Relationship between Localized Corrosion and Stress Corrosion Cracking of Nickel Based Alloys in HPHT Oil and Gas Environments

Abstract

Abstract Fatigue and fracture performance of UNS N07718 (718) and UNS N07716 (625+) in sour environments over a range of temperatures (300 to 400°F) and chloride concentrations (0.15wt% to 25wt% NaCl). Rising displacement fracture toughness test performed in a range of environments indicated that the fracture toughness was a strong function of chloride concentration and temperature. The initiation fracture toughness values were lower in environment compared to the in-air values. The initiation toughness decreased with increasing chloride concentration and temperature. At a given chloride concentration, the initiation fracture toughness at 400°F was significantly lower than at 300°F or 350°F. Increasing chloride concentration at 400°F led to a sharp decrease in the initiation toughness. Decreasing initiation toughness was associated with increasing intergranular cracking. Increasing temperature and chloride concentration resulted in a decrease in the repassivation potential of IN718. The decrease in repassivation potential was sharp at 400°F. Increasing chloride concentration at 400°F also led to a sharp decrease in the repassivation potential. Similar behavior was observed for 625+. There was no significant effect of the chloride concentration on the corrosion potential. The decrease in fracture toughness coincided with a decrease in the repassivation potential (in particular when the repassivation potential was below the corrosion potential), which suggest that in sour environments at elevated temperatures, the factors that are responsible for stabilizing pit growth also contribute to stabilizing local environments that favor environmentally assisted cracking.