Microscopic Surface Studies of Modified Nickel-Based Superalloy 718 after Corrosion Fatigue in Simulated Sour Environment
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 (pH2S) was 692 kPa and for CO2 (pCO2) 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/pH2S = 692 kPa/ pCO2=3458kPa indicates that this environment is more aggressive than 0.2% NaCl/ pH2S = 692 kPa/ pCO2=3458kPa.