Evaluation of corrosion behaviour and susceptibility to hydrogen-induced cracking in high-strength low-alloy X70 steels under sour gas conditions: Evaluation of corrosion behaviour and susceptibility to hydrogen-induced cracking in high-strength low-alloy X70 steels under sour gas conditions

High-strength low-alloy (HSLA) steels find extensive usage in the oil industry for manufacturing pipes due to their
requirement for materials with high mechanical and corrosion resistance. Among these steels, the HSLA X70 steels
are particularly of great interest to the petroleum industry because of its high mechanical resistance, ductility and weldability.
In this study, it was aimed to assess the corrosion behaviour of HSLA API X70 pipelines using electrochemical
impedance spectroscopy (EIS) technique and evaluate its resistance to hydrogen-induced cracking (HIC). The analyses
were conducted in non-sour and sour media, as per NACE TM0284 standard. Microstructural characterisations of
polished and corroded samples were also conducted using scanning electron microscopy (SEM) and optical microscopy
(OM). The results indicate that the resistance to HIC in API 5L X70 steel is compromised when exposed to surface corrosion
in sour gas media. This susceptibility may be attributed to the presence of microconstituents MnS and M/A along
the grain boundaries, which were found in the crack trajectory. The identified microstructural features are deemed to
exert a significant influence on the propagation of cracks and the consequent susceptibility to HIC. Moreover, the EIS
tests revealed the lowest corrosion resistance when exposed to H2S media, as MnS inclusions accelerate the active
regions in the matrix, consequently leading to the formation of a plethora of corrosion products and the dissolution
of iron.