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

Abstract

Scanning micropipette contact method (SMCM) suffers from droplet evaporation and crystallization which limits the use of most saline electrolyte solutions under natural air humidity. To overcome the limitations and expand the application of SMCM, we improve this technique by scanning the droplet under mineral oil that covers on the substrate surface. The hydrophobic and inert oil significantly enhances 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 SMCM. 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 SMCM 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.