Influence of atmospheric moisture on the corrosion of chloride-contaminated wrought iron

Wrought iron with a remaining metallic core recovered from marine and terrestrial archaeological contexts is unstable and has the potential for further corrosion after recovery or excavation. Where metallic iron remains within the object, it deteriorates as crystals of new, chloride-bearing, corrosion products form at the metal/corrosion interface, resulting in the loss of overlying corrosion. This is undesirable, for the information on the form of the original, uncorroded, artefact is commonly found within the overlying corrosion which is detached. A mechanism for this type of deterioration, based upon the retention of chloride counter-ions as part of the electrochemical corrosion process, was identified by Turgoose (1982) for archaeological iron and his research has informed the storage of chloride-contaminated wrought iron in museum collections ever since: Iron corrodes in the presence of ferrous chloride tetrahydrate but not its dihydrate. Precise data for the relative humidities at which these respective hydrates are stable, and at which iron in association with ferrous chloride corrodes, has been produced by the research presented in this thesis. The role of a commonly encountered chloride-bearing oxidation product of ferrous chloride tetrahydrate, pFeOOH (akaganeite), in the deterioration of wrought iron, and its relationship with atmospheric moisture, were also investigated. Corrosion of iron was observed at relative humidities as low as 22%RH in the presence of ferrous chloride and as low as 15%RH in the presence of unwashed pFeOOH. The effect of temperature on transitional RH values was demonstrated. The results of this research enable informed management decisions about environmental parameters for the storage of chloride-contaminated wrought iron recovered from maritime contexts and terrestrial archaeological contexts stored in museum collections