Atomic level characterization in corrosion studies

Dynamic dissolution of Cm3+ ions incorporated at the calcite-water interface: an ab initio molecular dynamics simulation study

The stability of actinide-mineral solid solution in a water environment is critical for assessing the safety of nuclear-waste geological repositories and studying actinide migration in natural systems. However, the dissolution behavior of actinide ions incorporated at the mineral-water interface is still unclear at the atomic level. Herein, we present metadynamics simulations of the reaction pathways, thermodynamics and kinetics of trivalent curium ions (Cm3+) dissolving from calcite surfaces. Cm3+ ions incorporated in different calcite surfaces (i.e., terrace and stepped surfaces) with distinct coordination environments have different reaction pathways, free energy barriers and free energy changes. We found that Cm dissolution from a stepped surface is more favorable than that from a terrace surface, both thermodynamically and kinetically. In addition, water molecules seem to promote the detachment of curium ions from the surface by exerting a pulling force via water coordination with Cm3+ and a pushing force via proton migration to the surface layer and water diffusion in the vacant Cm site. Thus, the findings from this work prove to be a milestone in revealing the dynamic dissolution mechanism of trivalent actinides from minerals and would also help predict the dissolution behaviors of other metal ions at the solid-water interface in chemical and environmental sciences.

Susceptibility of 316L Stainless Steel Structures to Corrosion Degradation in Salivary Solutions in the Presence of Lactic Acid

In the field of healthcare and dentistry, 316L stainless steel is widely used for its corrosion resistance. However, the presence of lactic acid in salivary solutions can affect its surface reactivity. This study employed electrochemical methods to investigate the influence of lactic acid on 316L stainless steel's corrosion resistance in Fusayama Meyer saliva and saliva doped with varying lactic acid concentrations. The results revealed a significant decrease in polarization resistance as the lactic acid concentration increased, despite a shift toward more positive corrosion potentials. Consequently, the study suggests that the lactic acid presence in salivary solutions should be considered when evaluating the corrosion susceptibility of 316L stainless steel devices.

The Oxygen Evolution Reaction Drives Passivity Breakdown for Ni-Cr-Mo Alloys

Corrosion is the main factor limiting the lifetime of metallic materials, and a fundamental understanding of the governing mechanism and surface processes is difficult to achieve since the thin oxide films at the metal-liquid interface governing passivity are notoriously challenging to study. In this work, a combination of synchrotron-based techniques and electrochemical methods is used to investigate the passive film breakdown of a Ni-Cr-Mo alloy, which is used in many industrial applications. This alloy is found to be active toward oxygen evolution reaction (OER), and the OER onset coincides with the loss of passivity and severe metal dissolution. The OER mechanism involves the oxidation of Mo4+ sites in the oxide film to Mo6+ that can be dissolved, which results in passivity breakdown. This is fundamentally different from typical transpassive breakdown of Cr-containing alloys where Cr6+ is postulated to be dissolved at high anodic potentials, which is not observed here. At high current densities, OER also leads to acidification of the solution near the surface, further triggering metal dissolution. The OER plays an important role in the mechanism of passivity breakdown of Ni-Cr-Mo alloys due to their catalytic activity, and this effect needs to be considered when studying the corrosion of catalytically active alloys.

Dynamic Polarization Behaviors of Equimolar CoCrFeNi High-Entropy Alloy Compared with 304 Stainless Steel in 0.5 M H2SO4 Aerated Aqueous Solution

Three corrosion potentials and three corrosion current densities are clearly identified before the passivation for both dynamic polarization curves of equimolar CoCrFeNi high-entropy alloy (HEA) and 304 stainless steel (304SS) in 0.5 M H₂SO₄ aerated aqueous solution, by decomposing anodic and cathodic polarization curves. The passivated current density of the former is greater than the latter, compliant with not only the constant of solubility product (k_sp) and redox equilibrium potential (E_eq) of each metal hydroxide but also the sequence of bond energy (E_b) for monolayer hydroxide on their facets derived from the first principle founded on density function theory. However, the total amount of ion releasing from HEA is less than 304SS, since the hydroxide/oxide film formed in the air of the latter containing greater amounts of Fe(Ⅱ) and Mn(Ⅱ) is less stable around corrosion potentials while they are further oxidized into more stable Fe(Ⅲ) and Mn(ⅢorⅣ) with much lower k_sp, leading to the much less increasing ratios of ion releases from 0.25 to 0.6 V.

Halide-induced Step Faceting and Dissolution Energetics from Atomistic Machine Learned Potentials on Cu(100)

Adsorbates impact the surface stability and reactivity of metallic electrodes, affecting the corrosion, dissolution, and deposition behavior. Here, we use density functional theory (DFT) and DFT-based Behler-Parrinello neural networks (BPNN) to investigate the geometries, surface formation energies, and atom removal energies of stepped and kinked surfaces vicinal to Cu(100) with a c(2×2) Cl adlayer. DFT calculations indicate that the stable structures for the adsorbate-free vicinal surfaces favor steps with <110> orientation, while the addition of the c(2×2) Cl adlayer leads to <100> step facets, in agreement with scanning tunneling microscopy (STM) observations. The BPNN calculations produce energies in good agreement with DFT results (root mean square error of 1.3 meV/atom for a randomly chosen set of structures excluded from the training set). We draw three conclusions from the BPNN calculations. First, Cl on the upper <100> step edges occupies the three fold hollow sites (as opposed to the four-fold sites on the terraces), congruent with deviations of the STM height profile for the adsorbate at the upper step edge. Second, disruptions in the continuity of the halide overlayer at the steps result in significant long-range step-step interactions. Third, anisotropic metal dissolution and deposition energetics arise from phase shifts of the c(2×2) adlayer at orthogonal <100> steps. This DFT-BPNN approach offers an effective strategy for tackling large-scale surface structure challenges with atomic-level accuracy.

Early-stage NiCrMo oxidation revealed by cryo-transmission electron microscopy

Hydroxide formation at the surface of corroded alloys is critical for understanding early-stage oxidation of many corrosion-resistant alloys. Many hydroxides are unstable in an ambient environment and are electron-beam sensitive, limiting the use of conventionally-prepared specimens for transmission electron microscopy characterization of these alloy-water interfaces. In order to avoid sample dehydration, NiCrMo alloys corroded in a Cl^--containing electrolyte solution were cryo-immobilized by plunge freezing. A cryo-focused ion beam microscope was used to thin the sample to electron transparency, while preserving the alloy-water interface, and the sample was then cryo-transferred to a transmission electron microscope for imaging and diffraction. The presence of rocksalt Ni_1-xCr_2x/3O and β-Ni_1-xCr_2x/3(OH)₂ phases and their orientational relationship to the underlying alloy were observed with electron diffraction, confirming the preservation of the surface structure through the fully-cryogenic sample preparation and analysis.

From Alloy to Oxide: Capturing the Early Stages of Oxidation on Ni-Cr(100) Alloys

The interaction of oxygen with Ni-Cr(100) alloy surfaces is studied using scanning tunneling microscopy (STM) and spectroscopy (STS) to observe the initial steps of oxidation and formation of the alloy-oxide interface. The progression of oxidation was observed for Ni(100) and Ni-Cr(100) thin films including Ni-8 wt % Cr(100) and Ni-12 wt % Cr(100), which were grown on MgO(100) in situ. These surfaces were exposed to between 1 and 150 L O₂ at 500 °C, and additional annealing steps were performed at 500 and 600 °C. Each oxidation and annealing step was studied with STM and STS, and differential conductance maps delivered spatially resolved information on doping and band gap distributions. Initial NiO nucleation and growth begins along the step edges of the Ni-Cr alloy accompanied by the formation of small oxide particles on the terraces. The incubation period known in oxidation of Ni(100) is absent on Ni-Cr alloy surfaces illustrating the significant changes in surface chemistry triggered by Cr-alloying. Step edge faceting is initiated by oxide decoration along the step edges and is expressed as moiré patterns in the STM images. The surface oxide can be characterized by NiONi(6 × 7) and NiO-Ni(7 × 8) coincidence lattices, which have a cube-on-cube epitaxial relationship. Small patches of NiO are susceptible to reduction during annealing; however, additional oxide coverage stabilizes the NiO. NiO regions are interspersed with areas covered predominantly with a novel cross-type reconstruction, which is interpreted tentatively as a Cr-rich, phase-separated region. Statistical analysis of the geometric features of the surface oxide including step edge heights, and NiO wedge angles illustrates the layer-by-layer growth mode of NiO in this pre-Cabrera-Mott regime, and the restructuring of the alloy-oxide interface during the oxidation process. This experimental approach has offered greater insight into the progression of oxide growth in Ni-Cr thin films and underscores the dramatic impact of alloying on oxidation process in the pre-Cabrera-Mott regime.