An Explanation for the Difference in Reported Iron Carbonate (Siderite) Precipitation Kinetics

In energy production, and carbon capture and storage processes which involve the transport of carbon dioxide (CO₂)-saturated aqueous solution in carbon steel pipework, the precipitation of iron carbonate (siderite) at the steel-solution interface is known to have a significant impact on the corrosion process. As a result, the precipitation kinetics of iron carbonate have been an area of significant research with several studies reported in literature. Unfortunately, the reported rates of precipitation can differ by 2 - 3 orders of magnitude depending on the experimental methodology.

In this study, two major categories of kinetic data are reviewed; first, the data generated from the growth of seed crystals of iron carbonate, and second, the data from the growth of iron carbonate on planar surfaces such as gravimetric coupons and quartz crystal microbalance sensors. In order to understand the 100-1000 fold difference in the reported precipitation rate between these two approaches, the underlying theoretical considerations of precipitation kinetics are reviewed and applied in detail to the experimental methods used.

The analysis shows that while the reaction kinetics in both cases are controlled by diffusion processes at or near the precipitating surface, it is the geometry of the surface that controls the dominant length scale for the diffusion process, and as a result, the precipitation rate. Therefore, to properly compare precipitation kinetics, not only should the environmental conditions be comparable, but so too should the governing geometry of the diffusion process.