Understanding the Influence of SO₂and O₂on the Corrosion of Carbon Steel in Water-Saturated Supercritical CO₂

Abstract

The general and localized corrosion behaviour of carbon steel (UNS G15130) in water-saturated supercritical CO 2 conditions containing O 2 and SO 2 at 35°C and 8 MPa is evaluated with a view to the effect this may have on pipeline integrity during dense-phase CO 2 transport. The results indicate that crystalline FeCO 3 forms in the presence of solely water and CO 2 . However, the combined introduction of small concentrations of O 2 and SO 2 (as low as 20 and 2 ppm (mole), respectively) change FeCO 3 crystal morphology. Increasing the concentration of SO 2 to 50 and 100 ppm whilst maintaining O 2 content at 20 ppm resulted in the formation of FeSO 3 ·3H 2 O as well as FeCO 3 . In conjunction with the change in corrosion product chemistry and morphology, general corrosion rates of samples increased from 0.1 mm/year to 0.7 mm/year as a result of the rise in SO 2 content from 0 to 100 ppm (based on 48 hour experiments), whilst localized corrosion rates (determined from surface profilometry) rose from 0.9 to 1.7 mm/year. The research demonstrates that localized corrosion measurements are a fundamental requirement when determining the threat posed to carbon steel pipelines during dense-phase CO 2 transport, exceeding the uniform corrosion rate by nearly one order of magnitude under certain conditions. Additional tests involving solution replenishment over 48 hours indicated that the higher corrosion rates observed in the presence of SO 2 did not present the worst case scenario corrosion rates and highlight the importance of having a system where the process fluid is continuously replenished. The corrosion product morphology and chemistry was identified through a combination of scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX) and X-ray diffraction (XRD).

Internal corrosion of carbon steel pipelines for dense-phase CO2transport in carbon capture and storage (CCS) – a review

“…FeSO3·3H2O has been observed on carbon steel surfaces by both Choi et al [4] , Xiang et al [80] and Hua et al [78,81] in SO2-containing dense phase CO2 experiments. Additionally, both…”

“…In general, three typical corrosion morphologies are reported in literature; a thin sulphur-rich cracked film [27,78,82,83] (Figures 6(a), (b) and (c)), columnar/spherical crystals of FeSO3/FeSO4 [4,78,84] (Figures 6(d), (e) and (f)), and globular crystals of FeCO3 [78,81] …”

“…[81,83] In terms of supporting the theory, the work of Farelas et al [83] demonstrated that the presence of flow (1000 rpm sample rotation speed) reduced corrosion rates of X65 steel by around an order of magnitude in specific dense phase CO2 environments. Farelas et al [83] performed tests at 8.…”

Impact of surface roughness and humidity on X70 steel corrosion in supercritical CO 2 mixture with SO 2 , H 2 O, and O 2

“…Hence, it is concluded that high O 2 concentration can reduce the critical relative humidity. Given that high O 2 concentration promotes the formation of large amounts of FeSO 4 , which is hygroscopic salt with the ability to retain water [45], corrosion remains significant at a low relative humidity of 50%, and then the critical relative humidity is reduced.…”

The effect of O 2 content on the corrosion behaviour of X65 and 5Cr in water-containing supercritical CO 2 environments

“…A schematic representation of the autoclave experimental system layout has been shown in a previous publication [18]. The entire system consists of a 1 l capacity autoclave, temperature controller, a CO 2 /O 2 mixed cylinder, a liquid CO 2 cylinder and a series of valves for CO 2 flow control.…”