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Carbon Capture and Storage (CCS) has been highlighted as a potential method to enable the continued use of fossil-fuelled power stations through the abatement of carbon dioxide (CO2). A complete CCS cycle requires safe, reliable and cost effective solutions for the transmission of CO2 from the capturing facility to the location of permanent storage. In subsequent sections, early laboratory and field corrosion experience relating to natural dense phase CO2 transport for the purposes of enhanced oil recovery (EOR) are summarised along with more recent research efforts which focus on identifying the role of anthropogenic impurities in the degradation processes. For each system impurity, the reaction rates, mechanisms and corrosion product composition/morphology expected at the steel surfaces are discussed, as well as each component's ability to influence the critical water content required to initiate corrosion. Potential bulk phase reactions between multiple impurities are also evaluated in an attempt to help understand how the impurity content may evolve along a long distance pipeline.The likelihood of stress-corrosion cracking and hydrogen-induced cracking is discussed and the various corrosion mitigation techniques which exist to control degradation to acceptable 2 levels are reviewed. Based on the current research performed in the context of impure dense phase CO2 corrosion, issues associated with performing laboratory experiments to replicate field conditions and the challenges such limitations present in terms of defining the safe operating window for CO2 transport are considered.

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14A systematic study is undertaken to establish the influence of sulphur dioxide (SO 2 ) 15 concentration on the critical water content required to avoid substantial levels of internal 21Analysis of corrosions products formed on the steel surface was performed using x-ray 22 diffraction, Raman spectroscopy and scanning electron microscopy. The results indicate that 23 the presence of SO 2 reduces the critical water content required to maintain a general 24 corrosion rate below 0.1 mm/year. Furthermore, the water content required to avoid 25 excessive localised corrosion is far less than that to prevent significant general corrosion. 26Localised corrosion rates close to 1 mm/year were observed in the absence of SO 2 when the 27 CO 2 system was water-saturated, but below water contents of ~1800 ppm (mole) and ~500 28 ppm, general and localised corrosion rates (respectfully) were found to be below 0.1 29 mm/year even in the presence of 100 ppm SO 2 . The research presented highlights that 30 reducing water content is a more favourable option compared to reducing SO 2 content to 31 minimise internal pipeline corrosion during transportation. Consideration is also afforded to 32 the consumption of impurities in the closed system experiments. 33

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The formation of FeCO3 and Fe3O4 on carbon steel and their protective capabilities against CO2 corrosion at elevated temperature and pressure. Corrosion Science, 157. pp. 392-405.

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).

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