No abstract

If all humans vanished tomorrow, almost every metal structure would collapse within a century or less, the metal converting to an oxide. In applications ranging from the mature technology of nuts and bolts to high technology batteries, nuclear fuels and turbine engines, protective oxide films are critical to limiting oxidation. To date models of these oxide films have assumed that they form thermodynamic equilibrium stable or metastable phases doped within thermodynamic solubility limits. Here we demonstrate experimentally and theoretically the formation of unusual nonequilibrium oxide phases, that can be predicted using a scientific framework for solute capture at a moving oxide/substrate interface. The theory shows that solute capture is likely a generic process for many electrochemical processes, and suggests that similar phenomena yielding nonequilibrium phases can occur and be predicted for a wide range of other processes involving solidfluid and solid-solid chemical reactions.

The instantaneous kinetics of oxide formation and growth, in competition with passive film dissolution and breakdown, were investigated for Ni-22 Cr and Ni-22 Cr-6 Mo (wt%) during single step passivation at +0.2 V SCE . Experiments were conducted in selected acidic and alkaline chloride-containing environments using simultaneous AC and DC electrochemistry; including on-line Inductively Coupled Plasma-Mass Spectrometry (ICP-MS). In parallel experiments, in-situ neutron reflectometry (NR) and ex-situ X-ray photoelectron spectroscopy (XPS) were utilized to characterize the formation of surface oxide films as a function of time. The specific roles of pH and Mo during passivation and breakdown kinetics are highlighted, providing an insight into the fate of the elements which comprise the alloys, and their effects on passivation behavior. It was observed that early oxidation of both Ni and Cr-species occurred in acidic electrolyte. Preferential dissolution of Ni 2+ at later times enabled gradual Cr 3+ enrichment within the surface film. However, greater relative stability of NiO and Ni(OH) 2 was observed in the alkaline condition. Upon alloying Ni-Cr with Mo, Cr 3+ became increasingly enriched in the surface film during anodic polarization. Oxides were interpreted to consist of non-stoichiometric solid solutions formed via solute capture.

The electrochemical stabilities of nanofilms of Ni oxides and hydroxides are of special importance to the diverse fields of catalysis, energy storage and conversion, and alloy corrosion resistance. Many coexisting intrinsic and environmental factors may simultaneously become significant when the material size is reduced to ultrathin dimensions, making it challenging to unravel the multiple interacting mechanisms active in complex nanoscale structural architectures. Here we establish a comparative theory–experiment approach to accurately study the stabilities of nanoscale Ni-based compounds against oxidation under various electrochemical conditions and use it to quantitatively reveal the roles of surface termination, thickness, water adsorption, and supporting substrate on phase stability. We use density functional theory to calculate the energies of Ni-based nanofilms at different thicknesses subjected to various boundary conditions and environments, including free-standing, suspended in water, and substrate-supported nanofilm geometries. We use this data to simulate the corresponding nanofilm electrochemical phase diagrams and comprehensively explain various reported electrochemical phenomena. Our theoretical findings are further validated by an electrochemical experiment designed here, where the potential-driven growth of (hydr)­oxide nanofilms on Ni substrates in different solutions is precisely characterized using in situ polarized neutron reflectometry. The obtained quantitative results and insights into the microscopic corrosion mechanisms will be useful for the design, synthesis, and application of other nanoscale transition-metal compounds; in addition, the comparative theory–experiment approach can be readily translated to accurately study the electrochemical properties of other complex nanoscale systems.