Corrosion behaviour of X65 carbon steel under flowing conditions

This thesis presents the outcome of a systematic study carried out to establish an understanding of the influence of flow conditions on the corrosion behaviour of X65 carbon steel in CO2 saturated environments. An assessment of both the corrosion behaviour of uncovered and film-covered surfaces of X65 is made under different flow conditions. All the experiments under flowing conditions were conducted using a newly designed flow loop. This was designed to provide a platform to study the effect of flow parameters and the physical/chemical properties of the solution on corrosion behaviour. A range of wall shear stresses that were gradually increased throughout the test section could be achieved and corrosion rate could be monitored in-situ. Firstly the corrosion behaviour of an uncovered surface under flowing conditions was examined at two values of solution pH of 6.6 and 4 at two temperatures 50ºC and 80ºC. Such an approach enabled the investigation of the effect of flow parameters (described by wall shear stress, Reynold`s number, mass transfer coefficient, and water chemistry/properties) on the corrosion behaviour of the X65 carbon steel. The results enabled the effect of these parameters on the corrosion to be unravelled and it demonstrated how a combination of these parameters may influence the corrosion rate. Furthermore, the parameter with which the corrosion rate shows a direct correlation at each pH level was determined. Furthermore, the influence of the flowing conditions on the Rate Determining Step (RDS) of the cathodic reduction process at pH 4 & 6.6 solution was investigated; the data analysis at temperatures 50ºC & 80ºC and pH 4 showed a threshold of a diffusion boundary layer thickness, at which the reduction reaction switches from diffusion-controlled to mixed diffusion and charge-transfer controlled. The second part of this study was the corrosion behaviour of the FeCO3 film-covered surface. Prior to the corrosion rate tests, the protective film was formed using static conditions in a high-pressure CO2 autoclave at different immersion times. The outcome of the formation tests series revealed the critical time required to achieve the optimum thickness of the iron carbonate film. The corrosion rate and film stability tests in static and flowing conditions were evaluated. Iron carbonate film stability was determined under static conditions through corrosion rate measurement and topography analysis of the protected surface at pH values of 6.6 and 4, at two temperatures of 50ºC and 80ºC. The outcome of the static tests showed that the stability of the iron carbonate film is a function of the solution chemistry and temperature. The critical pH level at which the formed corrosion product film starts to show clear sign of dissolution in static conditions was evaluated at both temperatures. The stability of the iron carbonate film was further investigated under flowing conditions in a range of shear stress values of 10 to 655 Pa. The wall shear stress levels where the protectiveness of the film starts to reduce were determined at both temperatures 50ºC and 80ºC. The film removal mechanisms were identified; the removal process was a strong function of mass transfer. The nature of the film after exposure to shear stress was evaluated using SEM/EDX and XRD. The removal mechanisms of the protective film were determined. The thesis contributes to the current debate of how flow would effect a carbon steel corrosion behaviour in the CO2 environment, and in particular how flow would affect corrosion in the absence/presence of an iron carbonate film.