Transgranular crack growth in the pipeline steels exposed to near-neutral pH soil aqueous solutions: The role of hydrogen

Corrosion fatigue crack growth behavior of pipeline steel under underload-type variable amplitude loading schemes

“…The crack growth rate in the frequency range higher than 10 À3 Hz under constant amplitude fatigue loading has been well studied. It is generally believed that the fatigue crack growth rate increases with lowering frequency, because of the increased time either for the interaction of corrosive chemicals with the metal or for hydrogen diffusion [6,7]. However, such a conclusion could not be extended to the frequency range lower than 10 À3 Hz, within which there is always sufficient time for hydrogen to diffuse to the weakest link in the material ahead of the crack tip, in accordance with the change of stress.…”

“…The average time interval between two neighboring cycles with R-ratios lower than 0.9 in the spectra shown in Fig. 1 was found to be about 36 h. The loading frequency of minor cycles was set to be at 5.0 Â 10 À3 Hz, which is often used in laboratory simulations [6,7,16]. The loading frequency actually measured during testing on a pneumatic loading frame was 5.4 Â 10 À3 Hz due to some execution errors in the test system, which yielded a total of 697 cycles for a 36-h loading interval of the minor cycles.…”

“…A gas mixture of 5% CO 2 balanced with nitrogen was purged through the C2 solution before and during the test to achieve a stable pH value in the solution of 6.29 [6]. The CT specimens were sealed in test cells filled with C2 solution and pin loaded on a horizontal pneumatic loading machine with computer load control [7]. The waveform shown in Fig.…”

“…(iii) Both time-and cycle-dependent processes, for example, may correspond to a maximum segregation of hydrogen to the crack tip that is both time and cycle dependent [6][7][8]. (iv) Time-dependent mechanical property attributes, such as room temperature creep [9][10][11] and high temperature creep [12], both of which could lead to crack tip blunting and mechanical damage to the crack tip materials.…”

“…(2) Scenario 2: The rate of pressure fluctuations, often referred as loading frequency, produces variable pressure fluctuations, which may yield different time-dependent contributions to the crack growth, which is categorized as time/frequency-dependent load interaction. These time-dependent contributions may include: the rate of corrosion, the rate of hydrogen segregation by diffusion to the crack tip [7], and the degree of crack tip blunting caused by room temperature creep [7,[9][10][11] and interactions between hydrogen and dislocations [21,22].…”