Investigating the treatment of chloride-infested archaeological iron objects

Archaeological iron objects become infested with chloride ions during burial. These enhance electrochemical corrosion, acidify the pore solution and form hygroscopic p-FeOOH. Controlling chloride-induced corrosion requires <15% relative humidity (RH) this is difficult to achieve in practice. Iron objects are at significant risk when dry storage is not maintained. Alkaline deoxygenated solutions remove chloride ions from objects. A large quantitative dataset was needed to evaluate the chloride extraction efficiency, the relationship between chloride concentration and corrosion rate and the risks posed by aqueous treatments. Using 120 archaeological iron nails as sample material, and recording chloride extraction behaviour for each individual object, three treatments were tested nitrogen-deoxygenated 0.1M NaOH (20 C) and alkaline sulphite (0.1M NaOH/0.05M Na2S03) at 20 C and 60 C. Objects were subsequently digested to measure residual chloride. Chloride extraction efficiency was 60-99% in most cases 87% of treatments resulted in residual chloride levels <1000 ppm. Accelerated corrosion tests showed that treated objects were more stable than untreated. Post-treatment scanning electron microscopy identified chloride ions in corrosion products or within slag inclusions, but none at the metal/corrosion interface. The transformation of synthetic P-FeOOH showed that 0.5M NaOH or 0.1M NaOH/0.05M Na2S03 (60 C) produced the maximum transformation (c.50%) to a-FeOOH and/or a-Fe203. It could not be confirmed whether this reaction occurs on objects. Risk to objects was evaluated by assessing physical damage during treatment and modelling chemical residues. Less than 2% of objects experienced total fragmentation 69% experienced no change. The most likely chemical residue from alkaline sulphite is Na2S04 this poses no threat to objects at RH