Study of the Effect of Isothermal Aging on API 5L Grade B Steel Through Charpy Energy Testing and Electrochemical Evaluation

Oil and gas pipelines are often threatened by mechanical degradation and corrosion damage. This mechanical degradation is attributable to the aging of the carbon steel, and corrosion damage results from the level of environmental aggressiveness combined with the material's metallurgical characteristics. Unfortunately, studying the natural aging of steel in in-service pipelines is difficult because many tests are destructive. For this reason, the present study used isothermal artificial aging to simulate the mechanical degradation of pipeline steel. Charpy energy tests were carried out to assess the changes in the material's ductility. It was found that this energy exhibited random behavior. Our findings indicate that, on average, the aged material exhibited lower Charpy fracture energy and a much higher variance. This evidence will help experts select the correct material properties while considering its degradation over time. In addition, according to the fractographic study, it was confirmed that the aged pipeline steel tended to be more brittle than the unaged material. Moreover, the dominant corrosion mechanism observed during electrochemical tests for aged and unaged steel was analyzed. We concluded that a moderate chloride ion content is insufficient to stimulate pitting corrosion. Corrosion rates with three different electrochemical techniques (LPR, EIS, and ENA) were obtained, showing that the unaged material tended to be more vulnerable to corrosion effects.