Failure Analysis and Stress Corrosion Cracking Mechanism of UNS N08825 Hydraulic Control Lines in Heavy Oil Thermal Recovery Wells

Corrosion cracking occurred in UNS N08825 hydraulic control lines during heavy oil thermal recovery at 350°C. In this study, failure analysis was conducted through fracture surface analysis, crack metallographic analysis, material property testing, and failure reproduction tests. Based on SEM images of the fracture and metallographic images of the cracks, it was found that the cracks in the hydraulic control line initiated from the outer surface of the pipe wall and propagated toward the inner wall in a dendritic morphology until fracture, with the fracture surface exhibiting brittle fracture characteristics. Based on fractography and crack analysis, the failure was identified as stress corrosion cracking (SCC). Corrosion tests were performed to reproduce and verify the failure mechanism. The corrosion tests further elucidated the role of oxygen in SCC, demonstrating that oxygen synergistically accelerates pitting with chloride ions, facilitates hydrogen sulfide penetration into the grain interior, and ultimately leads to SCC under applied stress.