Corrosion Processes in Metals Buried in Sediments: Effects of Fines Content, Mineralogy, and Degree of Saturation

Buried metals, such as driven pile foundations, ground anchors, pipelines, and metal casings, are in direct contact with native soils or compacted fill. Previous studies of soil corrosivity have generated case-specific data that often yield contradictory results due to unclear boundary conditions, limiting their ability to provide a unified understanding of corrosion in soils or identify the key governing soil properties. Soil research has highlighted the dominant influence of mineralogy on soil buffering capacity, the role of the fines fraction in governing pore size and capillarity-saturation trends, and the saturation-dependent connectivity of air and water for species transport through the sediment’s porous network. In light of these findings, the current experimental program investigates the interactions among fines mineralogy (with varying pH, plasticity, and buffering capacity), fines content (FF ranging from 0% to 100%), and degree of saturation (S from 0% to 100%) on soil corrosivity under controlled conditions. A total of 451 specimens, comprising metal coupons embedded in sand-fines mixtures, were tested under passive corrosion conditions. Experimental measurements included x-ray microtomography, mass loss, and detailed image analysis. The results underscore the critical role of mineral buffering capacity in preventing acidification. Oxygen reduction is predominant in neutral to high pH soils under unsaturated or open conditions, potentially contributing up to 1 kg of mass loss per cubic meter of air. In oxygen-limited environments, hydrogen reduction may drive corrosion in fines-controlled sediments with low pH and limited buffering capacity. Collectively, the findings emphasize the importance of using the revised soil classification system to characterize host soils for corrosivity, identify controlling soil fractions, and predict corrosion localization and the likelihood of heterogeneity-enhanced corrosion.