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.

In this study, we investigated the formation mechanism and chemical structure of melanin that results from the self-assembly of L-3,4-dihydroxyphenylalanine (L-DOPA). Using a combination of "top-down" and "bottom-up" approaches, and on the basis of state-of-the-art electrospray ionization mass spectrometry (ESI-MS) results, we propose a new formation mechanism and an alternative structure for melanin. Specifically, our study of the self-aggregation of L-DOPA based on L-DOPA clusters revealed that melanin is comprised partially of noncovalent supramolecular aggregate that is formed by self-aggregation of L-DOPA and with the individual monomers linked together by a combination of hydrogen bonds, π-π stacking, and ionic bonds. Furthermore, our study showed that unmodified L-DOPA may be part of the building block for melanin in addition to the previously proposed indole derivative based on L-DOPA cyclization. A similar self-aggregation phenomenon was also observed in other structurally related catecholamines, for example, adrenaline.

Quantitative scanning micropipette contact method measurements are subject to the deleterious effects of reference electrode interference. The commonly used Ag/AgCl wire quasi-reference counter electrode in the miniaturized electrochemical cell of the scanning micropipette contact method was found to leak Ag+ into the electrolyte solution. The reduction of these Ag+ species at the working electrode surface generates a faradaic current, which significantly affects the low magnitude currents inherently measured in the scanning micropipette contact method. We demonstrate that, during the microscopic corrosion investigation of the AA7075-T73 alloy using the oil-immersed scanning micropipette contact method, the cathodic current was increased by the Ag+ reduction, resulting in positive shifts of corrosion potentials. The use of a leak-free Ag/AgCl electrode or an extended distance between the Ag/AgCl wire and micropipette tip droplet eliminated the Ag+ contamination, making it possible to measure accurate corrosion potentials during the oil-immersed scanning micropipette contact method measurements.

In single-channel scanning electrochemical cell microscopy, the applied potential during the approach of a micropipette to the substrate generates a transient current upon droplet contact with the substrate. Once the transient current exceeds a set threshold, the micropipette is automatically halted. Currently, the effect of the approach potential on the subsequent electrochemical measurements, such as the open-circuit potential and potentiodynamic polarization, is considered to be inconsequential. Herein, we demonstrate that the applied approach potential does impact the extent of probe-to-substrate interaction and subsequent microscale electrochemical measurements on aluminum alloy AA7075-T73.

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.