Mechanisms and evidence of chloride-accelerated pitting in gas pipeline steel

Internal corrosion poses a significant challenge to the mechanical integrity and operational reliability of oil and gas transmission pipelines, often causing premature failures and environmental risks. This study offers a detailed investigation of localized pitting corrosion observed on a pipeline wheel segment after ten years of continuous service under field conditions. A range of materials characterisation techniques, including scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), and thermogravimetric analysis combined with mass spectrometry (TGA-MS), were utilised to examine the corrosion morphology, composition, and mechanisms. The results show that the main corrosion products are rich in chlorine and sulphur compounds, indicating the combined influence of these aggressive ions in the corrosion process. Chlorine appears to be the key factor initiating pit formation, with a clear radial propagation pattern that suggests autocatalytic growth of pits. The detection of chlorine-induced compounds such as akaganeite (beta-FeOOH), supports the theory of chloride-induced passivity failure and localised acidification. These findings confirm that pitting corrosion, mainly driven by chlorine attack, is the principal degradation mode in the pipeline section studied.