The corrosion of buried iron-based objects in soil is governed by coupled electrochemical, chemical, and physical processes and can be strongly influenced by sudden environmental transients caused by human activities or geological events. This issue is critical in applications requiring long-term stability. In nuclear engineering, steel canisters for radioactive waste disposal must withstand corrosion over hundreds of thousands to millions of years. In archaeology and conservation science, preserving iron artifacts in archaeological sites is essential for cultural heritage conservation. Such artifacts contain data on corrosion and environmental evolution, providing insights into the long-term corrosion behavior of buried iron objects.
In bicarbonate- and carbonate-rich soils and groundwaters, iron(II) carbonate or siderite scale commonly forms on the surface that protects the underlying metal from further corrosion. However, the scale promotes localized corrosion when discontinuous or damaged, highlighting the need for a mechanistic understanding of its formation, dissolution, and stability. Current knowledge of siderite scale is mainly based on carbon dioxide-induced corrosion studies from the oil and gas industry, typically conducted under acidic pH, and high temperature and pressure conditions that differ markedly from the near-neutral to basic pH and ambient environments of natural soils. Moreover, the combined effects of environmental transients, such as soil acidification, salinization, and aeration, on siderite stability and iron corrosion remain poorly understood.
This PhD research was conducted within the SNSF Sinergia project CORINT, which aims to elucidate iron corrosion mechanisms in porous media through a multiscale approach. The present work focuses on fundamental electrochemical and chemical processes at the metal/scale/electrolyte interfaces, isolated from soil complexity. The objectives are to clarify siderite growth and dissolution mechanisms and to assess the effects of environmental transients, including anaerobic-to-aerobic, chloride-free to chloride-containing, and basic-to-acidic pH conditions.
Experiments were performed on iron in 1 M sodium bicarbonate under ambient conditions. In-situ electrochemical techniques (OCP, polarization methods, LPR, EIS) were coupled with ex-situ characterization (SEM-EDS, FIB cross-sections, and XRD) to correlate electrochemical behavior with structural and compositional evolution.
Results show that dissolved oxygen has a limited effect on corrosion kinetics at fixed applied potentials but shifts the open-circuit potential into the passive region. Siderite scale growth is controlled by ferrous ion production and proceeds through iron corrosion, nucleation, island and lateral growth, coalescence, channel formation, and eventual channel closure and corrosion blockage. While both aerobic and anaerobic conditions produce siderite, aerobic conditions yield slightly lower crystallinity due to f
