A Combined Experimental and Modelling Approach to Elucidate FeCO3 Scale Formation Kinetics

In CO2 corrosion, when the local concentrations of Fe2+ and CO32- ions exceed the solubility limit, precipitation of iron carbonate (FeCO3) can occur internally within pipework, forming a protective corrosion barrier at the steel-electrolyte interface. Accurately quantifying the rate of precipitation of this film is important within the oil and gas industry as it can be implemented into corrosion prediction tools to provide a more reliable estimate of anticipated corrosion rates. \nExisting precipitation rate models are based on measurements conducted in a glass cell in static conditions where the kinetics of FeCO3 precipitation are accelerated by the addition of FeCl2.4H2O and correlated with bulk solution properties. They do not address the key aspects of FeCO3 formation in real corroding systems which relate to the local surface supersaturation produced as a result of the production of Fe2+ ions due to the corrosion process. \nIn the following thesis, a combined experimental and modelling approach is carried out to investigate the development in the morphology of the FeCO3 film under different environmental conditions and its consequent effect on the degradation rates of a pipeline. A thin channel flow cell is designed to extend the analysis to a fluid flow environment and a mechanistic model is developed to predict the nature of the near surface layer. \nIt is found from the experimental analysis that FeCO3 precipitation is a simultaneous nucleation and growth process and the characteristics of the surface film significantly changes under varying parameters. Results show that the existing precipitation models based on measuring the dissolved ferrous ions in the bulk solution overestimate the precipitation of iron carbonate by a large margin and the precipitation model developed through the direct weight change approach is limited to the experimental conditions in which it was carried out. The models are correlated with bulk solution properties and it has been clearly demonstrated within this work that the precipitation of FeCO3 is directly related to the conditions at the steel surface which can be very different from that in the bulk. A combined model and experimental analysis shows that a higher initial surface saturation ratio indicates a more protective film formation over time.