An Electrocrystallization Approach for Modeling Protective Scale Formation in CO2 Corrosion of Carbon Steel
Authors: Metri Vishal , Rai Sandhya , Laycock Nicholas J. 
Published: February 2024

The CO2 corrosion of carbon steel has been extensively studied and many models have been developed to estimate the corrosion rate in various industrially relevant conditions [1]. These models agree moderately well in non-scaling environments but can produce significantly different results under higher temperature and higher pH conditions where protective scales of iron carbonate may be formed [2,3]. The scaling effect is typically incorporated in empirical models using a ‘scaling factor’ [2,3]. Nesic and co-workers developed a model that attempts to calculate the corrosion rate in scaled conditions from first principles [4,5,6], later including a numerical model to describe the formation kinetics and growth morphology of the surface layer [7]. In this and other models, the driving force for scaling is the degree of supersaturation (S) of FeCO3 at the steel surface, and it is generally assumed that protective films are formed by precipitation from solution, at a rate dependent on S [7,8,9].
To investigate the formation and structure of these surface scales, Williams and colleagues [10-14] completed a series of experiments using in-situ synchrotron x-ray techniques. The majority of this work accelerated the corrosion process through the application of a small anodic potential step, and focused on solutions of moderate chloride content, saturated with CO2 at ~ 1 bar, pH adjusted to ~ 6 and at temperatures of ~ 80 °C. Small Angle X-Ray Scattering (SAXS) showed that the first stage in scaling was the formation of a precipitated layer of Amorphous Ferrous Carbonate (AFC) [13], while X-Ray Diffraction (XRD) revealed that the reduction of corrosion rate was associated with the slower formation of crystalline siderite (FeCO3) and sometimes chukanovite (Fe2(OH)2CO3) (see Figure 1). Hassan et al [14] then developed a model for this process in which the protective scale forms directly on the steel surface by an electrocrystallization mechanism, using an Avrami model to describe the crystal nucleation and growth. The relative volume of siderite as a function of time was fitted with an Avrami-like equation. The anodic current transients formed during potentiostatic experiments (see Figure 1) were then fitted with separate components due to iron dissolution and electrocrystallization, allowing various kinetic parameters to be extracted from the experimental data.
In this paper, we extend these previous ideas on the importance of considering electrocrystallization as a fundamental step in the scale formation process. There exist several models of the potentiostatic current time transient in the electrocrystallization literature [15,16]. Taking the potentiostatic data from Hassan et al [14], we fit the current time transient to some models from literature [15,16]. This fitting provides rich information about the kinetics of nucleation. We speculate on some differences from the models in Luo et al [15] by current transient fitting. We also provide expressions for surface coverage of siderite under different applied potentials with the Avrami equation to account for the scale protection factor more rigorously. Improved estimates of the protection factor by our methods can lead to reduced overestimation of corrosion rates thereby improving pipeline integrity assessments in existing software packages.

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Figure 1