Iron – a preservation headache!
Brunel’s ss Great Britain in dry dock in Bristol. Iron forms the fabric of this award-winning museum.
For millennia, iron and its alloys have been in ubiquitous use within home, industry, construction, transport, agriculture and warfare. Archaeological sites can contain thousands of iron objects, ranging from swords to nails. Iron can even form the fabric of the museum itself as with the ss Great Britain. Both these and more recent historic iron structures and artefacts are subject to inherent instability in the atmosphere, which constitutes a major challenge for their long-term preservation in museum stores. If ferrous metal heritage is to be available for future generations to enjoy it is necessary to understand and control its corrosion. Unfortunately, corrosion control can be very resource-intensive, and given the current economic situation museums need to make management decisions based on cost-effectiveness and risk-benefit analysis. To allow museums to use their resources more efficiently, we need much more detailed knowledge of corrosion rates of archaeological iron and how they can be reduced using minimal resources.
Chloride in iron: the problem
The original shape of this medieval iron nail is preserved not by the residual iron core (silver) but by the dense grey black corrosion product, surrounded by outer corrosion layers.
To the casual observer it may seem remarkable that any metallic iron survives after millennia in the ground; indeed, sometimes iron is entirely converted to corrosion products, but even in this form the corrosion retains information about object shape and is of use for research. Burial conditions, climate and alloying with other elements all contribute to determining archaeological iron corrosion rates, along with the chloride that has been pulled into the iron from the soil to balance the positive charge inside the object resulting from the corrosion process.
Once the object is excavated, the chloride ions accelerate corrosion processes. In damp atmospheres the chloride ions produce corrosion products such as akaganeite (β-FeOOH). These newly-formed compounds expand within the corrosion layers, causing rapid and catastrophic lamination which destroys the heritage value of the object. Reducing relative humidity (RH) below 35% slows corrosion, but akaganeite and the related salt ferrous chloride are hygroscopic, resulting in corrosion of iron at an RH of 15% and 21% respectively. Thus, even very dry atmospheres have the potential to corrode iron.
Destruction of the iron object occurs as corrosion layers laminate away from the residual metal core.
Weight gain of ferrous chloride and iron powder mixtures at varying relative humidity; corrosion occurs at low relative humidity.
Preserving iron in heritage collections – ideals, reality, current practice
The ideal situation for heritage iron is no corrosion at all, but pragmatic assessment exposes this goal as unrealistic. While we know in theory how to stop corrosion by eliminating moisture or oxygen or the chloride ions which are so damaging, in practice the complete removal of any of these is technically challenging, costly and fraught with management difficulties. Desalination treatments to remove chloride ions are unpredictable, with complete removal of electrolyte rarely being achieved, and removal of oxygen is generally impractical over the long-term (for more information please see Previous research).
Archaeological archives such as the Museum of London LAARC contain thousands of iron objects. Managing such large collections of unstable material is challenging. (Photo credit: Helen Ganiaris)
Current conservation practice therefore focuses on controlling the ambient relative humidity, sometimes in combination with desalination washes to reduce chloride levels in the iron. Since achieving the ‘no-corrosion’ RH threshold of 12% or less offers major technical, financial and management challenges, an alternative is to adopt a ‘minimal corrosion’ strategy employing a higher relative humidity in return for finite object life, but with a lower resource cost. These management decisions require quantitative knowledge of corrosion rates at different RHs and its impact on the heritage value of an object, as determined by cracking, lamination and loss of object shape. The goal of this AHRC/EPSRC Heritage Science Project is to deliver the knowledge that is needed to design a range of pragmatic, evidence-based and predictive preservation standards that allow managers to choose how best to utilize their resources.