The role of the bright orange akaganeite in promoting corrosion was first investigated in the 1980s.
The Department of Conservation at Cardiff University has a history of innovative research into the corrosion of archaeological and heritage iron. In the early 1980s work carried out by Dr Stephen Turgoose showed the importance of the role of chloride ions and β-FeOOH in the corrosion of archaeological iron. At the time, various desalination treatments for iron were being developed, but little quantitative work was carried out to measure and compare the efficiency of these treatments. In the early 1980s David Watkinson carried out a series of experimental treatments with the first quantitative measurements of treatment success, by digesting archaeological iron nails after the treatment to measure the residual chloride ions. He demonstrated that alkaline solutions were most efficient, and that some of the solvent treatments such as lithium hydroxide in ethanol were not as effective as aqueous solutions.
In the mid 1990’s Watkinson produced further evidence to quantify the use of alkaline treatments and this led to a more extensive study at Cardiff University by Dr Abdulnasser Al-Zahrani. The aim was both to compare the efficiency of various types of aqueous treatments, using the same digestion technique as Watkinson, and to look at the effect of these alkaline treatments on the β-FeOOH that had been identified as so damaging by Turgoose. The study showed that alkaline deoxygenated treatments were most efficient, and that β-FeOOH could be transformed by alkaline solutions in certain circumstances. However, it is not known whether this also occurs on objects.
Chloride-induced corrosion was breaking up the wrought iron hull of the ss Great Britain. This process was investigated by Cardiff University.
Although it was known that β-FeOOH and ferrous chloride were damaging to archaeological iron, it was not until research in the early 2000’s by Watkinson and Dr Mark Lewis to develop a preservation strategy for the ss Great Britain that the full impact of these compounds was recognised. By mixing β-FeOOH and ferrous chloride with iron powder and monitoring the corrosion under controlled environmental conditions, it became clear that both compounds could induce corrosion at very low relative humidity (RH). For the first time, a no-corrosion threshold for chloride-contaminated iron was defined at 12% RH. Washing experiments also confirmed that the removal of surface-adsorbed chloride ions from β-FeOOH made the compound stable with regard to the corrosion of iron.
Although removing chloride ions from objects was clearly beneficial in terms of reducing corrosion rates, in the museum world desalination treatments had fallen out of favour by the late 1990s, due to resource considerations and concerns about risks. A further study was needed to improve on the work of Al-Zahrani by looking at a much larger statistical sample of objects and considering the long-term stability of objects treated by alkaline solutions. An AHRC collaborative doctoral research project between Cardiff University and the BritishMuseum was carried out from 2007 to 2010. This treated 120 iron nails and digested them to calculate a statistically reliable efficiency for chloride extraction. Although almost all objects retained some chloride ions after treatment, the majority released a very significant proportion.
Objects exposed to accelerated aging conditions disintegrate rapidly. Treated objects are much more stable and retain their shape.
Accelerated corrosion testing of the treated objects confirmed that treated objects were significantly more stable than untreated objects. Work was also done on the risks to objects from chemical residues introduced during treatment, and found that these were much less damaging, even in the worst case scenario, than the chloride ions themselves. The research clearly showed that desalination treatments are a powerful tool in the conservation of iron objects.
The research carried out at Cardiff over the past 30 years has contributed significantly to our understanding of chloride-induced corrosion and desalination treatment, but questions remain. The most important of these is the relationship between chloride content and corrosion rate, and how the distribution and form of the chloride in objects affects both the corrosion and the treatment of the objects. These are questions that Cardiff continues to research, in order to develop our understanding and provide practical information for conservators and museum professionals to guide the preservation of their iron collections.