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Voids

Injection of Vacancy-Type Defects and Hydrogen by Metallic Corrosion Kurt R. Hebert, Iowa State University, DMR 0605957. AlH 3 + 3H 2 O + OH -  Al(OH) 4 - + 3H 2.

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Voids

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  1. Injection of Vacancy-Type Defects and Hydrogen by Metallic Corrosion Kurt R. Hebert, Iowa State University, DMR 0605957 AlH3 + 3H2O + OH-Al(OH)4- + 3H2 • Aluminum dissolution in reactive environments creates a subsurface region with elevated hydrogen chemical potential (μH) and tensile stress, and containing hydride (AlH3) and voids. • Hydride, voids and hydrogen can promote degradation • Voids act as localized corrosion sites when exposed by further dissolution • These events are attributable to formation of vacancy-hydrogen defects (e. g. Vac-H3) • First-principles calculations show that Vac-Hn defects favored at high μH conditions present during dissolution AlH3 AlH3 H2O/NaOH (μH small) H2 H2 Al (high μH σxx large) Al + 3H2O +OH-  Vac-H3 + Al(OH)4- Al + Vac-H3  AlH3 H H Voids ~ 100 nm In situ stress measurements Electron microscopy / positron measurements DFT / thermodynamic calculations H interstitial/Vac-H2 Vac-H/Vac-H2 Concentrations of Vac-H and Vac-H2 defects increase with μH EM and PAS reveal subsurface voids. In TEM, tilt response reveals that objects at white arrows are voids. Tensile stress shift correlates with appearance of hydride, indicates injection of vacancies

  2. Injection of Vacancy-Type Defects and Hydrogen by Metallic Corrosion Kurt R. Hebert, Iowa State University, DMR 0605957 We have developed a finite element simulation of transport in anodic oxide films by coupled electrical migration and viscous flow. The hypothesis of viscous flow was successfully validated by comparison to prior tracer measurements. Predictions of localized tensile stress near nanoscale ridges explains the formation of voids in the oxide film at these sites. Oxide voids such as these are known to act as sites for localized corrosion initiation (images from Houser JE, Hebert KR. Nature Mater. 2009;8:415) . Stress distribution at nanoscale ridges at oxide/metal interface, showing characteristic localized tensile regions. Comparison of W tracer profiles in cross-sectional TEM images and model simulation, during growth of porous alumina film Graduate students supported: Jerrod Houser (Ph. D. 2008); Saikat Adhikari (Ph. D. 2008); Newira Widharta (current). Postdoctoral researcher: Jiahe Ai (2007-08); Guiping Zhang (2008-). Undergraduates: Newira Widharta, Stephen Liu

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