Long-term temporal and spatial evolution of iron corrosion products at the pore scale in near-neutral pH

Predicting steel corrosion in porous media – e.g. soil, bentonite, concrete – remains challenging due to the limited understanding of the precipitation and evolution of corrosion products under realistic conditions. To shed light on these processes, we investigate the long-term temporal and spatial evolution of iron (hydr)oxides at the pore-scale, employing in-situ synchrotron-based chemical imaging including X-ray fluorescence, X-ray diffraction, X-ray absorption spectroscopy, and absorption contrast. A pore is simulated in a 300 μm diameter capillary, containing iron and stagnant electrolyte, and is aged for up to 2.7 years. The electrolyte is buffered at pH 8, with HCO 3 - , Cl - , and SO 4 2- ions, both under aerobic and near-anaerobic conditions. Results show that, in electrolytes containing HCO 3 - , ferrihydrite initially formed remotely from the iron and transformed into goethite over time. In the long-term, mixed-valence green rust and magnetite formed near the iron. The spatial distribution and oxidation states of corrosion products can be rationalized using a reactive transport conceptual framework: Fe(II) and O 2 release or consumption in electrochemical, diffusion, oxidation and additional reactions determine the concentration profiles and hence the location and time when corrosion products precipitate. Electrolyte composition – Cl - and SO 4 2- – further influences which corrosion product is formed and stabilized over a certain time (e.g. green rust and lepidocrocite). Our findings demonstrate preferential precipitation on pore walls and far from the iron, with prolonged green rust stability, showcasing the relevance of investigating iron (hydr)oxides in realistic pore models. These insights improve predictions of corrosion product evolution, pore clogging, and stresses in porous media.