Visualization of Coupling Current and Immersion Potential of Iron Surface Corroding Beneath the Ice in the Temperature Cycling under the Freezing Point

The test sample composed of 100 iron wires of 1 mmφ in diameter in the arrangement of 10 × 10 matrics was exposed to the temperature cycling between 0 and -20 °C. 0.1 w% NaCl solution was dropped on the surface to form an ice droplet, and the coupling current of each iron electrode against the other 99 electrodes was sequentially measured to obtain a coupling current map. The averaged coupling current of 100 electrodes fluctuated with temperature cycling. In the absence of an ice droplet, the coupling current increased at the relative humidity higher than ca. 65%, which was similar with atmospheric corrosion at the temperature higher than the freezing point. When an ice droplet exists, the coupling current increased with increasing temperature rather than the relative humidity. This behavior was interpreted that the thin solution layer of concentrated NaCl solution was formed at the interface between the electrode surface and ice due to the exclusion of NaCl from the growing ice crystal of pure water. In the coupling current map, an inner area of iron electrodes beneath the ice droplet tended to be a cathode, while an outer and a surrounding area tended to be an anode. An open circuit potential map was also measured using a quasi-Ag/AgCl electrode placed on the sample surface. The potential of the inner area was less noble against the outer and surrounding area and shifted with temperature cycling. The ice droplet shrank in the temperature cycling and left rust on the surface.