Effect of Passivity of the Oxide Film on Low-pH Stress Corrosion Cracking of API 5L X-65 Pipeline Steel in Bicarbonate Solution

Effect of oxide film passivity on low-pH stress corrosion cracking (SCC) of API 5L X-65 pipeline steel in bicarbonate solution was investigated using potentiodynamic polarization, optical microscopy, the abrading electrode technique, electrochemical impedance spectroscopy, scanning electron microscopy (SEM), and slow strain rate test (SSRT). A decrease in solution pH resulted in a narrower passive region and a lower breakdown potential caused by the formation of a less-protective film. Optical micrographs showed that reduced solution pH promotes the pitting phenomena. Potentiostatic current transients obtained from the moment just after interrupting the abrading action revealed that the repassivation rate of the pipeline steel is reduced. At the same time, the pit growth rate of the pipeline steel is increased with decreasing solution pH, which was ascribed to the formation of a less-protective oxide film in the early stage of repassivation and hence increased pitting susceptibility in low-pH solution. Moreover, the oxide film resistance decreased with decreasing solution pH as a result of the easy charge transfer through the less-protective oxide film. From the observation of the SEM fractograph after SSRT, it was found that stress corrosion failure mode in low-pH bicarbonate solution is transgranular SCC (TGSCC). Based on experimental results, it was inferred that in low-pH solution TGSCC is developed by a pit-to-stress corrosion crack transition mechanism.