Injection of Vacancy-Type Defects and Hydrogen by Metallic Corrosion

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TECHNICAL: Recent work in the PI's laboratory has revealed potentially important aspects of pit initiation on aluminum. In particular, positron annihilation studies have detected nanometer-scale voids near the oxide/metal interface, which because their metal surface is oxide-free, can function as highly reactive sites at which localized corrosion initiates. The observation of interfacial void formation by ambient-temperature corrosion processes has led to the hypothesis that voids result from the injection of vacancy-hydrogen defects. The project will determine the mechanism of void nucleation and growth using positron methods in combination with microscopy and surface analytical probes. Studies using high-purity Al will characterize the behavior of subsurface defects during a simple corrosion process, including effects due to trapping of defects by metal solute atoms. In tandem with the experiments, an atomistic-level simulation of void formation will be developed, based on calculated energies governing defect formation, binding, and mobility, derived from rigorous theoretical approaches. The combined experimental-theoretical approach will provide a fundamental basis of understanding for formation of corrosion-relevant defects, as affected by metal-phase composition. The results are expected to elucidate the mechanism of hydrogen entry into aluminum during corrosion, and its diffusion and reactions in the near-surface region. NON-TECHNICAL: Pitting corrosion, in which metal dissolution is confined to a small area on an otherwise oxide film-protected surface, is widespread among structural metals. Since the nature of inherent surface defects where pits form is not well understood, damage due to pitting cannot be effectively predicted. Results are also of critical interest for understanding of environment assisted cracking, and so the fundamental understanding obtained should impact this field. Through this work, students will be trained by an interdisciplinary team assembled from faculty in chemical engineering, physics, materials science and chemistry. They will learn to bring condensed matter physics modeling approaches into the realm of engineering science, in order to confront a problem of significant economic relevance. Even in today's atmosphere of interdisciplinary research, such educational experiences are unusual. Student training will take advantage of the strong presence at Iowa State of graduate and undergraduate programs promoting under-represented groups, both at the Departmental and University levels.