Investigation of chloride-induced depassivation of iron in alkaline media by reactive force field molecular dynamics
Summary¶
DorMohammadi et al. combine ReaxFF molecular dynamics, electrochemical measurements, and X-ray photoelectron spectroscopy to study chloride-induced depassivation of iron in strongly alkaline media (npj Materials Degradation, DOI 10.1038/s41529-019-0081-6). The scientific target is a longstanding infrastructure and materials question: how chloride destabilizes passive oxide films that otherwise protect steel in high-pH environments such as cementitious pore solutions. The authors align simulations with experiments on pure iron to reduce compositional ambiguity while preserving the Fe–oxide–electrolyte chemistry central to corrosion science. At the narrative level, the integrated story emphasizes local acidification near the oxide/electrolyte interface, iron dissolution, and cation vacancy dynamics in the passive film, with chloride acting as a catalyst for defect formation rather than necessarily as a species that must fully penetrate a thick barrier—consistent with point-defect and film breakdown pictures discussed in the corrosion literature.
Methods¶
Experiments (electrochemistry + XPS). The study reports electrochemical measurements and X-ray photoelectron spectroscopy (XPS) on 99.95% Fe specimens prepared/polished as described in the article, exposed to pH ~13.5 NaOH electrolytes with controlled chloride additions intended to connect to alkaline corrosion contexts (details of polishing, chambers, and electrochemical schedules are in the PDF).
Reactive MD (ReaxFF, LAMMPS, XSEDE). Engine / code: ReaxFF-MD is implemented in LAMMPS and run on XSEDE resources as stated in the article. Boundaries / periodicity: the Fe / passive film / electrolyte supercell uses 3D periodic boundary conditions (PBC) in the published setup (with vacuum spacing and slab geometry along the non-metallic direction as detailed in the PDF). Temperature / pressure framing: simulations are reported at room temperature (300 K) with “standard atmospheric pressure (1 atm)” noted as part of the simulation framing, while the thermostat/ensemble paragraph is written for canonical (NVT) sampling. Timestep: 0.1 fs with Velocity Verlet integration. Thermostat: Nosé–Hoover for NVT. COM constraint: the center of mass of the system is fixed to remove rigid-body translation. Initial state / electrolyte geometry: initial solution density and vacuum slab dimensions are set using solution density at 300 K as described in the article. Barostat: N/A — the excerpted protocol emphasizes NVT temperature control rather than NPT stress control.
Force-field provenance / validation hooks. The manuscript cites ReaxFF parameter sources for Fe/Na/O/H/Cl interactions drawn from prior parameterizations (Aryanpour, Rahaman, Psofogiannakis lines as named in the paper) and reports comparing selected surface formation energies and water adsorption energies on Fe(110) against DFT references as a consistency check.
Electric fields / enhanced sampling. N/A — not summarized as part of the core MD protocol text excerpted for this note.
Production length / staged sampling. The article discusses chloride exposure trajectories on multi-nanosecond scales in the Results/figures (e.g., PDF comparisons shown out to 2000 ps in the extracted figure caption region); use the npj Materials Degradation PDF for exact run lengths and any separate equilibration stages beyond energy minimization as stated.
Findings¶
The authors argue that depassivation begins with localized electrolyte acidification adjacent to the passive film, which promotes Fe release and drives iron vacancy accumulation within the oxide network. Chloride is associated with accelerated vacancy formation and migration toward the metal/oxide interface in the simulated mechanisms, supporting a view where chloride catalysis of defect chemistry can precede catastrophic breakdown even when macroscopic film thickness appears modestly perturbed. Experiments corroborate the broad electrochemical trends implied by the mechanistic sequence, though the manuscript also acknowledges simplifications such as pure Fe substrates versus engineering steels and the intrinsic time and length limits of MD relative to field corrosion. For the knowledge base, the paper is a concrete ReaxFF application bridging electrochemistry and reactive MD for oxide films in alkaline electrolytes.
Limitations¶
Simplified iron substrates versus industrial carbon steel; scale and time sampling limits of MD.
Relevance to group¶
ReaxFF application to corrosion/passivation chemistry in alkaline environments.
Citations and evidence anchors¶
https://doi.org/10.1038/s41529-019-0081-6