Conservation of Brunel’s steamship ss Great Britain
Corroding hull of ss Great Britain before environmental control
Brunel’s engineering masterpiece ss Great Britain was launched in 1843. It was the first screw driven ocean going liner with an iron hull. In 1970 it was salvaged from the Falkland Islands and returned to Bristol to rest in the original open air dry dock in which it was constructed. In 1999 ongoing corrosion meant that its estimated life span was 25 years. Desiccation was identified as the most controllable and predictable way to control corrosion of the chloride infested hull. To underpin this ambitious conservation project David Watkinson and Mark Lewis carried out research in the Department of Archaeology and Conservation in Cardiff University. This identified the corrosion mechanisms occurring on the hull and modelled them in climatic chambers to identify the effect of low relative humidity storage on the corrosion of the iron (Watkinson and Lewis 2004, 2005). This focussed on the influence of iron chloride and βFeOOH corrosion products on the corrosion of iron in low humidity. This data informed engineers and architects constructing the desiccated envelope around the hull of the ss Great Britain as to how dry the environment had to be to prevent corrosion and its rapidity if relative humidity exceeded no-corrosion points. The results are used to maintain a controlled relative humidity of 20% within the space that was created by construction of a glass waterline roof between the ship and the dock side. This reduces iron corrosion to a negligible rate and vastly extends the life of the ship.
Desiccation plant within dry dock
The project was funded by ss Great Britain Trust/Heritage Lottery fund to the value of £44,000.
Flooded roof to offer the impression of a floating ship
The ss Great Britain won the
Gulbenkian Museum of the Year award for 2006.
The leading judge, Professor Lord Robert Winston referred to it as a ‘…truly groundbreaking piece of conservation…’ making the ship ‘…accessible and highly engaging for people of all ages.’ Without the Cardiff University study of corrosion rates any attempt to desiccate would have been empirical, with unpredictable results accompanied by excessive energy expenditure. The data produced by Cardiff acts as a management tool to control the preservation of the ship (Watkinson and Tanner 2008).
Managing resources using corrosion research data.
Climatic chamber modelling of chloride corrosion of iron at low humidity.
Research Method - Classic weight gain methods were used to measure iron corrosion and hydration/desiccation of corrosion products in fixed relative humidity and temperature. At low humidity both β- FeOOH and FeCl24H2O corroded iron, but below 20% relative humidity, when FeCl22H2O was determined to be the stable phase, iron did not corrode. The β- FeOOH corrosion product corroded iron even at 15 % relative humidity. A storage relative humidity of 20% was chosen to offer good corrosion control in return for the energy required to run the desiccation plant. Preventing iron corrosion would require a relative humidity of 12% and significantly greater energy expenditure. Cardiff University continues to collaborate with the ss Great Britain Trust monitoring the hull environment.
Add on Value: Iron Storage in Museums – Considering how iron might corrode at low humidity, where ferrous chloride can be formed by drying chloride rich iron and where βFeOOH will be present from atmospheric corrosion of the same chloride matrix, reveals that corrosion is slow until 30% relative humidity is exceeded. This information impacts upon managing storage of archaeological iron in museums. Previously it was known that the storage of chloride contaminated archaeological iron should be dry, but this new data identifies that corrosion should be preventable if relative humidity is kept below 12%. Also, if this is exceeded by a small amount the ensuing slow corrosion rate is not critical for the preservation of the iron.
Current work – A more detailed study of low humidity corrosion is taking place at Cardiff to produce quantitative corrosion rates by determining oxygen consumption during oxidation of the iron (link to new project). Also, a study is currently examining the distribution of relative humidity over the hull of the ss Great Britain. Desiccation over the hull is provided by dried air that is exited from large jets aligned along the length of the hull. The air is dried to 0-3% relative humidity at source and exits the vents at around 16%. By recording relative humidity above, below and to the side of air jets, corrosion control over the hull surface is monitored. This is compared to external relative humidity and temperature to investigate their influence on the environment within the humidity controlled space. Resulting data will be used to manage output from the desiccation plant
Exit jets for the desiccated air travelling over the hull of the ss Great Britain.
- Watkinson, D. and Lewis, M. R. T (2008) Desiccated storage of chloride contaminated iron: A study of the effects of loss of environmental control. In Heritage Microbiology and Science: Microbes, Monuments and Maritime Materials. May, E., Jones, M. and Mitchell, J. (eds), Special publication 315, Royal Society of Chemistry, Cambridge 279-89. ISBN978-0-85404-141-1
- Watkinson, D and Tanner, M. (2008) ss Great Britain: conservation and access – synergy and cost. In Conservation and Access; contributions to the London Congress. 15-19 September 2008, Saunders, D., Townsend, J. and Woodcock, S. (eds), The International Institute for the Conservation of Historic and Artistic Works, London 109-114.
- Watkinson, D. and Lewis, M. (2008) ss Great Britain: Science and technology underpin enclosure design. Conservation Matters in Wales, On Display: Showcases and Enclosures, Henderson J. (ed). Federation of Museums and Art Galleries in Wales, 13-17
- Watkinson, D., Tanner, M., Turner R. and Lewis M. (2005) ss Great Britain: teamwork as a platform for innovative conservation, The Conservator, 29 73-86.
- Watkinson D. and Lewis M. (2005) Desiccated storage of chloride contaminated archaeological iron objects. Studies in Conservation, 50 241-252.
- Watkinson D. and Lewis M.R.T. (2005) The Role of βFeOOH in the Corrosion of Archaeological Iron, in Materials Issues in Art and Archaeology VII, Vandiver P.B., L. Mass J.L., and Murray A. (eds.), Material Research Society Symposium Proceedings 852, Warrendale, PA, 2005, 001.6.
- Watkinson D. and Lewis M. (2004) ss Great Britain iron hull: modelling corrosion to define storage relative humidity, Metal 04 Proceedings of the international conference on Metals Conservation, Ashton J. and Hallam D. (eds.), Canberra Australia 4-8 October 2004, 88-103 National Museum of Australia.