Assessing the effects of alkaline desalination treatments for archaeological iron using scanning electron microscopy

Archaeological iron objects are often vulnerable to severe post-excavation corrosion induced by chloride ions, a corrosion accelerator. To reduce this problem, alkaline deoxygenated desalination treatments may be used to remove chloride ions. There is very little information on the mechanisms and efficacy of such treatments and they are not in general use by British conservators. As part of a larger study, some iron objects were desalinated and a scanning electron microscope (SEM) equipped with energy-dispersive X-ray spectrometry (EDX) was used to analyse eight polished cross-sections of treated and untreated archaeological iron nails to investigate whether differences between treated and untreated nails could be detected with this method. The analysis showed clear differences between the treated and untreated halves of the iron nails. Untreated nails showed regions of high chlorine content that appeared to be mobile and caused corrosion on the polished surfaces of the samples, despite storage at low humidity. Chlorine-containing corrosion products were identified as akaganéite using Raman spectroscopy. The majority of the treated samples showed no fresh corrosion or areas of very high chlorine content except near slag inclusions surrounded by metal. These caused fresh corrosion, presumably due to the inability of the treatment solution to reach these deep-seated areas. The use of SEM-EDX showed that desalination of the objects had a positive impact on the stability of archaeological iron. It also showed some of the limitations of desalination, such as its inability to remove deepseated chloride ions. Although not an extensive study, this information is useful in understanding the effects of treatment on a detailed level and will complement existing data on the effectiveness of treatments