Hydrogen Diffusivity During Corrosion of High-Purity Aluminum

Ai, Jiahe; Scully, John R.

doi:10.5006/0826

Cited by 13 publications

(9 citation statements)

References 54 publications

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“…The effect of temperature on the attempt frequency (ν * As explained previously, only H diffusivity was studied. The activation energy value that was found (0.14 eV, fitted on the diffusivity) is slightly lower than the experimental one (0.17 eV, [18] and references therein). The activation energy we found includes both migration energies (t-o and o-t), that which explains why our value is slightly different from the migration energies.…”

Section: Diffusion Equationscontrasting

confidence: 59%

Diffusion of interstitials in metallic systems, illustration of a complex study case: aluminum

David

Connétable

2017

J. Phys.: Condens. Matter

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While diffusion mechanisms of interstitial elements in fcc systems are generally well-known, especially in the case of H atoms, we show in this work that even in the case of a simple metallic system (aluminum), the diffusion of interstitials exhibits a wide variety of paths and mechanisms that depend on the specie. We used an approach based on first-principles calculations associated with kinetic Monte-Carlo simulations and a multi-state diffusion formalism to compute the diffusion coefficients of five interstitial elements: hydrogen, boron, carbon, nitrogen and oxygen. For instance, at the atomic scale, whilst we find that C atoms prefer to be located in octahedral sites (labeled $o$) rather than in tetrahedral positions (labeled $t$), we find one additional stable position in the lattice ($M$). The diffusion through these three stable positions are thus studied in detail. In the case of B atoms, for which the tetrahedral site is found unstable, the diffusion path is between $o$-$o$ sites. Similarly, in the case of oxygen, $t$ positions are found to be the only stable positions ($o$ are unstable) and the path of migration, along $t$-$t$ direction, is found through a twice degenerated asymmetric transition state. In the case of H and N atoms for which $t$ and $o$ sites are stable, we explain why the only path is along the $t$-$o$ direction. Finally, we discuss explicit formulas to compute coefficients of diffusion of interstitials in fcc structures. .

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Section: Diffusion Equationscontrasting

confidence: 59%

Diffusion of interstitials in metallic systems, illustration of a complex study case: aluminum

David

Connétable

2017

J. Phys.: Condens. Matter

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“…such that after hydrogen exposure there should be superabundant vacancies inside the aluminium pillar. The diffusion should then involve the migration of hydrogen-vacancy complexes, which are energetically more stable and also move much more sluggishly than hydrogen interstitials343536. Such gas-stabilized vacancies and cavities are indeed well known in radiation damage37, which also cause plastic flow localization and embrittlement.…”

Section: Resultsmentioning

confidence: 99%

Hydrogenated vacancies lock dislocations in aluminium

Xie

et al. 2016

Nat Commun

151

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Due to its high diffusivity, hydrogen is often considered a weak inhibitor or even a promoter of dislocation movements in metals and alloys. By quantitative mechanical tests in an environmental transmission electron microscope, here we demonstrate that after exposing aluminium to hydrogen, mobile dislocations can lose mobility, with activating stress more than doubled. On degassing, the locked dislocations can be reactivated under cyclic loading to move in a stick-slip manner. However, relocking the dislocations thereafter requires a surprisingly long waiting time of ∼103 s, much longer than that expected from hydrogen interstitial diffusion. Both the observed slow relocking and strong locking strength can be attributed to superabundant hydrogenated vacancies, verified by our atomistic calculations. Vacancies therefore could be a key plastic flow localization agent as well as damage agent in hydrogen environment.

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“…Based on observations reported for other Al alloys, this structure would be between 0.5 μm and 1 μm in diameter, and is free of dislocations inside the cell [88]. The dislocation structure developed during crack tip straining is likely the prime candidate for H trapping that governs DH-EFF in Al alloys with Mg [72] or without Mg [68,87,89]. When spacing between β is shorter than the size of the LEDS, the ligament of Al-Mg between β should be dislocation free and thus H-trap free.…”

Section: The Role Of β In Developing Crack Tip Chemistrymentioning

confidence: 86%

“…Moreover, recent thermal desorption spectroscopy showed similar trapping states for pure Al and 5083-H131 at various DoS, with a broad desorption peak consistent with the 20-30 kJ/mol binding energy [72,87]. Speculatively, H traps at α-β and α-α interfaces with a similar binding energy of about 25 kJ/mol (EGB for α-α), though there is no evidence to suggest that trap sensitive permeation changes with DoS [72].…”

Section: The Role Of β In Developing Crack Tip Chemistrymentioning

confidence: 96%

“…With a higher volume of β dissolution, these regions are amplified and lead to the critical acidity required for a rapidly growing occluded crack tip and high ηH. [72,87], and (2) dislocation structure change in the local microstructure. Speculatively, during straining a low energy dislocation structure (LEDS) may develop ahead of a crack tip.…”

Section: The Role Of β In Developing Crack Tip Chemistrymentioning

confidence: 99%

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Validation of the Coupled Dissolution-Hydrogen Embrittlement Mechanism of IGSCC in Low Temperature Sensitized AA5083-H131

Crane¹

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Non-heat-treatable wrought Al-Mg alloys are commonly used in marine structures that require light weight, moderate strength, weldability, and corrosion resistance. These alloys become susceptible to intergranular corrosion (IGC) and intergranular stress corrosion cracking (IGSCC) when an active β phase (Al3Mg2) precipitates on grain boundaries during prolonged thermal exposure. The Degree of Sensitization (DoS) is quantified using the standard nitric acid mass loss test. Coupled dissolution of β and H embrittlement is the hypothesized mechanism of IGSCC in sensitized Al-Mg. The primary objectives of this research are to characterize the interactive crack tip electrochemical and mechanical factors that govern IGSCC in Al-Mg alloy 5083-H131, and particularly to test this hypothesized mechanism. Research is focused on low temperature, long time sensitization with discrete grain boundary β precipitation. High resolution subcritical crack growth experiments established a critical DoS of about 10 mg/cm 2 for IGSCC susceptibility in 5083-H131 stressed at near open circuit potential in neutral NaCl. Below this DoS, the alloy is essentially immune to IGSCC, while above this sensitization, crack growth rates increase as a unique function of DoS for several time-temperature conditions. This threshold is likely due to an acidic crack tip chemistry established by the dissolution of a critical volume of β, in combination with an increase in local stress for increased H concentration ahead of the crack tip due to β presence. As β volume increases and spacing decreases, these factors continue to increase crack growth rates. Crack growth measurements and elastic-plastic Jintegral analysis establish that this alloy is susceptible to H embrittlement in the absence of β phase, provided that an acidic chemistry is artificially generated in the crack tip, as an analog to β dissolution. The role of β is verified using polarization to either promote or preclude β dissolution. Anodic polarization significantly enhances crack growth, while cathodic polarization suppresses IGSCC because both β and matrix dissolution are limited. Results demonstrate the strong benefit of modest cathodic polarization as a means to control IGSCC. The role of H is separated from β dissolution through hydrogen precharging of tensile specimens stressed in the absence of a β-dissolving crack tip electrolyte. Results demonstrate that, though β is not essential for hydrogen environment embrittlement, β presence promotes H damage. The coupled dissolution H-embrittlement mechanism for IGSCC in 5083-H131 is validated by demonstrating that: a) β distribution controls crack tip chemistry and H localization at the fracture process zone, b) crack tip electrochemistry provides the H production and uptake for grain boundary decohesion, and c) H-diffusion, rate-limited embrittlement is the dominant mechanism for crack growth between β. Unexpectedly rapid IGSCC rates can be explained based on enhanced H diffusion in this ligament as trapping impedance is reduced. The...

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Hydrogen Diffusivity During Corrosion of High-Purity Aluminum

Cited by 13 publications

References 54 publications

Diffusion of interstitials in metallic systems, illustration of a complex study case: aluminum

Diffusion of interstitials in metallic systems, illustration of a complex study case: aluminum

Hydrogenated vacancies lock dislocations in aluminium

Validation of the Coupled Dissolution-Hydrogen Embrittlement Mechanism of IGSCC in Low Temperature Sensitized AA5083-H131

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