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Hydrogen Embrittlement of Magnesium and Magnesium Alloys

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Hydrogen Embrittlement of Magnesium and Magnesium Alloys ( hydrogen-embrittlement-magnesium-and-magnesium-alloys )

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C168 Journal of The Electrochemical Society, 160 (4) C168-C178 (2013) 0013-4651/2013/160(4)/C168/11/$31.00 © The Electrochemical Society Hydrogen Embrittlement of Magnesium and Magnesium Alloys: A Review Mariano Kappes,a,b,∗ Mariano Iannuzzi,a,z and Ricardo M. Carranzab aNational Center for Education and Research on Corrosion and Materials Performance (NCERCAMP), The University of Akron, Akron, Ohio 44325, USA bComisio ́n Nacional de Energ ́ıa Ato ́mica, Instituto Sabato (UNSAM/CNEA), Buenos Aires 1650, Argentina Magnesium and magnesium alloys are susceptible to stress corrosion cracking in various environments, including distilled water. There is compelling evidence to conclude that SCC is assisted, at least in part, by hydrogen embrittlement. This paper reviews the thermodynamics of the Mg-H system and the kinetics of hydrogen transport. Aspects of magnesium corrosion relevant to hydrogen absorption are also discussed. Crack growth mechanisms based on delayed hydride cracking, hydrogen adsorption dislocation emission, hydrogen enhanced decohesion, and hydrogen enhanced localized plasticity have been proposed and evidence for each of them is reviewed herein. © 2013 The Electrochemical Society. [DOI: 10.1149/2.023304jes] All rights reserved. Manuscript submitted October 22, 2012; revised manuscript received January 16, 2013. Published February 26, 2013. Pure magnesium is inherently susceptible to stress corrosion crack- ing (SCC)1–3 and many of its alloys suffer SCC in environments con- sidered innocuous for most other engineering alloys, e.g. distilled water.4,5 In order to prevent SCC, some authors suggest that the ap- plied stress has to be kept below 50% of the yield strength (YS),6,7 reducing the attractiveness of Mg alloys for structural applications. Pure magnesium1–3,8 and magnesium-aluminum alloys5,9–21 have received special attention in the literature. The susceptibility of mag- nesium alloys to SCC increases with increasing aluminum content,17 a trend that is opposite to the beneficial effect of Al in stress-free corrosion rates. Precipitation of β phase Mg17 Al12 occurs in alloys with more than 2.1 wt% Al.16 Since Mg17Al12 is nobler than the Mg-Al matrix,11,22 precipitation of Mg17Al12 phase along grain boundaries11,16,20 is thought to promote intergranular SCC (IGSCC) caused by prefer- ential galvanic dissolution of the surrounding matrix.16 The effect of second phase particles is discussed in detail in a separate section. Extremely fast crack growth rates (i.e. in the order of 10−5 m/s) have been often reported for pure Mg2 and Mg-Al alloys.14,23 If crack- ing was entirely controlled by faradaic anodic dissolution, such crack growth rates would require anodic current densities in the order of 50 A/cm2.14 Therefore, most SCC models include some contribution of mechanical assisted fracture. Even though there is compelling evidence supporting a hydrogen- assisted crack propagation mechanism,1,2,13 there is no agreement regarding the exact nature of the H-metal-atom interactions leading to embrittlement. Mechanisms based on hydride formation, hydrogen enhanced decohesion, localized hydrogen enhanced plasticity, and hy- drogen adsorption induced dislocation emission have been proposed and are reviewed in this paper. Validation of those mechanisms re- quires reliable hydrogen diffusion and hydrogen solubility data, which is also discussed herein. The next sections present a critical review of the current state of knowledge on SCC of magnesium and magnesium alloys, with emphasis on hydrogen embrittlement mechanisms. Thermodynamics of the Hydrogen–Magnesium System Magnesium, a hydride-forming metal.—Mg and H form MgH2,24–26 a stoichiometric compound27 that has a tetragonal structure28 and decomposes at temperatures above 287◦C25 under a H2 pressure of 101.3 kPa. In addition to its implications in SCC mech- anisms, the formation of magnesium hydrides is of great interest to solid state hydrogen storage research.24 The MgH2 dissociation tem- perature, as well as other features of the Mg-H phase diagram, depend on the H2 partial pressure.26,27 ∗Electrochemical Society Active Member. zE-mail: mi@uakron.edu The nucleation and growth of MgH2 during exposure of magne- sium to gaseous H2 at high pressure and temperature (∼5 MPa and ∼300–400◦C) has been extensively studied29–32 due to its application as a solid state hydrogen storage medium. Magnesium hydride has a high hydrogen absorption capacity, with a theoretical limit of 7.6 wt%. However, magnesium has a high temperature of hydrogen discharge24 and slow absorption/desorption kinetics.24,33,34 Magnesium is combined with alloying elements by ball milling to enhance its hydriding/de-hydriding kinetics and to reduce the stabil- ity of the hydride.24 Alloying with aluminum increases the kinetics of H absorption and desorption33 during gas charging at 400◦C and 3.8 MPa. Those properties were highest for a composition near that of Mg17 Al12 . This intermetallic compound, upon hydriding, decom- posed into MgH2 and Al, and the reaction was reversible upon dehydriding.33 Downloaded on 2016-10-19 to IP 129.241.220.220 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract). Hydrogen solubility in Mg.— There are several complications in determining hydrogen solubility in magnesium. First, MgH , 2 if formed as a surface layer, can limit the kinetics of hydrogen absorption35,36 due to the extremely low diffusivity of hydrogen in MgH2 (on the order of 10−16 m2 /s at 25◦ C 28 ). Second, magnesium has a high vapor pressure and is volatile.37,38 For example, as shown by Zeng and coworkers, a sample of magnesium can lose up to 1.5% of its weight during 1 h heating at 550◦C.37 Finally, a film of mag- nesium hydroxide, Mg(OH)2 often present due to unintentional at- mospheric exposure, can also hinder hydrogen absorption. Mg(OH)2 decomposes to magnesium oxide (MgO) and hydrogen gas above 440◦ C.28,39 Popovic39 showed that a pre-heating at 600◦ C was nec- essary for hydrogen absorption at lower temperatures. This suggests that the hydroxide is more effective in blocking hydrogen ingress than the oxide. San Martin and Manchester26 and Okamoto25 reviewed the Mg- H system in detail. The authors suggested that, at a H2 pressure of 1 bar and in the temperature range between 175◦C and the melting point (i.e. 650◦C), H solubility was between 0.005 and 0.07 at%. The dependence of solubility with temperature (in at%) was given by:26 􏰙 22,780 􏰚 S(at%) = 0.0023 + 1.28 · exp [1] R·T Extrapolation to room temperature yields a solubility value of 0.002 at%. Table I summarizes reported hydrogen solubility values in magnesium. Krozer and Kazemo40 and Popovic et al.39 reported that H sol- ubility in Mg followed Sievert’s law,41 which states that hydrogen solubility in the metal lattice is proportional to the square root of the hydrogen partial pressure. In a recent review, Zeng et al.37 stated that the reported heat of solution values for hydrogen in magnesium range between 20 and 24 kJ/0.5 mol H2 for temperatures between 196 and

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