Fatigue crack propagation of API 5L X70 steel in a modified simulated fuel-grade ethanol (MSFGE) environment

In recent years, the claim for renewable energy sources has increased worldwide, especially in Europe. In this context, fuel-grade ethanol has been more adopted as energy source. Therefore, the understanding of the carbon steel pipelines’ degradation mechanism when they are in contact with this renewable fuel is of great importance. In this work, fatigue crack growth tests were performed in order to evaluate the effect of modified simulated fuel-grade ethanol (MSFGE) on the fatigue crack growth of API 5L X70 pipeline steel. The tests were conducted under a constant force ratio of 0.1 and frequencies of 0.5 Hz (increasing ∆K) and 0.25 Hz (increasing ∆K and decreasing ∆K). The fatigue crack growth rates measured in MSFGE tests were higher compared to those obtained in typical conditions. This effect was more pronounced in the test performed at 0.25 Hz. The microstructure and fracture surface analysis of specimens by scanning electron microscopy confirmed the embrittlement effect of MSFGE. Some of the observations indicated hydrogen-induced cracking contribution.