Electrochemical Formation Mechanism of Microdroplets on Pure Iron

The electrochemical formation mechanism of micro-droplets formed around a primary droplet of 3.5% NaCl solution on an iron-plated film was investigated by quartz crystal microbalance (QCM) and concentric three-electrode array (CTEA) measurements. During the initial stage, the micro-droplets mainly originate from evaporation owing to cathodic polarization and electric current of the localized corrosion cell under the primary droplet. The maximal electrochemical potential difference between the anode and cathode was measured to be 0.36 V and acted as the driving force for the formation of micro-droplets. The maximums of anodic and cathodic electric current density of pure iron under the NaCl droplet are 764 and -152 μA/cm2, respectively. Propagation of micro-droplets in the developing stage attributes to horizontal movement of electrolyte, water evaporation, and re-condensation from primary and capillary condensation from moist air. The results of the study suggest that the initiation and propagation of micro-droplets could promote and accelerate marine atmospheric corrosion.