Stress corrosion cracking and corrosion fatigue analysis of API X70 steel exposed to a circulating ethanol environment

API X70 steel has been widely used to manufacture pipes to carry ethanol. However, a recognized problem in carbon steel pipes exposed to ethanol is stress corrosion cracking (SCC) and, during their operation, pipelines are also subject to pressure fluctuations, causing concern regarding fatigue behavior. In this study, slow strain rate (SSR), constant load (CL), staggered constant load (SCL) and fatigue crack growth (FCG) tests were performed on API X70 steel specimens in a circulating synthetic ethanol solution (CSES) and in air to evaluate SCC susceptibility and corrosion fatigue. Results revealed a significant strain decrease for notched specimens under SSR, indicating a significant SCC effect, but it showed to be minimized in unnotched specimens. Under CL, the steel appears not to be sensitive to SCC at stress levels below the yield point; however, under SCL, clear evidence of the phenomenon at the fracture surface was revealed. Alteration in initial portion of the da/dN-ΔK curves obtained in CSES was observed, shifting ΔKth (threshold) to higher values. The Paris-Erdogan linear regions in CSES exhibited a steeper slope relative to the air baseline, i.e., showed crack acceleration, thus indicating some influence of the corrosion fatigue mechanism. Changes caused by SCC in the linear region at certain levels of ΔK at 0.25 Hz were also revealed.