Corrosion resistance of ERW [electric resistance welding] seam welds as compared to metal base in API 5L steel pipes

The corrosion resistance of ERW seam welds and the base metal in API 5L X70 steel pipes was evaluated by Tafel tests, according to the ASTM G3 standard. The study was completed with metallographic and chemical characterization of the tested zones, i.e., the welded zone and the base metal away from the weld. All tests were made on the internal surface of the pipe to assess the internal corrosion of an in-service pipeline made of API 5L X70 steel. The test solution was acid brine prepared according to NACE Publications 1D182 and 1D196. The results showed that the ERW seam weld corrodes as much as three times faster than the base material. This behavior is attributed to a more heterogeneous microstructure with higher internal energy in the ERW seam weld zone, as compared to the base metal, which is basically a ferrite pearlite microstructure in a normalized condition. This result also indicates that pipeline segments made of ERW steel pipe where the seam weld is located near or at the bottom of the pipe are prone to a highly localized attack that may form channels of metal loss if there is water accumulation at the bottom of the pipeline.