Abstract
ABSTRACTThis study examined the impact of relative humidity (RH) on the corrosion rate of 129 archaeological iron nails from two sites. Oxygen consumption of individual nails in controlled RH was used as a corrosion rate proxy to deliver quantitative data on corrosion rate as a function of RH. This was negligible at 20% RH, slow up to 40% RH for both sites, and increased rapidly at 60% RH for Roman nails from Caerleon (Wales) and at 70% RH for medieval nails from Billingsgate (London). The nails were digested and their chloride content was determined and related to their oxygen consumption at specific RH values. While a generic pattern of corrosion as a function of chloride was identified, for any single concentration of chloride corrosion rate was not predictable. Desiccation is in common use to control post-excavation corrosion of archaeological iron; quantifying how differing levels of desiccation changed corrosion rate provided a scaled tool for identifying corrosion risk, estimating object longevity, and calculating cost benefit for storage options.
Highlights
Context of the studyThe occurrence and control of the post-excavation corrosion of archaeological iron present major challenges for professionals involved in the preservation of heritage
Results from this study unequivocally reveal that the ‘gold standard’ for the storage of archaeological iron is below 20% relative humidity (RH), which aligns with other studies that examine aspects of the mechanism of corrosion relative to RH (Ståhl et al 2003; Watkinson and Lewis 2005a, 2005b; Réguer et al 2009; Rimmer et al 2013)
Increasing RH increases the corrosion rate of archaeological iron, which is influenced by Cl− concentration within objects but not predictably so
Summary
Context of the studyThe occurrence and control of the post-excavation corrosion of archaeological iron present major challenges for professionals involved in the preservation of heritage. While there are many variables that influence the corrosion of archaeological iron, a nuanced scale recording how changing RH affects corrosion rate and its relationship with the Cl− content of iron objects is central to achieving this goal To this end, the study reported here uses a unique experimental design, employing a highly accurate and sensitive technique for remotely detecting oxygen concentration, to deliver quantitative data on the corrosion rate of a large number of archaeological iron objects as a function of ambient RH. The study reported here uses a unique experimental design, employing a highly accurate and sensitive technique for remotely detecting oxygen concentration, to deliver quantitative data on the corrosion rate of a large number of archaeological iron objects as a function of ambient RH It relates the Cl− content of these archaeological objects to their corrosion rate at specific RH values to provide insight into the relationship between the two variables
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