Corrosion reason analysis and countermeasures of buried oil unloading pipeline in an oilfield

•We combines various methods such as on-site inspection, laboratory analysis, and simulation experiments to study the corrosion cause of an unloading pipeline.•The morphology, composition, and structure of the corrosion product film have been thoroughly and meticulously studied, and verified through simulation experiments.•The study revealed the corrosion mechanism, the main influencing factors and proposed protective measures. It is of great significance for extending the service life of pipelines and improving the safety and reliability of pipeline operation. The material, corrosion morphology and characteristics, as well as the compositions of corrosion products of a 20# steel unloading pipeline were studied using on-site C-scan detection, laboratory corrosion morphology analysis, material metallographic analysis, chemical composition analysis and tensile testing. The uniform corrosion and pitting behavior of 20# steel were studied using a high temperature and high pressure autoclave under conditions of 50 °C and CO2 partial pressure of 0.05 MPa, 0.1 MPa, and 0.2 MPa, respectively. The microstructure and composition of the surface and cross-section corrosion product film were analyzed by SEM. The results show that the uniform corrosion rate of 20# steel increases with the increase of CO2 partial pressure. When the CO2 partial pressure reaches 0.2 MPa, there are a large number of corrosion pits on the surface of the sample, and the maximum local corrosion rate reaches 1.8719 mm/a. The corrosion product film has a multi-layer structure, with a surface layer of FeCO3 corrosion product films and an inner layer of (Ca, Mg)CO3 deposition layer. There are obvious corrosion pits at the interface between the corrosion product film and the substrate locally. The severe thinning of the on-site pipeline wall is mainly caused by CO2 corrosion. The defects of the corrosion product film and the deposition of scale layer accelerate local corrosion.