The Role of βFeOOH in the Corrosion of Archaeological Iron

Abstract

The chloride bearing corrosion product akaganéite (βFeOOH) can form during postexcavation corrosion of chloride infested archaeological iron and is able to corrode iron in contact with it. Its action on iron is examined using βFeOOH synthesized from ferrous chloride and iron powder. Using weight measurements the hygroscopicity of βFeOOH is established. The influence of relative humidity on the corrosion of iron powder mixed with βFeOOH is examined by dynamic mass change within a climatic chamber. At 20 o C and 12% relative humidity, iron in contact with βFeOOH did not corrode. At 15% relative humidity slight iron corrosion was detected after 160 hours, but at 35% relative humidity corrosion occurred after a few hours. Surface adsorbed chloride was removed from βFeOOH by aqueous washing and this reduced its hygroscopicity. The reported metastability of βFeOOH was examined via XRD of a 23 year old sample, which was found to be still entirely composed of βFeOOH. These results provide better understanding of βFeOOH corrosion of iron, corrosion control of chloride infested iron using dry storage and the effect of aqueous washing on archaeological iron.

The impact of chloride desalination on the corrosion rate of archaeological iron

“…The residual chloride in objects (up to 400 ppm) that complete treatment does not appear to have a strong effect on corrosion behaviour, and may therefore represent bound chloride in β-FeOOH tunnels (Stahl et al, 2003;Réguer et al, 2009). β-FeOOH which is washed to remove its surface-adsorbed chloride does not cause corrosion of iron in contact with it (Watkinson & Lewis, 2005). Flooding β-FeOOH with hydroxide ions during washing removes surface-adsorbed chloride, leaving the bound chloride which is detected during post-treatment digestion of the object.…”

“…Contact with atmospheric conditions after excavation produces rapid corrosion governed by relative humidity (RH) and oxygen to produce iron oxyhydroxides, including the chloride-bearing product β-FeOOH which is itself corrosive to iron. In the presence of β-FeOOH, iron begins to corrode at 15% RH (Watkinson & Lewis, 2005;Watkinson & Lewis, 2008) and increases rapidly above 35% RH (Thickett & Odlyha, 2010;Thickett, 2011). As a result, objects fragment and disintegrate (Loeper-Attia, 2007), with subsequent loss of archaeological value.…”

“…Although the consumption of oxygen can be used to monitor corrosion rate reactions (Matthiesen, 2007;Thickett et al, 2008;Matthiesen & Wonsyld, 2010) this was not available during the initial part of this study. Dynamic weight measurement has been used to measure corrosion reaction rates of powdered iron and iron compounds, as the metal reacts with oxygen and water to increase weight by forming iron oxides and adsorbing water (Watkinson & Lewis, 2005). This method can only measure one sample at a time, and is therefore not suitable for a short-term study seeking to produce enough data for statistical validity.…”

“…75% RH was chosen so that ferrous chloride (FeCl 2 ·4H 2 O) within the objects would become liquid through deliquescence and promote the most aggressive types of corrosion. Adsorbed surface chloride on β-FeOOH would also attract water and promote corrosion under these conditions (Watkinson & Lewis, 2005). Objects containing no soluble chloride should be easily distinguished.…”

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

“…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).…”