Crystallographic controlled dissolution and surface faceting in disordered face-centered cubic FePd
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esearch Letters
Crystallographic controlled dissolution and surface faceting in disordered face-centered cubic FePd D. J. Horton, A. W. Zhu and J. R. Scully, Department of Materials Science and Engineering, Center for Electrochemical Science and Engineering, University of Virginia, Charlottesville, Virginia 22904 M. Neurock, Department of Chemical Engineering, University of Virginia, Charlottesville, Virginia 22904 Address all correspondence to D. J. Horton at [email protected] (Received 28 July 2014; accepted 2 September 2014)
Abstract Electrochemical dissolution by congruent oxidation of Fe Pd in 1 M HCl solution was strongly controlled by crystallographic orientation. Anodic dissolution was characterized over a wide variety of grain surface plane orientations providing a detailed view of the crystallographic nature of oxidative dissolution and surface facet evolution as a function of grain orientation. Near {100}-oriented grains retained low surface roughness after corrosion and low dissolution rates. Grains with orientation within 2° of {111} were also topographically smooth after dissolution and were nearly as corrosion resistant as {100} grains. Overall dissolution depth depended linearly on crystallographic angle within 40° of {100} and within 10° of {111} planes. Post-corrosion surface faceting and dissolution were substantially increased at grain orientations near {110} and were highest between 10° and 20° from the {111} plane normal. Grains at these crystallographic angles roughened during oxidative dissolution by forming complex semi-periodic topographies. These finely spaced arrays of terraces and ledges likely consisted of combinations of more corrosion resistant low-index planes. Therefore, the overall corrosion depth within a grain possessing an initially irrational crystal orientation was determined by the amount of dissolution required to expose new, slowly dissolving surface facets with low-index orientations. Computations of Fe–Pd alloy surface energies and surface atom coordination as a function of crystal orientation are utilized to help support this explanation.
Introduction Many forms of electrochemical dissolution and deposition exhibit a rate dependence that depends on crystallographic orientation, but the governing structural factors are not well understood. The difference in corrosion behavior dependent on crystallographic orientations enabled fundamental explorations of the role of atomistic bonding, surface energy, surface relaxation and reconstruction, and other structural factors on electrochemically controlled dissolution[1–5] and also the metal–oxide–solution epitaxy interface during passivation and pitting corrosion.[6–10] True understanding of crystallographic behavior has broad implications toward the design of fine-scale surface structures by electrodissolution-based crystal and atomic-scale surface reorganization. There exist significant discrepancies regarding which orientations result in the least corrosion resistance, to either dissolution or pitting.[2–4,11] Older studies rel
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