Materials Science and Engineering Location: Wilsdorf Hall, Room 200
Add to Calendar 2019-02-15T11:00:00 2019-02-15T13:00:00 America/New_York PhD Proposal Presentation: William H. Blades Title:  From Alloy to Oxide: Capturing the Early Stages of Oxidation and Corrosion on Ni-Cr Alloys Wilsdorf Hall, Room 200

Title:  From Alloy to Oxide: Capturing the Early Stages of Oxidation and Corrosion on Ni-Cr Alloys

Abstract: The transition from alloy to oxide begins at the surface, where the early stages of oxidation and corrosion have a profound influence on the development of the geometric and electronic structure of the oxide. The general oxidation phenomena that manifest during this transition have been described using the Cabrera-Mott model, which considers the growth kinetics of oxide layers ranging from about 1 to 10 nm in thickness. However, discrepancies arise before a complete oxide layer is formed, particularly during the oxidation of transition-metal alloys. To this end, a synergetic experimental and theoretical approach aimed at understanding the critical details surrounding the early-stage oxidation and aqueous corrosion of Ni-Cr based alloys has been undertaken. The evolution of partial surface oxides can be captured by oxidizing Ni-Cr alloys and studying their growth with scanning probe microscopy, specifically scanning tunneling microscopy/spectroscopy (STM/STS) and atomic force microscopy (AFM).Theoretical modeling, primality with density functional theory (DFT), has also been employed, and when combined with experiment yields a complementary approach towards understanding the evolution of oxides within the pre-Cabrera-Mott regime.

The interaction of molecular oxygen with Ni(100) and Ni-Cr(100) thin films has been explicitly targeted. Ni(100), Ni-8wt.% Cr(100), and Ni-12wt.% Cr(100) thin films were prepared on MgO(100) in ultra-high vacuum and exposed to oxygen up to 400 L at 500 oC. Under these oxidation conditions, the pure Ni(100) prefers the Ni(100)-c(2x2)O reconstruction, which drives the steps to facet into {100} segments, subsequently limiting the growth of NiO at elevated temperatures. Our experiments demonstrate that once a nominal amount of Cr is added the Ni-Cr(100) surface will undergo a different oxidation reaction pathway. The nucleation and growth of NiO occurs along the step edges of the Ni-Cr(100) surface and manifests itself in the formation of two distinct superlattices. A NiO-Ni(6x7) cube-on-cube interfacial relationship is found on the terraces, while low-angle NiO wedges show a characteristically different superlattice spacing and take a NiO-Ni(7x8) match across the step edges. Unique surface reconstructions are also observed, and are analogous to Cr(100) oxidation, suggesting surface segregation and phase separation of BCC Cr. This restructuring of the surface provides insight into the initial geometry of alloy-oxide interfaces and the manner in which they mitigate strain within the pre-Cabrera-Mott regime. Each of these aforementioned surface oxides present a unique electronic structure and is reflected in both the STM images and STS spectra. These data are formatted into density of states and band gap maps, which lend themselves as powerful tools when studying these heterogenous alloy-oxide surfaces. The electronic heterogeneity observed on the alloy surface after oxidation is an indication that the assumption of a homogenous electric field at the alloy-oxide interface should be revisited. Additional experiments that target the oxidation rates of Ni-Cr(100) and Ni-Cr(111) surfaces with both oxygen and water will be also be identified, and STM/STS will be used to measure the effect surface orientation and alloy composition have on the transition from alloy to oxide.

The corrosion of binary Ni-Cr and ternary Ni-Cr-Mo alloys will be also considered under a variety of different electrochemical conditions and their surfaces measured ex-situ with AFM. The data analysis will be informed by in-operando single-frequency electrochemical impedance spectroscopy (SF-EIS) measurements, taken by the Scully group (UVA), on the electrochemically grown passive film in either a chloride or sulfate solution as a function of time. This will allow for a detailed assessment of the topography, where nanoscale fluctuations between the alloy and the oxide can be measured. From this analysis the effect of the processing conditions on the oxide growth and stability can be delineated and the alloy-film interface considered through the lens of a linear stability model put forth by the Voorhees group (Northwestern University). This combined experimental and theoretical approach has offered greater insight into the progression of early stage oxide growth on Ni-Cr alloys and underscores the dramatic impact of alloying on the oxidation process within the pre-Cabrera-Mott regime.

 

Committee: 

Petra Reinke, Advisor, UVA MSE

John R. Scully, Chair, UVA MSE

Líney Árnadóttir, OSU ChemE

Stephen J. McDonnell, UVA MSE

Ian Harrison, UVA Chemistry

 

All interested persons are invited to attend.