Microbiologically influenced corrosion of underground pipelines under the disbonded coatings

The microbiologically influenced corrosion (MIC) of underground pipeline was identified. Corrosion was typically found under the disbonded area of girth welds coating where cathodic protection (CP) current could not penetrate. Sulfate reducing bacteria (SRB) and fermentative acid producing bacteria (APB) were confirmed as the microbes involved in the corrosion process. This corrosion occurred despite the fact that the CP level was well below the criteria of -850 mV (Cu/CuSO4). Detailed field surveys and laboratory analysis revealed the presence of high numbers of microbes at these sites, metallurgical and mineralogical fingerprints of MIG. The results indicate that the synergistic effects of disbonding, the ineffective input of CP current under the disbonded coatings, and environments favorable to bacterial growth were the corrosion mechanism.