An Experimental Study of Corrosion for Long Distance Carbon Transportation Pipelines

This thesis presents results from a systematic study undertaken to establish the influence of sulphur dioxide (SO2 and O2) content on the critical water content required to initiate substantial levels of internal corrosion during the transport of supercritical CO2 for Carbon Capture and Storage (CCS) applications.
An assessment of both the general and localised corrosion behaviour of X65 carbon steel in water-containing CO2 environments is presented. Firstly, autoclave experiments were conducted in environments where the CO2 phase was either saturated or under-saturated with water. Such an approach enabled identification of the minimum water content in the system, below which no general or localised attack was observed.
Later, corrosion experiments were performed containing supercritical CO2 and the presence of 0, 50 and 100 ppm SO2 with a low level of O2 (20 ppm). The results highlights that reducing water content is a more favourable option compared to reducing SO2 content to minimise corrosion in the system as high corrosion rates can be observed in the absence of SO2 if water content is high enough, but below a minimum water content (~500 ppm), both general and localised corrosion is minimal despite the presence of 100 ppm SO2.
Analysis of corrosion products formed on the steel surface is performed using X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Energy-Dispersive X-ray spectroscopy (EDX), Transmission Electron Microscopy (TEM), Raman spectroscopy. Only FeCO3 was detected on the surface without SO2 and O2, whilst both FeCO3 and FeSO3.3H2O were identified on the surface of the carbon steel samples exposed to the environments containing 2 and 100 ppm SO2 and 20 ppm O2. The localised corrosion rates were determined by surface profilometry measurement.
The most important observation from this work is that the key degradation mechanism in all experiments was localised corrosion. Not only was the combination of SO2 and O2 shown to influence pitting severity, but the pitting rates recorded were nearly one order of magnitude greater than the uniform corrosion rate determined from mass loss measurements. The increase in SO2 content was shown to influence the shape of pits as well as their overall depth. In particular, the work highlights the importance of adopting a systematic approach when determining pitting behaviour of carbon steels exposed to impure dense-phase CO2.