Environmental Cracking of X-60 Pipeline Steel

Environmentally assisted crack (EAC) growth mechanisms affecting the external surface of API X-60 pipeline steel has been investigated. Low frequency cyclic tests at 4.63x10^-4 Hz were conducted in simulated groundwater (NS-4 solution) under near neutral (pH ≈7) conditions, similar to those for buried natural gas pipelines. The crack morphology and amount of growth were determined for two stress ratios, R (minimum/maximum stress), 0.5 and 0 and K_max=25.5 MPa√m under both constant and variable amplitude loading conditions. Examination of EAC was carried out using optical, scanning electron, focused ion beam (FIB) and transmission electron microscopy. Transgranular SCC occurred at the higher R-ratio of 0.5. The crack was relatively wide, filled with corrosion product. By contrast, stress corrosion cracking had taken place at R=0. The crack was transcrystalline. Its tip was extremely sharp, and the amount of growth was greater. For the sample subjected to variable amplitude loading conditions, the crack was less sharp, but significantly sharper than that the sample under constant load at R=0.5. The crack was open in many places, and the majority of the crack path was transgranular. Applying the superposition model to the variable amplitude test showed that the SCC growth rate accelerated when a single cycle at R=0 was combined with 159 cycles at R=0.5.