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Managing heritage collections requires understanding of their response to storage and display environments. For heritage iron artefacts, and especially those from marine \ud contexts, this environmental response is influenced by corrosion-instigating chlorides that remain after conservation treatments designed to remove them have been completed. Previous studies have shown that measuring chlorides extracted during treatment does not accurately inform on how many chlorides remain. Artefacts are then placed on display or in storage under the assumption it is safe to do so, when in fact their environment may be \ud causing them to deteriorate at a rate that compromises their structural integrity and cultural value. \ud \ud It is this problem that faces the 1248 cast iron cannonballs from the Mary Rose, Henry VIII’s Tudor-era warship that sank in 1545 and was excavated beginning in 1971. Many treatment methods have been employed on the cannonballs to mitigate their corrosion, yet they continue to show visible signs of corrosion in a range of humidities. Nearly all of the cast iron cannonballs are now in desiccated or passivating storage, robbing the public of crucial context their display would otherwise provide. \ud \ud This study set out to determine the corrosion rates of 56 variously treated cannonballs from the Mary Rose, to inform on and determine their best practices for their ongoing care. Using non-invasive oxygen consumption monitoring to determine their corrosion rates confirmed earlier studies on wrought iron showing the escalating risk faced with escalating humidity. All cannonballs exhibited negligible gains from 20–40% RH, followed by \ud substantial increases at 50% RH and again at 60% RH. Selected cannonballs were then \ud retreated in order to determine if their corrosion rates could be reduced. The results showed that not only did retreatment fail to reduce corrosion rates, doing so caused those rates to increase for most of the samples that underwent the procedure, thereby ruling out\ud retreatment as a viable option until new methods are assessed.\ud \ud By using the methodology and data presented in this study, the Mary Rose Trust is now able to ascertain the risk to each of their cast iron cannonballs post-treatment, and can use that knowledge to mitigate the deterioration of the assemblage.