External corrosion of carbon steel pipeline weld zones

Carbon steel pipelines are used extensively in the oil and gas industry for sub-sea applications. Usually they are cathodically protected and have exterior surface protective coatings. Where these are ineffective or not used, the evolution of weld zone corrosion with time may be of interest. Experience has shown that generally the heat-affected weld zone is more severely corroded by pitting than either the weld zone itself or the parent metal. However, quantitative data are scarce, particularly for longer-term exposures. Herein observations are reported of the pitting corrosion of API X56 Spec 5 L grade pipeline steel exposed continuously to natural Pacific Ocean seawater for 3.5 years, and extended to much longer exposure periods by comparison to similar steels exposed for up to 30 years. It is shown that the corrosion mass loss, maximum pit depth and pit depth variability are not simple linear functions of exposure time as often assumed in practice but are more complex functions. These functions are consistent with those observed previously for mild steels in various marine exposure conditions. However, considerable differences were noted in severity. The reasons for this are discussed and further work is outlined.