Measuring changes in wettability and surface area during micro droplet corrosion measurements

Abstract We report a new method for directly monitoring dynamic surface wetting that occurs during electrochemical droplet cell corrosion measurements on an Al alloy and Cu galvanic couple. The combination of a goniometer and potentiostat enabled the in situ observation of wettability during polarization of each metal and at the interface of the two metals in 0.4 μL NaCl aqueous droplets. Droplet spreading was heterogeneously dynamic for measurements at the interface of the two metals, where oxygen from the surrounding environment diffused into the droplet edges and reduced on the cathodic metal, thus increasing the wettability on this region, while the anodic metal served as a sacrificial anode and remained in a constant wetted state. Such dynamic wetting was inhibited by performing measurements under anaerobic conditions, where minimal changes in voltammetric response were observed in comparison the measurements performed in air. All voltammetry was normalized by the true surface area of the droplet footprint, which was measured after droplet evaporation to avoid overestimating corrosion current densities. This work highlights the potential for the combined goniometer-electrochemical setup to further enhance the corrosion community's knowledge about interfacial chemistry and the importance of considering the dynamic evolution of micro/nanoscale droplets on heterogeneous substrates when estimating corrosion rates.