The use of the electrochemical micro-cell for the investigation of corrosion phenomena

Study on Microelectrochemical Inhomogeneity of an SA508-309L/308L Overlay Welded Joint

The corrosion behavior of the dissimilar metal welded joint (DMWJ) is highly dependent on its heterogeneous microstructures. However, directly measuring the electrochemical properties of microstructures in different heat-affected zones (HAZs) is a formidable challenge, because traditional bulk electrochemistry can only offer an average signal. Herein, the microelectrochemical properties of an SA508-309L/308L DMWJ were measured in 3.5 wt % NaCl solution using lithography and capillary techniques. Specifically, high-throughput microelectrochemical tests, including open circuit potential (OCP), electrochemical impedance spectroscopy (EIS), and potentiodynamic polarization (PDP), were conducted on 168 spots (Φ 12 μm). Results revealed five typical EIS responses and seven varieties of PDP curves (different magnitudes of the current density). The maps of thermodynamic and kinetic metrics, such as polarization resistance derived from EIS, corrosion potentials, and corrosion currents extracted from potentiodynamic polarization curves, demonstrated good consistency. The uniform corrosion tendency of the SA508 HAZ subregions during the immersion tests is basically consistent with its Ecorr_avg order of subcritical HAZ (C5, -371 mV) < intercritical HAZ (C4, -546 mV) < fine-grained HAZ2 (C3, -579 mV) < fine-grained HAZ1 (C2, -593 mV). The random presence of inclusions leads to highly heterogeneous microelectrochemical properties of the DMWJ, thereby causing localized corrosion to occur preferentially. Moreover, the macroscopic corrosion behavior is affected by the corrosion products, which display a protective effect that modifies the local electrochemical activity of the SA508 HAZ. The combination of microelectrochemical properties allows for a more comprehensive understanding of the macroscopic corrosion behavior of metals and the galvanic effect between the heterogeneous microstructures.

Correlating Corrosion to Surface Grain Orientations of Polycrystalline Aluminum Alloy by Scanning Electrochemical Cell Microscopy

The study of grain-dependent corrosion behaviors of practical polycrystalline metals remains challenging due to the difficulty in eliminating the influences of other microstructural features, such as intermetallic particles and grain boundaries. In this work, we performed thousands of microscopic potentiodynamic polarization measurements on a polycrystalline aluminum alloy AA7075-T73 using the spatially resolved oil-immersed scanning electrochemical cell microscopy measurement. Data were extracted only from grain interior areas excluding intermetallic particles and grain boundaries. Based on the multiple potentiodynamic polarization measurements, the differences between grains can be revealed. Cathodic currents exhibited a strong grain orientation dependence with a decreasing order of {101} > {001} > {111}, agreeing with the prediction from the order of atomic planar density. By contrast, the dependence of anodic currents on grain orientation was weak, and pitting was independent of grain orientation, which could be due to the limited mass transport of ions within the surface oxide film. This work highlights the capability of oil-immersed scanning electrochemical cell microscopy in resolving small electrochemical differences, which will greatly promote the study of grain-dependent behaviors of practical polycrystalline samples.

Nanoscale electrochemistry in a copper/aqueous/oil three-phase system: surface structure-activity-corrosion potential relationships

Practically important metal electrodes are usually polycrystalline, comprising surface grains of many different crystallographic orientations, as well as grain boundaries. In this study, scanning electrochemical cell microscopy (SECCM) is applied in tandem with co-located electron backscattered diffraction (EBSD) to give a holistic view of the relationship between the surface structure and the electrochemical activity and corrosion susceptibility of polycrystalline Cu. An unusual aqueous nanodroplet/oil (dodecane)/metal three-phase configuration is employed, which opens up new prospects for fundamental studies of multiphase electrochemical systems, and mimics the environment of corrosion in certain industrial and automotive applications. In this configuration, the nanodroplet formed at the end of the SECCM probe (nanopipette) is surrounded by dodecane, which acts as a reservoir for oil-soluble species (e.g., O₂) and can give rise to enhanced flux(es) across the immiscible liquid–liquid interface, as shown by finite element method (FEM) simulations. This unique three-phase configuration is used to fingerprint nanoscale corrosion in a nanodroplet cell, and to analyse the interrelationship between the Cu oxidation, Cu²⁺ deposition and oxygen reduction reaction (ORR) processes, together with nanoscale open circuit (corrosion) potential, in a grain-by-grain manner. Complex patterns of surface reactivity highlight the important role of grains of high-index orientation and microscopic surface defects (e.g., microscratches) in modulating the corrosion-properties of polycrystalline Cu. This work provides a roadmap for in-depth surface structure–function studies in (electro)materials science and highlights how small variations in surface structure (e.g., crystallographic orientation) can give rise to large differences in nanoscale reactivity.

Probing Cu corrosion in an aqueous nanodroplet/oil/metal three-phase environment revealed unique patterns of surface reactivity. The electrochemistry of high-index facets cannot be predicted simply from the low-index {001}, {011} and {111} responses.

Oil-Immersed Scanning Micropipette Contact Method Enabling Long-term Corrosion Mapping

This work reports the development of an oil-immersed scanning micropipette contact method, a variant of the scanning micropipette contact method, where a thin layer of oil wets the investigated substrate. The oil-immersed scanning micropipette contact method significantly increases the droplet stability, allowing for prolonged mapping and the use of highly evaporative saline solutions regardless of ambient humidity levels. This systematic mapping technique was used to conduct a detailed investigation of localized corrosion taking place at the surface of an AA7075-T73 aluminum alloy in a 3.5 wt % NaCl electrolyte solution, which is typically challenging in the conventional scanning micropipette contact method. Maps of corrosion potentials and corrosion currents extracted from potentiodynamic polarization curves showed good correlations with the chemical composition of surface features and known galvanic interactions at the microscale level. This demonstrates the viability of the oil-immersed scanning micropipette contact method and opens up the avenue to mechanistic corrosion investigations at the microscale level using aqueous solutions that are prone to evaporation under noncontrolled humidity levels.