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

A test specimen, comprising 100 iron wires of 1 mm phi diameter arranged in a 10 x 10 matrix, was exposed to temperature cycling between 0 degrees C and - 20 degrees C. NaCl solution (0.1 wt%) was dropped on the surface to form an ice droplet, and the coupling current of each iron electrode against the other 99 electrodes was measured sequentially to obtain a coupling current map. The average coupling current of 100 electrodes varied with temperature cycling. The coupling current increased at a relative humidity > 65% in the absence of an ice droplet, which was similar to atmospheric corrosion at a room temperature higher than the freezing point. When an ice droplet exists, the coupling current increased with increasing temperature and did not depend on relative humidity. This behavior was interpreted as the formation of a thin solution layer of concentrated NaCl solution at the interface between the electrode surface and ice due to the exclusion of NaCl from the growing ice crystal of pure water. From the coupling current map, the inner area of the iron electrodes beneath the ice droplet tended to be a cathode, whereas the outer and surrounding area tended to be an anode. An open circuit potential map was also measured using a quasiAg/AgCl electrode placed on the specimen surface. The potential of the inner area was less noble than the outer and surrounding areas and shifted in a less noble direction with temperature. The ice droplet shrank during the temperature cycling and left rust on the surface