Understanding carbon steel corrosion in CO₂environments: influence of impurities

In carbon capture utilisation and storage systems, impurities in captured CO₂ 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 – H₂S, SO₂ and HNO₃ present in aqueous CO₂ 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 HNO₃ environments that proceeds by pit initiation followed by pseudo-passivation, and (ii) antagonistic effects in multi-impurity systems, where simultaneous presence of H₂S, SO₂ and HNO₃ produces corrosion rates comparable to or lower than HNO₃ alone, contradicting the prevailing assumption of additive aggressiveness. (iii) Sequential impurity exposure further revealed that H₂S can stabilise surfaces pre-corroded by HNO₃ through the formation of protective sulphide layers, while SO₂ consistently destabilises both sulphide and nitrate modified surfaces. Collectively, these findings provide essential mechanistic insights, refine the understanding of impurity interactions in CO₂ 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.