A Synchrotron-Based Study of the Mary Rose Iron Cannonballs

Post-excavation iron corrosion may be accelerated by the presence of Cl^- , leading to conservation methods designed to remove Cl. This study exploits a unique opportunity to assess 35 years of conservation applied to cast-iron cannon shot excavated from the Mary Rose. A combination of synchrotron X-ray powder diffraction (SXPD), absorption spectroscopy (XAS), and fluorescence (XRF) mapping have been used to characterise the impact of conservation on the crystalline corrosion products, chlorine distribution, and speciation. The chlorinated phase akaganeite, β-FeO(OH,Cl), was found on shot washed in corrosion inhibitor Hostacor IT with or without an additional reduction stage. No chlorinated phases were observed on the surface of shot stored in sodium sesquicarbonate (Na₂ CO₃ /NaHCO₃ ); however, hibbingite, β-Fe₂ (OH)₃ Cl, was present in metal pores. It is proposed that surface β-FeO(OH,Cl) formed in the early stages of active conservation owing to oxidation of β-Fe₂ (OH)₃ Cl at near-neutral pH.