Localized Electrochemical Corrosion Characterization in Sessile Droplets: A Powerful Tool for High-Throughput Screening of Material Reactivity?

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This work demonstrates a fast and localized approach to electrochemical corrosion measurements using small electrolyte droplets on metal surfaces using two different approaches. By integrating a potentiostat with a contact angle goniometer, we developed an in-house setup enabling simultaneous electrochemical and wetting characterization. This method is well-suited for microscale corrosion studies, where small working electrode areas allow rapid screening of surface-dependent reactivity. A custom bi-microelectrode probe introduces both the reference and counter electrodes into the droplet. Room-temperature ionic liquids (RTILs) were used to limit evaporation and ensure long-term droplet stability. Tests were conducted under both ambient and inert (glovebox) conditions, with droplet volumes of 2–3 µL depending on the environment. Initial validation on low-reactivity Ti-6Al-4V alloy showed stable contact angles, droplet shape, and corrosion potential over nine hours. Electrochemical impedance spectroscopy (EIS) confirmed reproducible and low-drift behavior. More reactive substrates, such as Al2024, showed lower initial contact angles and sustained droplet stability over 24 hours. EIS revealed a progressive decrease in polarization resistance, indicating a 2.5-fold increase in corrosion rate and highlighting the setup’s sensitivity to evolving surface reactivity. To investigate the variation of wettability during the contact time, additional experiments were performed with ethylammonium nitrate (EAN) with 0.1 M NaCl. Over time, droplets spread significantly, with increasing wetted area and decreasing contact angle. Impedance spectra showed complex features with both capacitive and inductive loops, pointing to charge transfer and adsorbed intermediate processes. These results underline the need to normalize electrochemical data to the actual wetted area. Moreover, experiments in a glovebox confirmed that oxygen and water significantly impact corrosion behavior. Under inert conditions, corrosion potentials shifted positively, and impedance increased over time, indicating slower degradation. These trends matched those observed in conventional electrochemical cells operated under the same conditions. A second experimental development of this work extends the method to molten salts by incorporating temperature control and in situ Raman spectroscopy of corrosion products. Despite current instrumentation limitations (e.g., sensitivity and laser wavelength at high temperature), a new electrochemical cell has been developed enabling operando measurements and the characterization for corrosion in molten salt. This study highlights the value of combining EIS with wettability monitoring to better assess localized corrosion mechanisms in confined geometries and the limitations of using open-circuit potential alone, especially for slow-reacting materials, as well as the combination with operando Raman spectroscopy for corrosion products analysis. Overall, the droplet-based setup offers a robust, scalable platform for studying corrosion in realistic micro-environments, serving both fundamental research and high-throughput material screening. Acknowledgements: This work has benefited from a government grant managed by the Agence Nationale de la Recherche under the France 2030 program; PEPRH2 – A-Dream reference ANR-22-PEXD-0003.

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