Microscopic Surface Studies of Modified Nickel-Based Superalloy 718 after Corrosion Fatigue in Simulated Sour Environment

The Unites States predicted 60% growth in energy demand by 2030 makes oil and natural gas primary target fuels for energy generation. The fact that the peak of oil production from shallow wells (< 5000 m) is about to be reached, thereby pushing the oil and natural gas industry into deeper wells. However, drilling to depths greater than 5000 m requires materials with high strength-to weight ratio, excellent corrosion resistance in H₂S-contating sour environments, and high toughness.

Ni-based superalloy, also called Alloy 718, has been considered for use as a bottom-hole component during offshore drilling due to its good general corrosion resistance and lack of magnetism. However, its susceptibility to environmentally assisted cracking (EAC)  has not been studied for microstructures with increased high strength. To fill this technology gap, the National Energy Technology Laboratory in collaboration with DNV is currently studying mechanisms of fatigue crack growth rate (FCGR) for Alloy718, oil grade version, in simulated sour well environments.

The FCGR behavior of oil grade Alloy 718, was investigated by monitoring crack growth rate in deaerated 0.2% and 20 % NaCl solutions, respectively, in contact with H₂S and CO₂ as a function of frequency. The partial pressures for H₂S (p_H2S) was 692 kPa and for CO₂ (p_CO2) was 3458 kPa in each solution. Each fatigue experiment was performed at 177°C at ΔK=1392 Nmm^-3/2 and R=0.2 in the frequency sequence 1Hz/ 0.3Hz/ 0.1Hz/ 0.03Hz/ 0.01Hz/ 0.003Hz followed by ΔK=522 Nmm^-3/2 and R=0.7 in the sequence 1Hz/ 0.3Hz/  0.1Hz/ 0.03Hz/ 0.01Hz in an autoclave. Surface investigations were augmented by scanning electron microscopy (SEM) and energy dispersive x-ray spectroscopy (EDS). In this study, research focused on surface analyses supported by the fatigue crack growth rate measurements.

SEM backscatter electron (BSE) micrograph of the fracture surface after the fatigue experiment carried in 0.2% NaCl containing CO₂ and H₂S revealed no accumulation of the corrosion products The EDS chemical analysis near the crack tip found mostly nickel. In general, only transgranular crack growth was observed over the investigated ΔK and frequency scans.

SEM investigations of the fracture surface of the sample after the fatigue test in the 20% NaCl containing CO₂ and H₂S. sour environment at 177°C revealed the presence of secondary cracks, bright inclusions, and beach marks. The EDS chemical analysis of the bright inclusion, showed a (Nb, C)-rich phase. Also, corrosion products were not detected near the crack tip.

It appears that the passive layer is disturbed by the cyclic stress in each solution. The presence of secondary cracks at the crack tip of Alloy 718 in 20% NaCl/p_H2S = 692 kPa/ p_CO2=3458kPa indicates that this environment is more aggressive than 0.2% NaCl/ p_H2S = 692 kPa/ p_CO2=3458kPa.