When a saline droplet sits on bare iron, the metal surface beneath it splits into coupled anodic and cathodic regions whose locations can exchange over time, driven by oxygen gradients, rust-film growth, and galvanic currents. Tang, Ma, Orazem, You and Li (Electrochimica Acta 354, 2020, 136631) used a concentric three-electrode array (CTEA) to measure position-resolved polarization curves and electrochemical impedance spectra (EIS) under 3.5% NaCl droplets of 25–340 μL on pure iron, and extracted local kinetic parameters (anodic/cathodic faradaic impedances, diffusion resistance, effective capacitance, interfacial oxygen concentration) — but only at the self-corrosion (mixed) potential and only within an initial window of about two hours. The authors explicitly delimited what remains unresolved: the dynamics of anode/cathode identity switching and its feedback with local chemistry, product formation, and coupling currents; local EIS under imposed anodic and cathodic polarization with a model extended to those states; generalization of the impedance model to partially blocking, time-evolving corrosion-product films; and a statistical parameter map across droplet sizes, chemistries, and longer times capable of supporting predictive rules for atmospheric-corrosion initiation and propagation.

Background

Atmospheric corrosion — metal degradation under thin electrolyte layers formed by condensation or hygroscopic salt deposits — is estimated to account for roughly half the annual cost of all metal corrosion, and it commonly proceeds under discrete droplets rather than under bulk immersion. In the classic Evans picture of a drop of salt water on a metal, oxygen diffuses more readily to the thin droplet edge than to the thick center, so the edge becomes the cathode (oxygen reduction) and the center becomes the anode (metal dissolution), with the two regions short-circuited through the metal as a galvanic couple. This 'differential aeration' pattern is not static: as corrosion products (rust) accumulate and local chemistry evolves, the anodic and cathodic roles assigned to a given surface position can switch over time. Electrochemical impedance spectroscopy (EIS) probes such interfaces by applying a small sinusoidal voltage over a range of frequencies and analyzing the response with an equivalent circuit, separating charge-transfer kinetics (faradaic impedance), diffusion through the electrolyte (often represented by a Warburg element, the impedance signature of semi-infinite diffusion), and interfacial capacitance; EIS is normally recorded at the open-circuit, or self-corrosion (mixed), potential at which anodic and cathodic partial reactions balance. To study droplet corrosion with spatial resolution, researchers use electrode arrays — the wire beam electrode (WBE), multi-electrode arrays (MEA), and coupled multi-electrode arrays (CMEA) — in which many small electrodes embedded flush in the surface are measured individually or monitored as a coupled set, mapping local potentials and galvanic currents. Tang, Ma, Orazem, You and Li developed a concentric three-electrode array (CTEA), in which small working-electrode units at different radial positions under a droplet share a counter and reference arrangement, and used it in two companion papers (Electrochimica Acta 354, 2020, 136633 and 136631): Part I mapped corrosion-potential and galvanic-current distributions under 25, 110, and 340 μL droplets of 3.5% NaCl on pure iron and showed how droplet size controls the symmetry of the electrochemical distribution and the tendency toward localized attack; Part II recorded local polarization curves and local EIS at selected electrode units under those droplets and regressed a mathematical impedance model to obtain position-dependent local corrosion kinetics. Part II found that the rate-controlling step varies strongly with position and droplet volume — under 25 μL both anodic and cathodic sites were oxygen-diffusion controlled, whereas under larger droplets the center was mass-transfer determined while the periphery mixed charge-transfer and diffusion control — and that some peripheral units required Warburg-type elements and very large diffusion resistances, signaling corrosion-product films that partially block transport. The study was, however, confined to spectra at the self-corrosion potential and to an initial period of roughly two hours, and its parameterization covered only three droplet volumes and selected units. Related work since 2019 has advanced neighboring pieces: dynamic-droplet measurements on iron (Electrochemistry Communications 101, 2019), micro-three-electrode measurements of noise and polarization resistance under single droplets of varying volume and chloride concentration (Corrosion Science 235, 2024, 112171), multi-electrode-array visualization of corrosion evolution beneath condensed droplets (Colloids and Surfaces A 684, 2024, 133252), coupled multi-electrode-array studies of spatio-temporal corrosion under a single droplet (Corrosion Science 267, 2026, 113881), high-throughput droplet-microarray experiments with finite-element modeling on carbon steel (npj Materials Degradation 9, 2025), and droplet-condition inhibitor screening (Corrosion Science 257, 2025, 113340). None of these replaces the missing quantitative, impedance-based local-kinetic description in the states the original authors identified as decisive.

Problem Statement

For pure iron under a saline (3.5% NaCl) droplet, convert the static, open-circuit snapshot of local corrosion kinetics obtained by Tang et al. (2020) into a quantitative dynamic and polarization-state-resolved description of the coupled droplet-metal system. The single overarching objective is a validated, position-resolved account of local corrosion kinetics under a droplet that can support predictive rules for atmospheric-corrosion initiation and propagation; its components are: (1) time-resolved, spatially resolved measurement and quantification of the switching of anodic and cathodic roles at individual positions under the droplet, and of its feedback with local corrosion chemistry, corrosion-product formation, and galvanic coupling currents, beyond the initial window of about two hours; (2) local electrochemical impedance spectra recorded while individual positions are held under imposed anodic and cathodic polarization potentials (rather than only at the self-corrosion mixed potential), with the impedance model extended and revalidated for those polarized states; (3) generalization of that model to represent partially blocking, time-evolving porous corrosion-product films and nonlinear transport at edge and film-covered sites, where Warburg-type elements and very large diffusion resistances appear; and (4) a statistical mapping of the resulting local kinetic parameters across droplet sizes, droplet chemistries, environmental conditions, and longer exposure times. All four components are jointly required, and no proper subset of them resolves the task. Each component is bounded: (1) role switching and its feedback with chemistry, product formation, and coupling currents are quantified at the array positions under 3.5% NaCl droplets of 25, 110, and 340 μL, from the end of the source's initial window of about two hours through droplet dry-out; (2) local EIS is recorded at those positions under a finite set of imposed anodic and cathodic polarization potentials fixed before measurement, and the extended model reproduces those spectra; (3) the generalized model fits the edge and film-covered positions that the source's model cannot represent, including time-evolving, partially blocking films and nonlinear transport; (4) the parameter map covers the source's droplet volumes (25, 110, and 340 μL) and 3.5% NaCl chemistry extended to at least one further chloride concentration and one further relative humidity, over the exposure times of component (1), reporting each local kinetic parameter with its variability. 'Validated' means that the model-derived local kinetic parameters and the polarization-state-resolved impedance predictions reproduce independent replicate local measurements at held-out positions, polarization states, and exposure times, with the error metric and its tolerance fixed before the model is fitted and reported together with the result.

References

  1. Tang, Xiao; Ma, Chaoran; Orazem, Mark E.; You, Chen; Li, Yan. 'Local electrochemical characteristics of pure iron under a saline droplet II: Local corrosion kinetics.' Electrochimica Acta 354 (2020) 136631. DOI: 10.1016/j.electacta.2020.136631. https://doi.org/10.1016/j.electacta.2020.136631
  2. Tang, Xiao; Ma, Chaoran; Orazem, Mark E.; You, Chen; Li, Yan. 'Local electrochemical characteristics of pure iron under a saline droplet I: Effect of droplet size on electrochemical distribution.' Electrochimica Acta 354 (2020) 136633. DOI: 10.1016/j.electacta.2020.136633. https://doi.org/10.1016/j.electacta.2020.136633
  3. Tang, Xiao; Ma, Chaoran; Zhou, Xiaoxiao; Lyu, Xiaolei; Li, Qiang; Li, Yan. 'Atmospheric corrosion local electrochemical response to a dynamic saline droplet on pure Iron.' Electrochemistry Communications 101 (2019) 28-34. DOI: 10.1016/j.elecom.2019.01.011. https://doi.org/10.1016/j.elecom.2019.01.011
  4. Pavlič, Simon; Kosec, Tadeja; Legat, Andraž. 'Spatio-temporal corrosion processes under a single droplet studied by coupled multi-electrode array (CMEA).' Corrosion Science 267 (2026) 113881. DOI: 10.1016/j.corsci.2026.113881. https://doi.org/10.1016/j.corsci.2026.113881
  5. Wang, Mingyu; Zhao, Xiaoyu; Gao, Shan; Zhu, Yesen; Zheng, Yu; Huang, Yi; Xu, Yunze. 'Visualizing and understanding corrosion evolution beneath a condensed droplet using the multi-electrode array.' Colloids and Surfaces A: Physicochemical and Engineering Aspects 684 (2024) 133252. DOI: 10.1016/j.colsurfa.2024.133252. https://doi.org/10.1016/j.colsurfa.2024.133252
  6. Rahimi, Ehsan; Zhang, Keer; Kosari, Ali; Van den Steen, Nils; Homborg, Axel; Terryn, Herman; Mol, Arjan; Gonzalez-Garcia, Yaiza. 'Atmospheric corrosion of iron under a single droplet: A new systematic multi-electrochemical approach.' Corrosion Science 235 (2024) 112171. DOI: 10.1016/j.corsci.2024.112171. https://doi.org/10.1016/j.corsci.2024.112171
  7. Wen, Hanyu; Tang, Xiao; Liu, Qinghao. 'Multiphysics simulation and EIS verification of dynamic droplet corrosion under external atmospheric humidity.' SSRN preprint (2026). DOI: 10.2139/ssrn.6876911. https://doi.org/10.2139/ssrn.6876911
  8. Du, Shiyao; Ren, Chenhao; Ma, Lingwei; Wu, Dequan; Zhou, Kun; Li, Zongbao; Wang, Jun; Zhang, Dawei. 'High-throughput investigation and finite element modelling of atmospheric corrosion on carbon steel under varying droplets.' npj Materials Degradation 9 (2025) 145. DOI: 10.1038/s41529-025-00695-3. https://doi.org/10.1038/s41529-025-00695-3
  9. Samarawickrama, Chathumini; Deng, Qiushi; Eiden, Philipp; White, Paul; Keil, Patrick; Cole, Ivan. 'Impact of droplet conditions on corrosion inhibitor mechanisms: A case study on small azole molecules.' Corrosion Science 257 (2025) 113340. DOI: 10.1016/j.corsci.2025.113340. https://doi.org/10.1016/j.corsci.2025.113340