Chemical reactions in cracks of aluminum crystals: Generation of hydrogen from water

Watanabe, Minoru

doi:10.1016/j.jpcs.2010.05.002

Cited by 21 publications

(17 citation statements)

References 6 publications

(9 reference statements)

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“…Although both anodic dissolution (AD) and grain boundary sliding (GBS) with apparent activation energies of 80 through 100 kJ/mol and a hydrogen diffusion based process with apparent activation energies around 20 kJ/mol are candidate processes, it is difficult to identify processes with apparent activities ranging from 35 to 40 kJ/mol, other than a water exchange reaction for a hydrated Al(III) complex, Al(H 2 O) 6 3+ (aq), involving a de-protonation of a single water molecule to generate the complex AlOH(H 2 O) 5 2+ [99,100] and recently published values for the generation and decomposition of aluminum hydride, AlH 3 . [101] It is possible that although crack propagation in Al-Zn-Mg-Cu alloys occurs by a hydrogen related process, the rate-controlling process under specific circumstances is determined by local environmental and alloy microstructural conditions, which are influenced by the alloy chemistry, and/or the prevailing loading conditions and temperature. For instance, the supply of hydrogen may be dictated by an electrochemical reaction and/or the availability of local reactive surfaces generated by GBS and be rate limiting (E a , 80 through 100 kJ/ mol) or a process within the alloy microstructure to utilize locally available hydrogen could be the ratecontrolling process (E a~4 0 kJ/mol), as recently proposed for crack growth in distilled water.…”

Section: -T7351 -~10mentioning

confidence: 99%

Stress Corrosion Cracking in Al-Zn-Mg-Cu Aluminum Alloys in Saline Environments

Holroyd

Scamans

2012

Metall Mater Trans A

108

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Stress corrosion cracking of Al-Zn-Mg-Cu (AA7xxx) aluminum alloys exposed to saline environments at temperatures ranging from 293 K to 353 K (20°C to 80°C) has been reviewed with particular attention to the influences of alloy composition and temper, and bulk and local environmental conditions. Stress corrosion crack (SCC) growth rates at room temperature for peak-and over-aged tempers in saline environments are minimized for Al-Zn-Mg-Cu alloys containing less than~8 wt pct Zn when Zn/Mg ratios are ranging from 2 to 3, excess magnesium levels are less than 1 wt pct, and copper content is either less than~0.2 wt pct or ranging from 1.3 to 2 wt pct. A minimum chloride ion concentration of~0.01 M is required for crack growth rates to exceed those in distilled water, which insures that the local solution pH in crack-tip regions can be maintained at less than 4. Crack growth rates in saline solution without other additions gradually increase with bulk chloride ion concentrations up to around 0.6 M NaCl, whereas in solutions with sufficiently low dichromate (or chromate), inhibitor additions are insensitive to the bulk chloride concentration and are typically at least double those observed without the additions. DCB specimens, fatigue pre-cracked in air before immersion in a saline environment, show an initial period with no detectible crack growth, followed by crack growth at the distilled water rate, and then transition to a higher crack growth rate typical of region 2 crack growth in the saline environment. Time spent in each stage depends on the type of precrack (''pop-in'' vs fatigue), applied stress intensity factor, alloy chemistry, bulk environment, and, if applied, the external polarization. Apparent activation energies (E a ) for SCC growth in Al-Zn-Mg-Cu alloys exposed to 0.6 M NaCl over the temperatures ranging from 293 K to 353 K (20°C to 80°C) for under-, peak-, and over-aged low-copper-containing alloys (<0.2 wt pct) are typically ranging from 80 to 85 kJ/mol, whereas for high-copper-containing alloys (>~0.8 wt pct), they are typically ranging from 20 to 40 kJ/mol for under-and peak-aged alloys, and based on limited data, around 85 kJ/mol for over-aged tempers. This means that crack propagation in saline environments is most likely to occur by a hydrogen-related process for low-copper-containing Al-Zn-Mg-Cu alloys in under-, peak-and over-aged tempers, and for high-copper alloys in under-and peak-aged tempers. For over-aged high-copper-containing alloys, cracking is most probably under anodic dissolution control. Future stress corrosion studies should focus on understanding the factors that control crack initiation, and insuring that the next generation of higher performance Al-Zn-Mg-Cu alloys has similar longer crack initiation times and crack propagation rates to those of the incumbent alloys in an over-aged condition where crack rates are less than 1 mm/month at a high stress intensity factor.

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Section: -T7351 -~10mentioning

confidence: 99%

Stress Corrosion Cracking in Al-Zn-Mg-Cu Aluminum Alloys in Saline Environments

Holroyd

Scamans

2012

Metall Mater Trans A

108

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“…The situation for crack propagation may differ in that the presence of a sharp crack itself may provide local conditions needed to facilitate the generation of local triaxial stresses ahead of the propagating crack tip or provide a local nanoscale environment for chemical reactions that do not readily occur on a free surface. [15,16] Support for the strain-rate dependency and a role for an internal process occurring ahead of a crack tip during intergranular sustained-load crack growth is provided by Ciaraldi et al [17] They generated intergranular cracking in a peak-aged high-purity Al-5.6Zn-2.6Mg alloy after subjecting test samples to several days pre-exposure in water vapor saturated air (WVSA) at 343 K (70°C) and, subsequently, tensile testing using a nominal strain rate of~10 À4 /s in dry argon. Their detection by electron diffraction of a thin layer of aluminum hydride (AlH 3 ) on the intergranular fracture surfaces and the lack of any intergranular cracking in the samples tested at a higher nominal strain rate of 10 À2 /s led them to conclude that the process leading to the intergranular embrittlement must have occurred while the specimen was being slowly strained and the observed embrittlement was associated with the generation of strain-induced aluminum hydride in the grain boundaries.…”

Section: -T651mentioning

confidence: 99%

“…[15,16] One such chemical reaction is aluminum hydride (AlH 3 ) generation within the confines of sharp cracks in aluminum at temperatures above~313 K (40°C), whereas the expected hydrated alumina surface film immediately forms on all external surfaces. The formation of AlH 3 introduced local stresses in the crack-tip region, [15] and cracks continued to grow when left exposed to moist air at room temperature after exposure to water at temperatures above 313 K (40°C). [16] These findings strongly support the proposal made by Ciaraldi et al [17] in the early 1980s that intergranular slow crack growth for high-purity Al-Zn-Mg alloys exposed to moist air involves the formation and rupture of a stressinduced intergranular aluminum hydride.…”

Section: Introductionmentioning

confidence: 99%

Crack Propagation During Sustained-Load Cracking of Al-Zn-Mg-Cu Aluminum Alloys Exposed to Moist Air or Distilled Water

Holroyd¹,

Scamans

2011

Metall Mater Trans A

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Intergranular sustained-load cracking of Al-Zn-Mg-Cu (AA7xxx series) aluminum alloys exposed to moist air or distilled water at temperatures in the range 283 K to 353 K (10°C to 80°C) has been reviewed in detail, paying particular attention to local processes occurring in the crack-tip region during crack propagation. Distinct crack-arrest markings formed on intergranular fracture faces generated under fixed-displacement loading conditions are not generated under monotonic rising-load conditions, but can form under cyclic-loading conditions if loading frequencies are sufficiently low. The observed crack-arrest markings are insensitive to applied stress intensity factor, alloy copper content and temper, but are temperature sensitive, increasing from~150 nm at room temperature to~400 nm at 313 K (40°C). A re-evaluation of published data reveals the apparent activation energy, E a for crack propagation in Al-Zn-Mg(-Cu) alloys is consistently~35 kJ/mol for temperatures above~313 K (40°C), independent of copper content or the applied stress intensity factor, unless the alloy contains a significant volume fraction of S-phase, Al 2 CuMg where E a is~80 kJ/mol. For temperatures below~313 K (40°C) E a is independent of copper content for stress intensity factors below~14 MNm À3/2 , with a value~80 kJ/mol but is sensitive to copper content for stress intensity factors abovẽ 14 MNm À3/2 , with E a , ranging from~35 kJ/mol for copper-free alloys to~80 kJ/mol for alloys containing 1.5 pct Cu. The apparent activation energy for intergranular sustained-load crack initiation is consistently~110 kJ/mol for both notched and un-notched samples. Mechanistic implications are discussed and processes controlling crack growth, as a function of temperature, alloy copper content, and loading conditions are proposed that are consistent with the calculated apparent activation energies and known characteristics of intergranular sustained-load cracking. It is suggested, depending on the circumstances, that intergranular crack propagation in humid air and distilled water can be enhanced by the generation of aluminum hydride, AlH 3, ahead of a propagating crack and/or its decomposition after formation within the confines of the nanoscale volumes available after increments of crack growth, defined by the crack arrest markings on intergranular fracture surfaces.

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“…Watanabe [48] studied the mechanism of the aluminium-water reaction in aluminium powders with particle sizes in the micron and sub-micron range, obtained by mechanical grinding, and came to the following conclusions. Micro-cracks formed at grain boundaries at the surface of the metal particles grow inward, due to corrosion by water.…”

Section: Performance Of the Aluminium-water Reactionmentioning

confidence: 98%