Flow enhanced corrosion of tee in gas transmission pipeline

•The main cause of failure of the tee is peeling off of inner coating.•Flow pattern change at the variable diameter position exacerbates the erosion.•Under the effect of flow enhanced corrosion, galvanic corrosion, leakage occurs. A tee failed due to flow enhanced corrosion at an oil field was investigated and analyzed by macroscopic analysis, nondestructive tests, material tests (including chemical composition, metallurgical analysis) and microstructure analysis (including microtopography, SEM, EDS and XRD) in this paper. It was concluded that the chemical composition and metallurgical properties of the tee accord with related standards, and the anti-corrosion coating on the inner wall peeled off is considered as one of the main failure factors of tee. The corrosion failure mechanism analysis results shows that the change of flow pattern caused the sudden change of the inner diameter between weld reinforcement and nickel based coating results in water gathering and flow enhanced corrosion at the concave area. Also, galvanic corrosion occurs at the concave area formed by weld reinforcement and nickel based coating between the carbon steel tee and nickel based coating due to high electrode potential difference. The galvanic corrosion further accelerated the corrosion process. Under the combined action of erosion, galvanic corrosion and flow enhanced corrosion, perforation leakage finally occurs.