Effect of CO2 and R-ratio on near-neutral pH stress corrosion cracking initiation under a disbonded coating of pipeline steel

This study investigates the role of CO2 and cyclic stress R-ratio (R = minimum stress/maximum stress) on near-neutral pH SCC initiation mechanism(s) under a disbonded coating of pipeline steel protected by cathodic protection (CP). It was found that depending on CO2 concentration and level of CP, different localized environments with various pH could be formed under the disbonded coating. When cyclic loading was applied, different SCC initiation mechanisms were involved depending on the pH of the localized environments. Reducing the R-ratio had different effects on the initiation mechanisms.

► Different localized environments can be formed under a coating disbondment.
► Both the level of CP and CO2 affect the crack initiation mechanisms.
► Cracks are mainly initiated at regions with limited CP, not completely free from CP.
► R-ratio may promote or not affect the SCC initiation depending on the level of CO2.