Simon, Hayley; (2021) Characterising marine archaeological iron degradation and the efficacy of treatments to date: worth a shot? Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Archaeological iron corrosion is a series of complex reactions that proceed differently throughout the lifetime of an object. Altering the environmental conditions, for example by excavating, conserving or displaying an artefact, can change the material stability, and result in a seemingly stable object degrading in a new environment. To develop effective and cost-efficient conservation strategies, it is necessary to characterise the rate and mechanism of corrosion before, during and after treatment. This thesis uses a combination of in situ experimentation and material characterisation to exploit a unique opportunity to study the corrosion and conservation of the cast iron cannon shot from the Mary Rose shipwreck (c. 1545), which began to severely degrade in storage following active conservation. The initial stages of marine burial were investigated using multimodal synchrotron tomography – µ-XRD-CT and µ-XRF-CT – which was applied to iron corrosion for the first time to visualise a ~10 µm particle of Mary Rose shot in situ during corrosion in oxygenated 0.6 M NaCl solution over 9-hours. This revealed that α-Fe reacts directly to magnetite, Fe3O4 with no observable intermediate. Corrosion occurred uniformly and reached equilibrium in 2-3 hours, with Cl- acting as a spectator ion that was not directly involved in the corrosion reaction. A comparison of the impact of treatment approach on the current-condition of the shot was investigated through characterisation of 22 shot from 4 different conservation histories using a suite of synchrotron-based techniques – synchrotron X-ray powder diffraction (SXPD), X-ray absorption spectroscopy (XAS) and X-ray fluorescence (XRF) mapping. This showed that no chlorine or chlorinated phases were present on the surface of shot passively stored at pH 10 in Na2CO3/NaHCO3 solution since excavation, however sub-surface hibbingite, β-Fe2(OH)3Cl, was observed in metal pores. In contrast, the chlorinated corrosion phase akaganeite, β-FeO(OH, Cl), was found on the surface of all shot actively treated to remove chlorine using a neutral pH water washing method, with or without an additional pH 12-13 NaOH/Na2SO4 chemical reduction treatment, even though the shot had been immersed in sesquicarbonate solution for a substantial period before active conservation and display. Previous conservation treatments involving the high temperature reduction at 800 °C in the presence of hydrogen gas was found to have resulted in the transformation of the cast iron into a new, light-weight α-Fe based material, which would not now be considered acceptable from a conservation ethics viewpoint. Building on these results, a novel long-term experiment was developed to directly observe reactions that occur on archaeological iron during long-term storage in Na2CO3/NaHCO3 solution. 4 samples (α-Fe metal standard, a freshly excavated cast iron shot, a previously conserved Mary Rose shot and a β-FeO(OH, Cl) standard) were immersed in Na2CO3/NaHCO3 (0.15 M, pH 10) for 19 months and the corrosion monitored by SXPD. The results showed that β-FeO(OH, Cl) was not present on the freshly excavated sample before treatment. Instead, the phase formed over the first week (~200 hours) of immersion, before transforming to goethite, α-FeO(OH) by day 84 (~2000 hours). β-Fe2(OH)3Cl was observed on the previously conserved shot, and it reacted to form Fe3O4 and α-FeO(OH) in a two-step reaction via a chlorinated green rust 1, GR1(Cl), intermediate, with varying ratios of magnetite:goethite depending on the O2 availability. The metal sample did not react during the 19 month experiment, as expected for an iron alloy passivated at pH 10, indicating that sesquicarbonate solution can provide a stable low-cost environment for aqueous storage of marine iron artefacts. However, although iron is stable during immersion, this project has demonstrated that the time point which poses the greatest risk to archaeological iron is when the object is removed from passive storage and taken for active treatment and drying prior to display.

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