Quantifying effectiveness of chloride desalination treatments for archaeological iron using oxygen measurement

Alkaline deoxygenated aqueous treatments are employed to remove damaging chloride ions (Cl⁻) from excavated archaeological iron objects; however, their effectiveness is either qualitatively assessed or anecdotal. A novel oxygen measurement technique is used to assess pre- and post-treatment corrosion rates of individual archaeological iron objects; these rates are related to their Cl⁻ content before and after treatment. Ten archaeological iron nails were individually sealed in reaction vessels conditioned to 80% relative humidity (RH) at 20 ºC. The oxygen partial pressure inside each vessel was measured remotely over the course of 37 days using an OxyMini fibre-optic meter and a sensor spot inside each vessel. Control vessels containing nitrogen gas revealed negligible leakage. Objects were removed and treated for two weeks in an alkaline sulphite (0.1 M NaOH / 0.05 M Na2SO3) solution at 60 ºC and the extracted Cl⁻ was recorded quantitatively and then the objects were placed back in the oxygen measurement vessels (80% RH and 20 ºC). After recording oxygen consumption, nitric acid digestion was used to determine the residual Cl⁻ content. Half of the ten objects that were treated had their oxygen consumption rate reduced by 91% or more, with the oxygen consumption rate of the remaining reducing by 49-71%. Object Cl⁻ contents reduced from between 336-3487 parts-per-million (ppm) before treatment to a maximum of 364 ppm after treatment. A linear correlation between pre-treatment Cl⁻ content and oxygen consumption rate existed but was not apparent post-treatment, suggesting that the readily accessible soluble Cl⁻, which is removed by a single two-week bath, is the most significant driver for corrosion. Half of the objects were still measurably consuming oxygen after treatment but it is expected that this slower corrosion rate should significantly increase overall lifespan. A single, brief, alkaline deoxygenation treatment is a useful addition to preventive conservation strategies for vulnerable iron objects. Use of oxygen partial pressure measurement to assess corrosion rates provides a new insight into the effectiveness of treatments for quantifying corrosion risk, determining the success of conservation strategies, developing management procedures and cost benefit analysis