Desiccated Storage of Chloride-Contaminated Archaeological Iron Objects

Abstract

Desiccation has long been used to store chloride-contaminated archaeological iron but there are no precise guidelines on the degree of desiccation required to prevent corrosion occurring. Akaganéite (β-FeOOH), ferrous chloride tetrahydrate (FeCl2·4H2O) and ferrous chloride dihydrate (FeCl2·2H2O) have been recorded on archaeological iron. Iron corrodes in the presence of FeCl2·4H2O and β-FeOOH but not in the presence of FeCl2·2H2O. The rate of desiccation of FeCl2·4H2O at various levels of relative humidity (RH) was determined by experiment and found to be an exponential relationship. The point at which FeCl2·2H2O first becomes a stable hydrate was established. Rates of corrosion for iron mixed with FeCl2·4H2O and with β-FeOOH were examined for a range of RH. The hygroscopicity of β-FeOOH and the RH at which it ceases to cause iron to corrode were established. Corrosion of iron in contact with FeCl2·4H2O and β-FeOOH speeds up as RH rises and is appreciable at 25% RH and above. On the basis of these results, recommendations are made that 12% should be the maximum allowable RH for long-term storage of archaeological iron from chloride-bearing soils. Low RH requirements raise problems for long-term monitoring of storage microclimates.