Corrosion of coated pipelines with cathodic protection.

摘要

In this work, fundamental models were constructed to investigate corrosion of pipelines. Four cases were studied: (1) corrosion with oxygen alone, carbon dioxide alone, and oxygen and carbon dioxide together on bare pipe, (2) corrosion with cathodic protection (CP) on bare pipe, (3) corrosion with a defective coating, and (4) corrosion with coating and CP together.; For the case of bare pipe corrosion with oxygen alone, the oxidation of the corrosion product, ferrous ion, in the solution boundary layer was found to decrease the corrosion rate of iron, by up to 13.9%, a value that is independent of oxygen pressure, boundary layer thickness, pH and temperature. The neglect of the effect of ferrous ion oxidation on the iron corrosion rate results in an overestimate of the corrosion rate. For bare pipe corrosion under a solution boundary layer with dissolved carbon dioxide, the corrosion rate of steel is greater than the rate in hydrochloric acid at the same pH. When carbon dioxide is present with oxygen, at a fixed oxygen partial pressure, carbon dioxide has nearly no effect on the corrosion rate at low partial pressures but increases the corrosion rate at higher pressures. This result reconciles a historical conflict on the effect of carbon dioxide on corrosion.; In a thin, narrow channel crevice between the pipe surface and the coating, corrosion can be caused by oxygen diffusion through a holiday in the coating when the imposed CP is insufficient. The oxygen concentration gradient within the crevice then will induce an internal current flow. Compared to normal corrosion, the effect of this internal current flow is to reduce the rate of corrosion near the holiday and to increase it away from the holiday. A higher solution resistivity or smaller crevice length increases the corrosion rate near the holiday. As the crevice becomes long and oxygen permeability in the coating becomes considerable with coating deterioration, oxygen diffusion through the coating may significantly increase the corrosion rate within the crevice, where imposed CP was found to be ineffective.