Understanding the Formation of Protective FeCO3 on to Carbon Steel Pipelines during CO2 Corrosion

In the oil and gas industry, internal corrosion of carbon steel pipelines is commonly encountered during production and transportation of carbon dioxide (CO2)-containing salt water and hydrocarbons. The growth of iron carbonate (FeCO3) on the internal walls of carbon steel pipelines can reduce internal corrosion by blocking active sites on the steel surface and creating a diffusion barrier. Two of the key aspects to consider when predicting pipeline corrosion rates in CO2 environments are the kinetics of film formation and level of protection afforded by the film under different operating conditions. This research focuses on understanding the factors governing the rate of precipitation, formation kinetics and protective properties of the FeCO3 layer. This is an important step in order to take advantage of the positive attributes of FeCO3 film formation and reduce the occurrence of localised corrosion attack. An electrochemically-integrated Synchrotron Grazing Incidence X-Ray Diffraction (SR-XRD) flow cell for studying corrosion product formation on carbon steel in CO2-containing brines typical of oil and gas production has been developed. The system is capable of generating flow velocities of up to 2 m/s at temperatures in excess of 80ºC during SR-XRD measurements of the steel surface, enabling flow to be maintained over the course of the experiment while diffraction patterns are being collected. The design of the flow cell is presented, along with electrochemical and diffraction pattern transients collected from the Diamond Light Source Synchrotron facility located in Oxford, UK. The flow cell is used to follow the nucleation and growth kinetics of FeCO3 using SR-XRD linked to the simultaneous electrochemical responses of the steel surface which were collected in the form of Linear Polarisation Resistance (LPR) measurements to for in-situ corrosion rates. The results show that FeCO3 nucleation could be detected consistently and well before its inhibitive effect on the general corrosion rate of the system. In-situ measurements are compared with ex-situ Scanning Electron Microscopy (SEM) observations showing the development of an FeCO3 layer on the corroding steel surface over time confirming the in-situ interpretations. The results presented demonstrate that under the specific conditions evaluated, FeCO3 was the only crystalline phase to form in the system, with no crystalline precursors being apparent. Solution chemistry can be considered to be one of the most influential factors with regards to the kinetics, morphology and protection of the FeCO3 film. The cell was used to follow the nucleation and growth kinetics of corrosion products on X65 carbon steel surfaces in a CO2-saturated 3.5 wt.% NaCl brine at 80ºC and a flow rate of 0.1, 0.5 and 1 m/s over a range of solution pH values (6.3, 6.8 and 7) under different electrochemically controlled methods. The purpose of the work was to establish whether correlations exist between the kinetics of FeCO3 formation, the quantity of film, and the level of protection facilitated and to identify the presence of any addition corrosion product phases or precursors. In all conditions tested in this work, FeCO3 was identified as the only crystalline phase to form on the steel surface in all conditions and was detected in all tests well before its inhibitive effect on general corrosion rate was detected using electrochemistry. With increasing pH, the FeCO3 precipitation rate and protectiveness increased, while the film thickness, crystallite size and induction time decreased. A critical observation was that the quantity of FeCO3 (measured from the in-situ diffraction data) on the steel surface is not a good indication of the protection of the film formed across different pH values. However, the results in this work have shown that at higher pH, larger portions of the surface become covered faster with thinner, more protective films consisting of smaller, dense and more compact crystals. Ex-situ SEM images are used to evaluate the surface coverage and size of the FeCO3 crystals at various time intervals. The comparison between XRD main peak intensities and the surface coverage indicate a qualitative relationship between the two parameters at each pH, providing valuable information on the kinetics of films growth.