Atmospheric-Corrosion Electrochemistry of NaCl Droplets on Carbon Steel

To understand the mechanism of marine atmospheric corrosion on steel, the electrochemistry of NaCl droplets formed by the deliquescence of pre-deposited NaCl particles upon exposure to high humidity on carbon steel surfaces was explored. The concentrations of the newly deliquesced NaCl droplets were near saturation. The pH of the droplets was in the range of 8–9 during the first several minutes in the humidity chamber. The open-circuit potential (OCP) of the steel was in the range of −0.10–0.0 V SCE for small droplets (i.e., D < ∼100 μm) and −0.50–0.30 V SCE for larger droplets (i.e., D > ∼100 μm). Anodic polarization showed that the steel passivated under the small droplets and corroded actively under the larger droplets. Cathodic polarization indicated oxygen reduction as the primary cathodic reaction under NaCl droplets of different sizes. The diffusion-limited oxygen reduction current density increased as the droplet size decreased and was inversely proportional to the droplet diameter in the range of 103–426 μm.