H218O) in combination with depth profiling secondary ion mass spectrometry (SIMS)12,13. This work revealed inward oxygen and hydrogen film growth behavior for both AZ31B and ZE10A. Unexpectedly, this work also suggested that hydrogen penetrated tens of microns into the alloy after only short-term (4 h) exposures in water at room temperature, particularly for the Zr- and Nd- containing ZE10A. The goal of the present work was to confirm this possible hydrogen uptake behavior, and elucidate the mechanism of hydrogen ingress. 2. MATERIALS AND METHODS 2.1 Alloys and Exposure Conditions A series of cast model Mg-X (X =Al, Nd, Zr) alloys was selected for study, with compositions based on wrought commercial alloys AZ31B and ZE10A, along with cast high-purity (HP) Mg as a baseline control. Analyzed compositions and estimated average grain sizes (lineal intercept method) of the materials studied are presented in Table 112-14. Test sample preparation and D2O exposures followed previous procedures12,13 and are summarized here.
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Commercial wrought plates ∼28cm x 28cm x 2.5cm of AZ31B and ZE10A (Elektron® 717) were obtained from Magnesium Elektron North America (MENA), Madison, IL USA. Test samples were electric discharge machined (EDM) cut from the same alloy plates as were used in references 12 and 13. The model Mg-X alloys were laboratory scale castings from MENA: ∼5cm x 5cm x 7.6cm blocks for Mg-2.1Al, Mg-0.14Nd, and Mg-0.19Zr wt.%, and a 2.5cm diameter rod casting for Mg-0.46Zr wt.%. The HP Mg was from a 15mm diameter rod casting, nominal six 9s purity molecular beam epitaxy grade Mg, from United Mineral & Chemical Corp., Lyndhurst, NJ USA (same source casting as used in references 12 and 13). Disk test samples 9 10mm diameter and 1 to 1.5mm thick were EDM cut, wet ground to a P1200 grit finish using SiC paper, cleaned (acetone and deionized water), dried (air stream), and stored in a dessicator at least 24 h prior to D2O exposures.
The Mg disks were immersed in 5 ml D2O (Cambridge Isotope Laboratories, Inc., Andover, MA USA, 99.96 at.% isotope purity) for 4 h. The disks were laid flat on the bottom of a plastic beaker half-filled with D2O, open to ambient air and loosely covered (no significant evaporation of the D2O was observed). Characterization was only pursued on the top-exposed disk sample faces. It should be noted that the pH increased from ∼7 to ∼10 during this exposure protocol due to dissolution and rapid saturation of Mg in the small volume of test solution12,13. The higher pH leads to conditions favoring film growth over dissolution10,12,13. This exposure protocol repeated our initial D216O and H218O tracer studies of Mg corrosion film growth, which yielded the first indications of hydrogen penetration into the alloy12,13. In that work (and the present work), buffering of the test solution with Mg(OH)2 was not used due to concerns regarding additional introduction of
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O or H in the tracer water exposures.
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Table 1. Estimated average alloy grain size (GS) (lineal intercept method) and chemical composition by inductively coupled plasma atomic emission spectroscopy (ICP-AES) (ZE10A, AZ31B, and HP Mg data from references 12, 13, 27). Additional impurities < 0.01 wt.% not reported. The ZE10A contains < 0.5 wt.% Nd, the exact Nd level is proprietary. w=wrought, c = as-cast, bal. = balance. Alloy
ZE10A (w) AZ31B (w) HP Mg (c) Mg0.46Zr (c) Mg0.19Zr (c) Mg0.14Nd (c) Mg2.1Al (c) Mg foil ZE10A foil
GS (µm)
Composition Wt.% Al
