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 (NS4 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, at
Kmax=25.5 MPavm 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 stress corrosion cracking (SCC) occurred at
the higher R-ratio of 0.5. The crack was relatively wide, filled with corrosion product. By
contrast, corrosion fatigue had taken place at R=0. The crack was also 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 of 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.
Keywords: pipeline steel, stress corrosion cracking, corrosion fatigue, variable amplitude
loading, crack propagation, focused ion beam microscopy