Unsupervised 3D segmentation and statistical corrosion-layer stratigraphy of long-term corrosion in carbon steel archaeological analogs using co-registered bimodal tomograms

Abstract Accurate prediction of long-term carbon-steel corrosion remains a major challenge in safety assessments for deep geological disposal of high-level radioactive waste. Because laboratory and in situ experiments span comparatively short periods, archeological iron offers access to corrosion states developed over centuries to millennia under natural burial conditions. Here, we present a quantitative three-dimensional analysis of corrosion stratigraphy in 10 Roman iron nails, buried for approximately 1700–2000 years, using co-registered neutron and X-ray computed tomography combined with unsupervised clustering in joint attenuation space. Voxel-wise neutron-X-ray attenuation pairs were segmented by K-means clustering at ( k = 4), (6), and (8) to test the reproducibility and useful granularity of attenuation-defined corrosion domains across specimens. At ( k = 4), all specimens exhibited a consistent first-order architecture comprising residual metal, dense product layer, and transformed medium, with preserved spatial ordering around the metallic core. An operational inner-envelope proxy indicated apparent metal loss of 43–65%, with a cohort mean of approximately 53%, corresponding to millimeter-scale penetration and low long-term average corrosion rates. At ( k = 6), reproducible sub-domains emerged within the dense product layer and transformed medium in both attenuation and tomogram space. Targeted Raman measurements showed recurring mixed mineral assemblages rather than phase-pure layers. At ( k = 8), finer subdivision remained possible, but cross-specimen correspondence decreased, particularly within the dense product layer, indicating limited robustness of the finest partition. This framework enables non-destructive, three-dimensional comparison of corrosion stratigraphy and establishes archeological iron as a constrained analog for long-term corrosion behavior in repository-relevant systems across natural burial timescales that are inaccessible to conventional experiments.