Corrosion Product Growth Kinetics/Characteristics and Inhibitor Performance in CO2 Environments

The application of corrosion inhibitors is extremely common in the oil and gas industry as it enables the use of carbon steel thereby reducing project initial capital expenditure. In many cases, once the use of corrosion inhibitors has become requirement, corrosion products have already precipitated onto the pipeline steel surfaces. The presence of Ca2+ ions can facilitate precipitation of mixed carbonates such as iron-calcium carbonate (FexCayCO3 – where x+y=1). This results in a scale/corrosion product with different chemical and morphological properties to pure FeCO3 which normally forms when CO2 corrosion occurs in the absence of Ca2+ ions. It is therefore of great importance to evaluate any possible antagonistic or synergistic effects associated with the use of corrosion inhibitors in the presence of such corrosion products.
This thesis addresses these issues by evaluating the role of Ca2+ ions on the corrosion behaviour of carbon steel in CO2-corrosion environments at 80⁰C and 150⁰C using in-situ electrochemistry assessment of corrosion in an autoclave. The ability of an imidazoline derivative to provide corrosion protection for X65 carbon steel in CO2-saturated sodium chloride (NaCl) brines at 80ºC was investigated and the role of pre-corrosion on the performance of an imidazoline corrosion inhibitor was evaluated using a two-stage testing procedure with autoclave pre-corrosion and glass cell inhibition testing of X65 carbon steel. The extent of localised/pitting corrosion was assessed using NPFLEX 3D optical non-contact profilometry and was found to be increasing through the addition of Ca2+ ions indicating that Ca2+ plays a role in promoting localised corrosion. XRD based calculations, along with FIB/EDX-line scans and SEM/EDX-point scans confirmed that FexCayCO3 corrosion products had precipitated. A concentration gradient was measured across the depth of the deposited layer.
This thesis demonstrates that the optimum inhibitor concentration required for suppression of uniform corrosion is significantly lower than that for localised corrosion protection. Pre-corrosion of X65 carbon steel was found to cause and in some cases promote severe localised attack especially in the presence of crystalline FeCO3 where the corrosion inhibitor was found to only provide 25-58% of the overall corrosion protection on pre-corroded specimens depending on the characteristics of the corrosion product.