Abstract
A section of 24 inch oil pipeline experienced pitting and corrosion problems. The cause of pitting and the corrosive nature of solids observed in the pit on the base metal were investigated. The analysis of the solids showed the presence of multiple layers. The layering was typically an adherent layer followed by a porous layer. The corrosion products from the pits were analyzed by XRD, SEM, EDS, and optical microscopy. At least one sample showed presence of organic matter under SEM. However no further evidence of organic (bacteria) matter was observed on other samples. Electrochemical tests were performed to investigate the corrosive nature of solids using zero resistance ammeter (ZRA) and cyclic polarization methods. ZRA testing revealed that the sample covered with solids is not cathodic to sample without solid deposition. On the contrary the sample without deposited solids showed a nobler potential (40 mV higher) than sample covered under solids. The corrosion current (cyclic polarization) of samples with solids was 15 times higher than sample without solids. Thus this investigation concluded that under the experimented conditions, the presence of deposits caused an increase in corrosion of base metal below the deposits due to a galvanic corrosion mechanism.
