The main goal of this work is to integrate bulk polycrystalline and single crystal electrochemical measurements with nanoscale characterization to advance the fundamental understanding of the structure, molecular, and electronic properties of oxides formed during aqueous passivation and uncover the role of Mo on surface stability in corrosive environments.
The alloys under study are Ni-11Cr, Ni-11Cr-6Mo, Ni-22Cr, and Ni-22Cr-6Mo, wt%, polycrystalline solid solutions with additional focus on individual grain orientations in isolation from grain or twin boundaries. DC and AC electrochemical techniques were utilized for exploring the controlled growth of passive oxide films on these materials in aqueous chloride and chloride-free solutions of various acidity.
The repassivation kinetics of polycrystalline and single grain crystal surfaces were determined potentiostatically and galvanostatically after previously formed air oxides were reduced. A major challenge to analysis of the oxidation rate from electrochemical data is that film thickening rate must be assessed independently from the total oxidation rate. Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) results indicate that a significant portion of the applied anodic charge during oxidation is associated with direct cation ejection and/or dissolution rather than film thickening.
Single frequency electrochemical impedance spectroscopy (SF-EIS) and ICP-MS were thus applied as novel characterization methods to model the electrochemically grown oxides and measure in-situ changes in oxide thickness and elementary dissolution reaction rates during passivation and film dissolution and/or cation emission. The oxide thicknesses calculated using these methods were related to the charge that goes toward oxide film growth and the related oxidation current density.
Atomic force microscopy (AFM) topography images and profiles across single grain surfaces at intermediate stages of potentiostatically and galvanostatically controlled oxide growth were obtained to give insight on the nucleation and morphology of oxides formed. The surface roughness was derived from these images in order to quantify the influence of the minor alloying elements, and chloride presence on the morphology and degree of film breakdown during the stages of passivation. 3D atom probe tomography and X-ray photoelectron spectroscopy analysis of the oxides following potentiostatic and galvanostatic growth provided additional information regarding the location and chemical identity of elements across the films.
This pairing of operando mesoscale electrochemical techniques with ex-situ microscopy describes the competitive oxidation and dissolution processes in aqueous environments and achieves an atomic-scale understanding of the oxide surface morphology along with the influence of minor alloying elements. The fate of all elements during fast repassivation was tracked operando for analysis of the significant impact of dissolution and cation ejection during passivation using ICP-MS, SF-EIS, and AFM measurements. The role of kinetics on passivation was found to play a crucial role over thermodynamics, resulting in non-conformal and unexpected oxide growth dictated by the minimization of interfacial energy and stresses. Finally, Mo was found to serve a number of roles during passivation and collects at the film/solution interface as well as throughout oxides.
ACKNOWLEDGEMENTS
This work was funded by the U.S. Office of Naval Research Multi-University Research Initiative under a subcontract from Northwestern University SP0028970-PROJ0007990 with Dr. David A. Shifler.
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