Cast iron cannon balls excavated from the wreck of King Henry VIII flagship, the Mary Rose, have been actively corroding on mixed material display at 55% relative humidity (RH). A Cardiff University study has examined corrosion rates of cannon balls treated by hydrogen reduction, alkaline sulfite and Hostacor IT. Oxygen consumption of six cannon balls has been measured at five relative humidities between 20% and 60% RH as proxy corrosion rate. Results show a noticeable increase in corrosion rate at 50% to 60% RH and also returned differences in corrosion rates of cannon balls based on previous treatment. Display RH of 55% for showcases containing organic materials and cast iron is found to be unsuitable for the cannon balls and their display in these conditions is not recommended. This has implications for contextual display at the Mary Rose Trust and elsewhere. Further study with greater sample numbers is providing statistical evidence of the effect of treatment regime on corrosion rate to inform decision making for future treatment, display and storage
