Salt‐induced iron corrosion under evaporating sessile droplets of aqueous sodium chloride solutions

Abstract

The corrosion process induced by evaporation of sessile droplets from aqueous sodium chloride solutions on planar iron surfaces was quantitatively investigated. The spatial distribution of the salt inside the evaporating sessile droplet, which is influenced by the initial salt concentration in the droplet bulk, is correlated to the localization of the anodic and cathodic reactions at the electrolyte–metal interface over the footprint droplet area. At low salt concentration, the inverse of the classical well‐accepted Evans model is observed due to the hydrodynamic flows linked to evaporation: the anode area is established near the three‐phase contact line region. Increasing salt concentration leads to a more uniform pitted sessile droplet: we observed locally anodic and cathodic areas over the droplet surface at the metal–electrolyte interface, where local variations in chloride concentrations occur. In addition, the presence of corrosion products and some pits demonstrate that pitting corrosion takes place if the salt concentration reaches a threshold value, experimentally determined by an optical set‐up.