Fatigue and fracture of line pipe steels in a sour inhibited environment is presented. Fatigue crack growth rate (FCGR) was investigated as a function of frequency at several different ΔK values at a pH of 3.5 and 0.21psia (0.00144MPa) H2S. The role of rise time and hold time were explored in a limited set of conditions. A plateau in the FCGR behavior at low frequency was observed and the value of FCGR was about 10 times higher than the in-air value. At low frequencies, introduction of hold times to the low frequency cycling did not lead to any appreciable change in the crack growth rate (CGR). Transitioning the crack to a constant K of 44MPa√m (i.e. no cycling) led to a steady sustained crack growth rate of 1.1×10-7mm/s. Changing the pH of the test solution from 3.5 to 7.8, by switching from a sour gas purge to pure N2 purge resulted in an eight fold decrease in the CGR. There was no significant variation in the CGR with changing K in the range of 41.25 to 48.4MPa√m.

The above results suggest that under carefully controlled experimental conditions, it is possible to transition from an environmentally assisted fatigue crack growth to sustained low static crack growth behavior in C-Mn line pipe steels in low pH environments.

Subject
Crack growth, Piping, Inhibitor performance, Sour environments, Crack propagation, Waves, Fatigue crack growth rate, Stress cracking, Lead, Constants, Sodium chloride solution, Steel, Hydrogen
Keywords:
Sour Service, Fatigue Crack Growth Rate, Static Crack Growth Rate, Frequency Scans
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2017
Association for Materials Protection and Performance (AMPP)
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