Steel Corrosion under a Disbonded Coating with a Holiday—Part 2: Corrosion Behavior

Corrosion of a pipe surface in a channel crevice has been modeled assuming oxygen diffusion through a holiday in a permeable coating. The crevice was thin and narrow with a finite length. The crevice solution pH was 9.2 controlled by ferrous hydroxide (Fe[OH]₂) precipitate, external oxygen pressure was 0.21 atm, the crevice gap between the coating and the pipe surface was 0.5 mm, and the coating thickness was 0.05 mm. Without imposed cathodic protection (CP), an oxygen concentration cell formed inside the crevice. The effect of this cell was that the pipe surface was less anodically polarized near the holiday and more anodically polarized away from the holiday. This internal polarization (IP) resulted in less corrosion near the holiday and a broad extension of corrosion within the crevice. Imposed CP weakened the effect of IP and decreased corrosion at the pipe surface because it consumed oxygen. Oxygen diffusion through the coating weakened the effect of IP and increased corrosion in the crevice. The greater the solution resistance and the smaller the total crevice length, the weaker the IP and the greater the corrosion current near the holiday.