The impact of aqueous washing on the ability of βFeOOH to corrode iron

Abstract

Controlling the corrosion of historical and archaeological ferrous metal objects presents a significant challenge to conservators. Chloride is a major corrosion accelerator in coastal areas for historic ferrous metal structures and for the many chloride-containing archaeological objects within museums. Corrosion reactions involve the formation of akaganéite (βFeOOH) which incorporates chloride within its crystal structure and adsorbs it onto its surface. The mobility of the surface-adsorbed chloride in aqueous systems and atmospheric moisture means βFeOOH can itself cause iron to corrode. The extraction of chloride from βFeOOH by aqueous Soxhlet hot wash and aqueous room temperature washing is measured. The impact of this washing on the ability of βFeOOH to corrode iron is quantitatively investigated by determining the oxygen consumption of unwashed, Soxhlet-washed and room temperature-washed samples of βFeOOH mixed with iron powder and exposed to 80 % relative humidity. This acts as a proxy measurement for the corrosion rate of iron. The results are discussed relative to climatic factors for outdoor heritage objects and the treatment of archaeological iron in museums. Delivering better understanding of the properties of βFeOOH supports the development of evidence-based treatments and management procedures in heritage conservation.

A Synchrotron‐Based Study of the Mary Rose Iron Cannonballs

“…The size of individual β‐FeO(OH) crystals depends on the growth conditions, but is typically small, around 0.15×0.03 μm, resulting in a large proportion of Cl sur . During a washing‐based conservation treatment, mobile Cl sur is removed, while Cl str is thought to be unaffected . X‐ray diffraction of akaganeite gives two low‐angle peaks, which at the wavelength used in this study ( λ =0.82578 Å) appear at 2 θ values 6.35° and 8.95°, corresponding to the (101) and (200) planes, respectively.…”

“…Having been produced in bulk, the shot were buried together and their relative uniformity maintained until excavation, when they were treated by a number of conservation methods and exposed to varying environmental conditions . Immediately after excavation, all of the shot were immersed in a high pH solution (either NH 3 , NaOH, or an equimolar mixture of Na 2 CO 3 /NaHCO 3 ) until undergoing active conservation, though some shot have remained in passive storage to the present day.…”

A Synchrotron‐Based Study of the Mary Rose Iron Cannonballs

“…[22] During aw ashing-based conservation treatment, mobile Cl sur is removed, while Cl str is thought to be unaffected. [14,22,23,27] Xray diffraction of akaganeite gives two low-angle peaks,which at the wavelength used in this study (l = 0.82578 )appear at 2q values 6.358 8 and 8.958 8,c orresponding to the (101) and (200) planes,r espectively.T he insets in Figure 2a highlight the location of these peaks (dotted grey lines) and show that, somewhat surprisingly,a kaganeite is not observed in the surface corrosion products of any of the SS shot, while it is present on the surface of all HW and HWAS shot. Phase identification of the powder diffraction profiles,F igure 2b, instead shows ac ombination of phases from the burial environment:c alcite (CaCO 3 ), quartz (SiO 2 ), and aragonite (CaCO 3 ); related to the microstructure of the metal:graphite and cementite (Fe 3 C);a nd commonly reported [3,14] marine iron corrosion products:goethite a-FeOOH, magnetite Fe 3 O 4 and lepidocrocite g-FeOOH.…”

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

“…βFeOOH is insoluble and has Cl − retained in its crystal structure and adsorbed on its surface (Mackay 1960;Ståhl et al 2003). This surface adsorbed Cl − is highly mobile in water (Réguer et al 2009;Watkinson and Emmerson 2017) and it makes βFeOOH hygroscopic (Kaneko and Inouye 1979;Watkinson and Lewis 2005a). The hygroscopicity mobilises surface adsorbed Cl − , causing it to corrode iron down to 15% RH (Watkinson and Lewis 2005b;Thickett and Odlyha 2014;Watkinson and Emmerson 2017).…”

“…This surface adsorbed Cl − is highly mobile in water (Réguer et al 2009;Watkinson and Emmerson 2017) and it makes βFeOOH hygroscopic (Kaneko and Inouye 1979;Watkinson and Lewis 2005a). The hygroscopicity mobilises surface adsorbed Cl − , causing it to corrode iron down to 15% RH (Watkinson and Lewis 2005b;Thickett and Odlyha 2014;Watkinson and Emmerson 2017). Solid FeCl 2 .2H 2 O does not corrode iron but above 20% RH it hydrates to form FeCl 2 .4H 2 O which supports the corrosion of iron in contact with it (Turgoose 1982;Watkinson and Lewis 2005b) and deliquesces at 55% RH to provide a strong electrolyte.…”