Research Associate, The Ohio State University
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“The Subsurface Structure of Abraded Al Alloys and its Influences on Corrosion”
Al alloys subjected to surface abrasion exhibit an altered surface layer (ASL) with unique microstructure as the result of heavy shear deformation. This ASL microstructure is extremely unstable even at room temperature (RT), and its evolution is strongly affected by sample geometry (bulk or TEM sample) and temperature. In the ASL on freshly-abraded Al-Zn-Mg-Cu samples, pre-existing η′/η precipitates and grain boundaries are deformed and bent toward the abrasion direction. After 7-day natural aging of the abraded bulk sample, ultrafine subgrains form locally in the ASL, while unusual Al2Cu (θ) and MgxZny phases precipitate at these subgrain boundaries; however, some deformed η′/η precipitates decompose. Repeated observation of this TEM specimen with storage at RT shows that more ultrafine subgrains and θ particles form in the ASL, and Zn diffuses out of MgxZny particles to form a Zn particle. These unusual Zn and θ phases continued to grow and coarsen during 42-month natural aging of the TEM specimens. Artificial aging of abraded bulk samples at 50 °C for 1 h significantly accelerate the formation of subgrains and θ particles in the ASL. Because of the unique ASL microstructure, abraded alloy surfaces exhibit a different corrosion resistance than the underlying substrate. The ASLs on Al-Zn-Mg-Cu alloys are preferentially attacked at lower potentials than the underlying substrates during potentiodynamic polarization in an NaCl solution, resulting in surface layer attack that would undermine any protective coating system. Long-term natural aging or short-period artificial aging increases the breakdown potential and improves the corrosion resistance of the ASL. The ASLs on AA2024 and sensitized AA5083 are more resistant to corrosion owing to the solute redistribution in the ASL. Clearly the details of the ASL microstructure and its evolution play a critical role in the corrosion properties of abraded Al alloys.
Shanshan Wang, Research Associate, The Ohio State University
Shanshan Wang is currently a research associate in materials science and engineering and Fontana Corrosion Center at The Ohio State University (OSU). She earned her Ph.D. and M.S. in materials science from Harbin Institute of Technology in China, and B.S. in materials science and engineering from Kunming University of Science and Technology in China. She also came to the United States as a visiting student at OSU between 2011 and 2013. After receiving her Ph.D. in 2015, she came back to the Fontana Corrosion Center at OSU, where she has continued to perform research as a postdoctoral researcher (2015-2018) and research associate since 2018. Shanshan’s primary research interests are the relationships between processing, microstructure and localized corrosion of metals and alloys, galvanic corrosion of mixed-metal systems, and protective coatings.