Influence of Anodic Current on Corrosion Protection Conditions of Buried Steel Pipeline under Cathodic Protection

摘要

Cathodic protection is commonly used to protect buried steel pipelines from corrosion. In general, the polarized potential of pipelines is maintained at a value less noble than the protection potential criterion, which is -0.85 V vs. Cu/CuSO_4. However, an instantaneous anodic current has been infrequently observed in pipelines whose polarized potential is maintained at a value less noble than -0.85 V. In the present study, the influence of this instantaneous anodic current on the corrosion protection conditions of buried steel pipelines under cathodic protection was investigated. Anodic dissolution was not observed on the anodic polarization curve of steel and the maximum corrosion rate of steel was found to be only 1.8 µm-year~(-1), under 20% of the generally accepted corrosion rate for buried steel pipelines. Therefore, it seems that the instantaneous anodic current observed at the potential less noble than -0.85 V does not influence the corrosion protection conditions of buried steel pipelines. Furthermore, most of the anodic current did not seem to be attributed to the corrosion reaction, and a part of the anodic current at -0.85 V was estimated to be attributed to the oxidation of various components in soil such as sulfur or organic matter.