ABSTRACT

This work proposes an in situ electrochemical analysis procedure for characterizing iron carbonate (FeCO3) development on carbon steel at elevated pressures. X65 carbon steel was exposed to an unbuffered carbon dioxide (CO2) saturated brine in an electrochemical autoclave at 80°C and 5.5 bar pCO2 until a protective layer of FeCO3 had formed. Open Circuit Potential (OCP) and Linear Polarization Resistance (LPR) were coupled with Electrochemical Impedance Spectroscopy (EIS) and ex situ surface and cross-sectional imaging to identify key stages in corrosion layer development. The impedance response shows clear transitions in the nature of the interface which coincides with trends observed in both LPR and OCP data. Cross-sectional SEM images reveal that FeCO3 forms almost entirely within the exposed Fe3C matrix after ferrite depletion. The result is a thick (100 μm) but highly porous layer that presents a finite length (transmissive) diffusion barrier between the substrate and electrolyte.

INTRODUCTION

Carbon steel exposed to aqueous CO2 environments can be conducive to the formation of naturally protective corrosion products, namely iron carbonate (FeCO3)1, 2. Understanding how FeCO3 develops across a range of conditions is a critical step in enabling the optimization of corrosion products as a natural form of corrosion mitigation3. To date, most studies investigating FeCO3 development focus on near-neutral pH solutions conducive to fast precipitation while test pressures are generally atmospheric to simplify in situ electrochemical measurements4-8.

Highly protective layers of FeCO3 have been observed in autoclave experiments at lower initial bulk pH and elevated pressures and temperatures9, 10. This has been attributed to a rapid increase in surface pH and super-saturation for FeCO3 locally due to enhanced corrosion kinetics but limited in situ data is available in these environments.

Establishing exactly how corrosion layers evolve and protect the substrate steel in more demanding CO2 environments requires the use of in situ techniques such as Electrochemical Impedance Spectroscopy (EIS). EIS can provide an insight into the behavior of a coated interface over time, but extraction of meaningful properties relies on a strong understanding of the physical system11. In corrosion studies, system parameters such as charge transfer resistance (Rct) are commonly estimated by fitting the modelled response of an Equivalent Electrical Circuit (EEC) to the raw data. The limitation with this approach is that a good fit of experimental data can be achieved with arbitrary elements in a multitude of circuit configurations of varying complexity. Despite notable efforts, across the literature4, 6, 12-15 there is a lack of consistency in the EEC selection which makes comparison of circuit parameters difficult. The most methodical and substantive approach to applying EIS is to prioritize graphical and physical interpretation to justify circuit fitting. As a result, a robust set of parameters can be obtained, allowing straightforward comparisons to be made on the properties of similar corrosion product layers.

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