Experimental and Simulation Study of Compositional and Atomic-Scale Ordering Effects on Passivation in Fe-Cr and Ni-Cr Alloys

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NON-TECHNICAL ABSTRACTStainless steels and iron-chromium and nickel chromium alloys are well known for their excellent corrosion resistance. This behavior stems from a protective atomic-scale protective oxide layer that forms on the surface of stainless steels which if scratched or otherwise breached in some manner reforms within seconds on the surface of the alloy. This self-healing behavior serves as a model for the development modern corrosion protection coating schemes for other families of important alloy systems including aluminum and magnesium. The main objective of the work is to develop simple physically based models for the behavior of stainless steels in order to develop design schemes that can rationalize existing data and identify alloy compositions and atomic-scale effects that can be used to engineer new families of alloys with superior corrosion resistance. The scope of research, which involves laboratory and computational work, affords both undergraduate and graduate students the opportunity for fundamental training in materials electrochemistry and corrosion science. Results of this research are expected to contribute to important issues of national concern connected to both the well being of our infrastructure as well as the development of light-weight metallic alloys used in automotive applications for which corrosion is a major problem.TECHNICAL ABSTRACTWhile it is well known that 10-13 at.% chromium is required for stainless-like behavior of iron alloys, a long standing unanswered question in corrosion science is why? Early work connected "critical" Cr compositions for stainless-like behavior to long-ranged percolation thresholds for random structures but recent density functional calculations that incorporate magnetic interactions have shown that Fe-Cr alloys are unique owing to an inversion in the sign of the heat of mixing occurring at approximately 10 at. % Cr within a solid solution. Importantly, Mössbauer and neutron scattering studies have confirmed the predictions of these calculations for Fe-Cr alloys and revealed that atomic scale ordering can be "tuned" by suitable heat treatment. In order to unravel this mystery the atomic scale structure of Fe-Cr and Ni-Cr alloys is being examined as a function of composition and short-range order. At fixed alloy composition, short-range order is tuned by varying the heat treatment of the alloy. The work will characterize short-range order by atomic-scale chemical imaging of well-prepared alloy surfaces using scanning tunneling microscopy. Computer simulations are performed in order to extract regular solution theory parameters that are then used in Monte Carlo renormalization group calculations to determine how ordering modifies standard percolation thresholds. Electrochemical methods are used to measure how composition and heat treatment affect passivation behaviors in Fe-Cr and Ni-Cr alloys.