Substrate Protection with Corrosion Scales: Can We Depend on Iron Carbonate?

Controlling corrosion with naturally occurring corrosion scales is potentially a more environmentally sustainable alternative to current approaches, including dosing of organic corrosion inhibitors. We report operando grazing incidence X-ray diffractograms correlated with electrochemical measurements to elucidate the growth and corrosion protection properties of a corrosion scale composed of FeCO₃ crystallites, which is encountered in various key energy industry applications. Data, acquired as a function of time from high-purity iron immersed in CO₂-saturated deionized H₂O at pH 6.8 and T = 80 °C, show that the FeCO₃ scale not only prevents corrosion of the covered substrate but also acts as a significant interfacial diffusion barrier for corrosion reagents and/or products once sufficient coverage is achieved. Most notably, from a corrosion engineering perspective, however, it is determined that corrosion occurring in gaps between scale crystallites remains appreciable; this important insight is gained through the analysis of electrochemical impedance spectra to estimate the variation in electrochemically active surface area as scale coverage increases. These results indicate that naturally occurring FeCO₃ scales are not a tenable solution for corrosion protection, as even in their intact state they are highly likely to be, at best, semiprotective.