New Collaboration between Chemistry and Mechanical Engineering

Professors Dan Buttry (Chemistry) and Demitris Kouris (Mechanical Engineering) were recently awarded a three-year, $826K program from AFOSR to study the combined influence of mechanical and chemical stress on pit initiation for Al and high strength Al alloys of relevance to the USAF air fleet. The study is motivated by two facts: pitting corrosion is the most frequent cause of failure of high-strength Al alloys such as AA2024, a high strength alloy used in the construction of older military and commercial aircraft, and no previous studies of pit initiation have considered well-defined samples under the "real world" conditions of combined mechanical and chemical stress.

Corrosion influences many areas of DoD activity, from the materials used in new aircraft systems to strategies for corrosion inhibition, to air fleet mainte-nance schedules. It has enormous financial impact on DoD operations. We be-lieve that controlling pit initiation is the key to controlling pitting corrosion of high strength Al alloys. This program involves a combined experimental and theoretical study to understand how pit initiation is influenced by combinations of mechanical and chemical stress, a key unexplored influence on pit initiation. Experiments will involve the preparation of Al and Al alloy samples with both native and synthetic defects. These samples will be exposed to combined me-chanical and chemical stress and the formation of metastable pits, as well as their evolution will be monitored to see whether or not they transition to stable pits. Mechanical and (electro)chemical stress will be applied using a unique in situ fluidic cell that will allow simultaneous application of mechanical stress, exposure to various solutions and imposition of an applied potential. Metasta-ble pits will be monitored using both in situ electrochemical and fluorescence measurements. Theory and simulations will be used to predict and understand the mechanical behavior at the surface of the samples, with an emphasis on how stress induces breakdown in the protective surface oxide, thereby driving me-tastable pit formation. The impact of the work will be significant in providing an understanding of how the combination of mechanical stress and chemical in-sult affect pit initiation in Al and its alloys under conditions of use. The long term goal is to provide information that will a) enhance DoD's ability to ration-ally schedule maintenance on its air fleet, b) provide a basis for design of envi-ronmentally compliant corrosion protection coatings, and c) move the science and engineering of corrosion forward into a new area.