Carbon Dioxide Corrosion of Steel: Subtleties of Anodic Electrocrystallization and Boundary-Layer Precipitation

The crystal growth and spreading of siderite (FeCO3) over a steel surface are critical factors controlling corrosion in hot, CO2-saturated brine, as found in oil and gas production pipelines. Electrochemical studies, including in-situ synchrotron X-ray diffraction and small-angle scattering, have uncovered the complexities of the process, in which iron dissolves into solution through a thin layer of amorphous iron carbonate (AFC), colloidal FeCO3 precipitates in the solution close to the electrode, and crystalline siderite forms on the electrode surface. In this Perspective, we reinterpret literature results, making an important distinction between electrocrystallization - crystals nucleated directly on the metal surface and simultaneously growing and spreading laterally - and boundary-layer precipitation followed by crystal growth. We discuss nucleation in electrocrystallization as a transformation of an amorphous precursor film, note the possibility of spallation of this film directly to particles in solution, modified by the composition of the steel, and give a quantitative model for the anodic electrocrystallization that satisfactorily unifies electrochemical and crystal growth rate studies. We identify five different boundary layers at the metal solution interface: hydrodynamic, concentration, and reaction boundary layers; surface roughness; and surface deposit. They show their effects at different stages of the reaction as the surface evolves from an initial smooth and uncovered state. Which is dominant in controlling electrocrystallization and boundary layer precipitation, particle growth and crystallization depends on solution flow, microstructure, and time. Outstanding questions concerning the details of effects of hydrodynamics, pH, surface roughness, and metal microstructure on morphology of surface layers should be resolvable through this approach. We discuss a particle size population balance model in a diffusion boundary layer as a first step for discussing the formation of surface deposits of varying degrees of crystallinity.