Desiccated Storage of Chloride-Contaminated Archaeological Iron Objects

Watkinson, David; Lewis, M.

doi:10.1179/sic.2005.50.4.241

Cited by 32 publications

(17 citation statements)

References 22 publications

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“…26,62,63 Information on the DRH of FeCl 2 .2H 2 O is lacking, but upon humidification of this salt from near 0%RH, it has been observed to convert to the four hydrate at ∼15% RH. 26 Both Turgoose 26 and, later, Watkinson 64 reported active corrosion of iron in contact with FeCl 2 .4H 2 O crystals down to 20% RH, albeit at a slow rate. Watkinson attributed the corrosion under the solid salt to waters of hydration.…”

Section: Discussionmentioning

confidence: 99%

Effect of Relative Humidity on Corrosion of Steel under Sea Salt Aerosol Proxies

Schindelholz

Risteen

Kelly

2014

J. Electrochem. Soc.

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This paper is the first of two that examine the relationship between relative humidity, the hygroscopic behavior of sea salt aerosol proxies, and the atmospheric corrosion of mild steel contaminated with them. Here, the association between the deliquescence phase transitions (solid-aqueous) of a single salt, NaCl, and the corrosion response of steel contaminated with this salt is examined. Specific focus is given to mechanisms that allow corrosion below the deliquescence relative humidity of NaCl (76% RH). Mild steel loaded with aqueous NaCl drops or crystals (9 μg · cm −2 ) was subjected to isohumidity exposures for up to 300 days. The resulting damage was quantified by optical profilometry. The corrosion chemistry that developed was identified using EDS and Raman spectroscopy. The wetting and drying of simulated corrosion chemistries were characterized by impedance measurements across an interdigitated electrode sensor. Taken together, these results show that corrosion can initiate by adsorbed water on NaCl crystals at as low as 33% RH. At 53% RH and above, attack proceeded at rates comparable to that observed above the deliquescence RH due to the development of hygroscopic corrosion chemistry. The governing role of the hygroscopic properties of this chemistry on attack rate and morphology is discussed.

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Section: Discussionmentioning

confidence: 99%

Effect of Relative Humidity on Corrosion of Steel under Sea Salt Aerosol Proxies

Schindelholz

Risteen

Kelly

2014

J. Electrochem. Soc.

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“…They act as corrosion accelerators after excavation and exposure of iron objects to damp oxygenated atmospheres (Turgoose, 1982(Turgoose, , 1985(Turgoose, , 1993. Chloride-bearing akaganéite (β-FeOOH) is a post-excavation corrosion product (Zucchi et al, 1977;Selwyn et al, 1999;Réguer et al, 2006Réguer et al, , 2007a that is capable of promoting iron corrosion at only 15% relative humidity (RH) (Watkinson & Lewis, 2005a, 2005b. The corrosion rate of chloride-contaminated excavated iron increases with rising RH (Watkinson & Lewis, 2005a, 2005b with rapid corrosion above 60% RH when adsorbed water films thicken (Garverick, 1994, p. 5).…”

Section: Introductionmentioning

confidence: 99%

“…Any failure of the desiccation system allows corrosion to restart, as the objects still retain their chloride ions. Desiccation, although a proven method of both corrosion prevention and control, therefore also presents high risks if its failure goes undetected for long periods (Watkinson & Lewis, 2004, 2005a, 2005b.…”

Section: Introductionmentioning

confidence: 99%

The efficiency of chloride extraction from archaeological iron objects using deoxygenated alkaline solutions

Rimmer

Watkinson

Wang

2012

Studies in Conservation

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Chloride-contaminated archaeological iron is unstable and problematic to store and display within museum collections. Reducing its chloride ion content using aqueous desalination followed by storage in controlled relative humidity offers one treatment option. This study reports a quantitative assessment of chloride extraction by aqueous deoxygenated alkaline desalination solutions from 120 individual archaeological iron nails. The three treatment methods comprised alkaline sulphite solution (0.1 M NaOH/0.05 M Na 2 SO 3 ) at room temperature and at 60°C and sodium hydroxide solution (0.1 M) deoxygenated using a nitrogen gas positive pressure system at room temperature. Chloride extraction was monitored using a specific ion meter. The nails were digested after treatment to measure their residual chloride content. A wide range of extraction patterns emerged, with the majority of individual treatments extracting 60-99% of the chloride present. Residual chloride levels for 87% of the objects fell below 1000 ppm and 42% were below 200 ppm. Although no treatment extracted 100% of the chloride in the object, alkaline desalination produced very significant reductions in chloride content. The impact of this on future corrosion of the objects is discussed. This quantitative and statistically viable assessment of deoxygenated desalination treatments provides evidence to support their use in conservation practice, which will impact on procedures for the preservation and management of archaeological heritage.

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“…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.…”

Section: Chloridementioning

confidence: 99%

“…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. Therefore, it is normally considered necessary to store Cl − contaminated archaeological iron below 12% RH to prevent it corroding (Watkinson and Lewis 2005a;Thickett and Odlyha 2014).…”

Section: Chloridementioning

confidence: 99%

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

Watkinson

Rimmer

Emmerson

2019

Studies in Conservation

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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.

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Desiccated Storage of Chloride-Contaminated Archaeological Iron Objects

Cited by 32 publications

References 22 publications

Effect of Relative Humidity on Corrosion of Steel under Sea Salt Aerosol Proxies

Effect of Relative Humidity on Corrosion of Steel under Sea Salt Aerosol Proxies

The efficiency of chloride extraction from archaeological iron objects using deoxygenated alkaline solutions

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

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