The Influence of Relative Humidity and Intrinsic Chloride on Post-excavation Corrosion Rates of Archaeological Wrought Iron

Abstract

This 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.

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

“…These are available in volumes of 250cm 3 to 3000cm 3 and have been used with silica gel to condition the internal vessel environment (see Sect. 8.1) [11,13,15,22,30].…”

“…Successful research projects have used Roman wrought iron nails from different archaeological sites in Britain [11,13]. The ubiquity of these objects on Roman sites, their broadly similar physical form and their limited value for archaeological study makes them an ideal sample set for oxygen consumption.…”

“…It is well known that the corrosion rate of metals is influenced by relative humidity and numerous studies have been conducted to examine this [12,13,35]. This is important for two main reasons.…”

“…Understanding of treatment outcomes has been transformed by the oxygen sensing technique, which has enabled treatment design [10] to be linked to treatment success [11,12]. Similarly, quantitative measurement of post-excavation corrosion rates [13] has produced more granular understanding of the practical application of desiccation for preserving archaeological iron using microclimates [14]. The performance of protective coatings and surface preparation techniques on wrought iron has also been assessed and is used by practitioners to guide their treatments [15,16].…”

“…Based on the Matthiesen method [1][2][3], the Cardiff application of the technique has been refined over its decade of application to ferrous metal corrosion studies. Numerous publications [11,13,15,[21][22][23][24] have reported the method used but have not discussed in detail the rationale behind the selection of parameters for the hardware, environmental factors and oxygen meter settings.…”