External Corrosion of Carbon Steel Pipeline Weld Zones

摘要

Carbon steel pipelines are used extensively in the oil and gasindustry for sub-sea applications. Usually they are cathodicallyprotected and have exterior surface protective coatings. Where theseare ineffective or not used, the evolution of weld zone corrosionwith time may be of interest. Experience has shown that generallythe heat-affected weld zone is more severely corroded by pittingthan 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 APIX56 Spec 5 L grade pipeline steel exposed continuously to naturalPacific Ocean seawater for 3.5 years, and extended to much longerexposure periods by comparison to similar steels exposed for up to30 years. It is shown that the corrosion mass loss, maximum pitdepth and pit depth variability are not simple linear functions ofexposure time as often assumed in practice but are more complexfunctions. These functions are consistent with those observedpreviously for mild steels in various marine exposure conditions.However, considerable differences were noted in severity. Thereasons for this are discussed and further work is outlined.