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Autumn 2008 Seminar Series
Friday, December 5, at 3:30 p.m.
Room 264 MacQuigg Labs
Kevin Ralston
PhD Candidate advised by Dr. Rudy Buchheit
Materials Science and Engineering
The Ohio State University
Chromate-Free Corrosion Inhibition Of Aluminum Alloys: Vanadates And Anionic Exchange Clay Pigments
Abstract
Aluminum 2024-T3 is a high strength aluminum alloy commonly used in the aerospace industry. The microstructure of Al 2024 is heterogeneous, and this heterogeneity makes the alloy inherently susceptible to localized corrosion. Historically, chromate-based coatings and pigments, which are carcinogenic, have been used to successfully increase corrosion resistance. Recently vanadates have garnered attention as non-carcinogenic chromate replacements and have shown promise as inhibitors in pigments and conversion coatings. Although generally accepted as inhibitors, vanadates have a complex aqueous chemistry, dependent on both pH and concentration, which prevents an easy explanation of which species are involved and the mechanism of inhibition. There is a body of evidence that suggests that vanadates are inhibitors of corrosion and, it seems likely that inhibition depends on speciation. However, this relationship has not been fully characterized. Additionally, the presence of intermetallic particles, and in particular Cu-bearing intermetallics, play a large role in the corrosion processes of Al 2024. An understanding of how inhibiting vanadates interact with these particles and the matrix will be discussed.
Unfortunately, the use of vanadates as a drop in replacement for chromates is not possible. In addition to strong corrosion inhibition, a convenience of many chromate-based pigments is that they have appropriate solubility and inhibitor efficiency to provide inhibition without leading to coating blistering. Vanadates do not possess the same inhibitor efficiency, and generally vanadate salts are too soluble for direct use as pigments in an organic coating. Studies using anion-exchange clays, known generically as hydrotalcites, have provided a way to incorporate highly soluble inhibitor salts directly into organic coatings. Previous work has shown vanadate-bearing hydrotalcite pigments to be inhibitors on organically coated Al 2024. However, these pigments have yet to be optimized, and the possibility of using hydrotalcites to deliver other non-vanadate inhibitors which would otherwise be too soluble will be discussed.
Bio
Kevin received his B.S. and M.S. in Materials Science and Engineering from the Ohio State University in 2003 and 2006, respectively. Kevin has worked within the Fontana Corrosion Center since 2002 and is currently a graduate student advised by Dr. Rudy Buchheit pursuing a PhD in Materials Science and Engineering.
