A thorough characterization of nickel-based UNS N09955 was performed in saltwater with cathodic protection (SWCP) environment for high-pressure high-temperature (HPHT) subsea applications. The test environment was deionized water with 3.5% NaCl, pH of 8.2, CP potential of −1,050 mV vs. saturated calomel electrode, and at 40 °F (4.4 °C).
Environmentally assisted cracking susceptibility of the alloy was evaluated by performing fracture toughness test using compact tension test specimen and rising displacement method in air and in SWCP as well as fatigue crack growth rate (FCGR) and static crack growth rate (SCGR) in SWCP. Fracture toughness test in SWCP conditions indicated a significant reduction in the initiation fracture toughness value of the alloy in SWCP versus in-air values. FCGR by performing frequency scan at various ΔK values and SCGR of the alloy was studied in SWCP condition. SCGR was obtained at different load holds of 55, 66, 75, and 99 MPa√m and various CP potentials of −1,050, −1,000, −950, and −900 mV. No crack growth was observed at 55 MPa√m, and a crack growth rate (CGR) of 2 × 10−7 mm/s was established at −1,050 mV CP potential. By changing the load and electrode potential, CGR was varied. The obtained results were also compared with other precipitation hardened nickel alloy behavior.
Finally, scanning and transmission electron microscopy (SEM and TEM) were used to characterize the fracture surface morphology, grain structure, and grain boundary precipitates, as well as the shape, size, and distribution of the precipitates.
Precipitation hardened (PH) Ni-based alloys have been utilized in oil and gas industry for decades. Among them, UNS1 N07718 because of its performance in sour wellbore fluids and in hydrogen charging environments has received the most attention for multiple upstream applications such as tubing hangers, production stab, multi-phase flow meter bodies, valve stems, etc.