Environmentally Assisted Cracking of High Strength Nickel Based Alloys Under Cathodic Polarization

Environmentally assisted cracking of various high strength nickel based alloys used for subsea components was studied from the perspective of fatigue crack growth rate (FCGR) and static crack growth rate (SCGR) as a function of several critical variables in 3.5 wt% NaCl (pH = 8.2) under cathodic polarization. This paper compiles data from earlier sources as well as more recent work that has been performed in this area. The crack growth rate measured under various loading modes, i.e. cyclic, static, and varying stress intensity (K) profiles revealed evidence of susceptibility to hydrogen embrittlement. The measured crack growth rates under cyclic loading exhibited a strong dependence on loading frequency, and no evidence of a plateau in FCGR at low frequencies. The SCGR for various alloys was a function of not only the applied value of K but also of the applied strain rate K ̇/K. In general, alloy 725 exhibited the highest susceptibility to hydrogen embrittlement, while alloy 718 exhibited the highest resistance as reflected in both the values of threshold stress intensity factor (Kth) and SCGR values. SCGR was also measured as a function of the applied potential, and first principles modelling suggested that it was related to water adsorption under the crack tip conditions. A crack tip strain rate based model was applied to rationalize the effect of various loading modes on the measured crack growth rates.