The precipitation and growth of AgCl on silver in physiological NaCl solution were investigated. AgCl was found to form at bottom of scratches on the surface which may be the less effective sites for diffusion or the favorable sites for heterogeneous nucleation. Patches of silver chloride expanded laterally on the substrate until a continuous film formed. The ionic transport path through this newly formed continuous film was via spaces between AgCl patches. As the film grew, the spaces between AgCl patches closed and ion transport was primarily via micro-channels running through AgCl patches. The decrease of AgCl layer conductivity during film growth were attributed to the clogging of micro-channels or decrease in charge carrier concentration inside the micro-channels. Under thin AgCl layer, i.e. on the order of a micrometer, the dissolution of silver substrate was under mixed activation-Ohmic control. Under thick AgCl layer, i.e. on the order of tens of micrometers, the dissolution of silver substrate was mediated by the Ohmic resistance of AgCl layer.

Hydrogen environment-assisted cracking (HEAC) of Monel K-500 is quantified using slow-rising stress intensity loading with electrical potential monitoring of small crack propagation and elastoplastic J-integral analysis. For this loading, with concurrent crack tip plastic strain and H accumulation, aged Monel K-500 is susceptible to intergranular HEAC in NaCl solution when cathodically polarized at À800 mV SCE (E A , vs saturated calomel) and lower. Intergranular cracking is eliminated by reduced cathodic polarization more positive than À750 mV SCE . Crack tip diffusible H concentration rises, from near 0 wppm at E A of À765 mV SCE , with increasing cathodic polarization. This behavior is quantified by thermal desorption spectroscopy and barnacle cell measurements of hydrogen solubility vs overpotential for planar electrodes, plus measured-local crevice potential, and pH scaled to the crack tip. Using crack tip H concentration, excellent agreement is demonstrated between measurements and decohesion-based model predictions of the E A dependencies of threshold stress intensity and Stage II growth rate. A critical level of cathodic polarization must be exceeded for HEAC to occur in aged Monel K-500. The damaging-cathodic potential regime likely shifts more negative for quasi-static loading or increasing metallurgical resistance to HEAC.

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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.

The effect of prior cold work (10, 20, and 40% reduction in thickness) on hydrogen diffusion and trapping was investigated using both Devanathan permeation and thermal desorption methods. The first rise transient during diffusion-controlled permeation marks the slowest Deff (1.8×10−7 cm2/s in as-received API X-70 steel), which is indicative of the greatest degree of trapping by both irreversible and reversible traps. Faster Deff during all subsequent decay and rise transients (3.0 to 4.2×10−6 cm2/s in as-received API X-70 steel) indicates partial trap filling/release from reversible traps and permanent filling of irreversible traps after the first rise transient. Cold work substantially increased trapping as evident from both slower permeation and reduced Deff (2.0 to 4.2×10−7 cm2/s in cold-worked API X-70 steels) as well as by thermal desorption. Thermal desorption spectroscopy indicates one relatively reversible and one room temperature irreversible trap state in the cold-worked steels with desorption activation energies of 13.9±0.8 and 19.9±0.8 kJ/mol, respectively. The reversible trap state was the dominant absorber of H in the cold-worked materials.

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