The Use of Neutron Analysis Techniques for Detecting The Concentration And Distribution of Chloride Ions in Archaeological Iron

Abstract

Chloride (Cl) ions diffuse into iron objects during burial and drive corrosion after excavation. Located under corrosion layers, Cl is inaccessible to many analytical techniques. Neutron analysis offers non-destructive avenues for determining Cl content and distribution in objects. A pilot study used prompt gamma activation analysis (PGAA) and prompt gamma activation imaging (PGAI) to analyse the bulk concentration and longitudinal distribution of Cl in archaeological iron objects. This correlated with the object corrosion rate measured by oxygen consumption, and compared well with Cl measurement using a specific ion meter. High-Cl areas were linked with visible damage to the corrosion layers and attack of the iron core. Neutron techniques have significant advantages in the analysis of archaeological metals, including penetration depth and low detection limits.

Refining the use of oxygen consumption as a proxy corrosion rate measure for archaeological and historic iron

“…Assuming that the calculations can be made and the values obtained are reliable, what do they mean for heritage artefacts? Study has shown that the corrosion of chloride-infested archaeological iron, for example, is not a process that occurs evenly across the metal surface [40]. Instead, it is localised in pits and this localised corrosion can cause catastrophic damage and break-up of an artefact long before the metal core has been converted by corrosion processes.…”

Influence of microstructure on the corrosion of archaeological iron observed using 3D synchrotron micro-tomography

“…cross-sectional slices. Individual cross-sections may consist of X-radiographs, [20] diffraction patterns, [21] neutron radiographs [22] or spectroscopic data [21] depending on the material and system being studied. The most common tomography, X-ray computed tomography (CT), has been shown to be a highly effective non-destructive method for studying archaeology, [23][24][25] corrosion [19,26,27] and metallography [28,29], particularly when used in combination with 2D analytical methods.…”

Integration of neutron-based elemental analysis and imaging methods and applications to cultural heritage research

“…A similar methodology was used in another study, where it was possible to establish a correlation between the longitudinal chlorine profile and the degree of corrosion in archaeological iron nails (Watkinson et al, 2014). After excavation, oxidation forms ferrous chloride and hydroxide, which cause cracking, fragmentation and break-up of the objects.…”