Understanding carbon steel corrosion in CO ₂ environments: influence of impurities

In carbon capture utilisation and storage systems, impurities in captured CO 2 streams can strongly alter corrosion behaviour, yet their interactive mechanisms remain poorly understood. This study investigated the corrosion behaviour of API L80-1Cr carbon steel under impurities – H 2 S, SO 2 and HNO 3 present in aqueous CO 2 in simulated conditions relevant to carbon capture utilisation and storage systems but at ambient pressure and temperatures of 5 °C and 30 °C. The individual and combined effects of these impurities on corrosion rates of carbon steel and its morphology were examined by using potentiodynamic polarisation, linear polarisation resistance, scanning electron microscopy and X-ray diffraction. The results showed key novel findings: (i) corrosion in HNO 3 environments that proceeds by pit initiation followed by pseudo-passivation, and (ii) antagonistic effects in multi-impurity systems, where simultaneous presence of H 2 S, SO 2 and HNO 3 produces corrosion rates comparable to or lower than HNO 3 alone, contradicting the prevailing assumption of additive aggressiveness. (iii) Sequential impurity exposure further revealed that H 2 S can stabilise surfaces pre-corroded by HNO 3 through the formation of protective sulphide layers, while SO 2 consistently destabilises both sulphide and nitrate modified surfaces. Collectively, these findings provide essential mechanistic insights, refine the understanding of impurity interactions in CO 2 transport environments and highlight the importance of considering impurity interplay, not only single-species effects, in the design of resilient carbon capture utilisation and storage infrastructure.