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
Rimmer, Melanie B.
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
investigating treatment archaeological iron object archaeological iron object become ion burial enhance corrosion solution form induced corrosion require relative humidity difficult achieve practice iron object significant risk dry storage maintain solution remove ion object large quantitative dataset need evaluate extraction efficiency relationship concentration corrosion rate risk pose treatment using archaeological iron sample material_record extraction behaviour_individual object treatment test nitrogen objects subsequently measure residual extraction efficiency case treatment result residual level ppm corrosion test show treated object stable post treatment scan_electron_microscopy identify