Micro Droplet Corrosion: Measuring Changes in Wetting and Surface Area during Electrochemical Measurements

Abstract

Dynamic surface wetting that occurs during electrochemical droplet cell corrosion measurements on an Al alloy and Cu galvanic couple was monitored using the combination of a goniometer and potentiostat. The in situ observation of wettability during polarization of each metal and at the interface in 0.4 μL, 0.05 M NaCl aqueous droplets was performed. Droplet spreading was heterogeneously dynamic for measurements at the interface of the two metals due to the diffusion of oxygen from the surrounding environment into the droplet edges that was reduced on the cathodic metal, thus increasing the wettability on this region. The anodic metal served as a sacrificial anode and remained in a constant wetted state. Performing measurements under anaerobic conditions, where minimal changes in voltammetric response were observed, inhibited such dynamic wetting. All voltammetry was normalized by the surface area of the droplet footprint, which was measured after droplet evaporation instead of overestimating the corrosion current densities by assuming the droplet size to be same diameter as the pipette aperture. This work highlights the potential for the combined goniometer-electrochemical setup to further enhance the corrosion community’s knowledge about interfacial chemistry, the influence of surface preparation on droplet footprint size, and the importance of considering the dynamic evolution of micro/nanoscale droplets on heterogeneous substrates when estimating corrosion rates.