Electrochemical behaviour of carbon steel in aqueous amine solvents for post combustion carbon capture

Abstract

Post combustion CO2 capture (PCCC) through amine scrubbing is one of the few commercially deployed CO2 emissions abatement technologies to date. Though essential for attaining global net zero anthropogenic CO2 emissions, widescale deployment is being hampered by technical issues, one of which is the high costs due to corrosion in PCCC plants. Monoethanolamine (MEA), the benchmark amine solvent, has been demonstrated to become increasingly corrosive with elevated temperatures and CO2 enrichment, so requiring plant construction with costly stainless steel (SS 316L), ca. 5 - 6 times the cost of carbon steel (CS). Methyldiethanolamine (MDEA), widely used in natural gas processing, has been reported to promote the formation of FeCO3 on CS, which is thought to provide corrosion protection to the underlying substrate, particularly at high temperatures and when saturated with CO2. Therefore, this project aimed to assess the use of pre-treated CS to eliminate the requirement for SS in areas of the PCCC plants particularly susceptible to corrosion, and explain the divergent behaviour exhibited by CS in MEA(aq) and MDEA(aq) solutions. Voltammetry, chronoamperometry and electrochemical impedance spectroscopy, together with ex-situ surface characterisation by scanning electron microscopy, energy dispersive spectroscopy, and X-ray diffraction, were used to measure FeCO3 film formation time, resistance to charge transfer, composition and morphology as a function of MDEA concentration at 80 °C. Protective crystalline FeCO3 films were found to be formed optimally on CS (1018) within 72 h in 1 M MDEA(aq) solutions. Subsequent stability tests in MEA(aq) solutions demonstrated promising results, with pre-treated C1018 being 10 times more resistive to charge transfer than untreated C1018. Amine solutions were deaerated with N2 and loaded with CO2 prior to experiments and were purged with CO2 during experiments. However, adventitious ingress of aerial O2 caused iron oxide(s) to form on surface films in several experiments, accelerating localised attack, which has implications for implementing this strategy in PCCC plants, since feed gas streams typically contain O2. Using these results, pre-treated C1018 coupons were tested in the PCCC pilot plant at Imperial College London. Though tested under non-ideal conditions, the surface film formed on coupons in the dilute 1 M MDEA(aq) pre-treatment solution remained intact and provided protection to the underlying substrate for up to 225 h in MEA(aq) solutions. A diffusion reaction model was built to determine the effect of the concomitant H+ release with FeCO3 formation on the stability of surface films. A substantial local pH decrease was estimated from the model during the potentiostatic formation of FeCO3, creating unfavourable conditions for its stability. Based on the observed presence of these films produced in MDEA(aq), a mechanism for local pH stabilisation was proposed, involving the effective protonation of MDEA and carbonate ions, thus providing an explanation for the divergent behaviour exhibited by CS in these two amines.