Saving a steam ship

Maria Burke discovers the chemical voyage being taken to rescue a once great ship and the state of the art home that will help preserve it

Launched in 1843, ss Great Britain was the world’s first iron-hulled, screw propeller-driven, steam-powered passenger liner. A remarkable salvage operation in 1970 brought the wreck back from the Falkland Islands to a dry dock in Bristol, UK. From then until 1999, conventional techniques were used to try to save the ship’s rusting lower hull, but none worked.   

Now, thanks to an innovative ?11.3 million conservation programme, the ship’s lower hull lies under a specially designed glass plate, which will officially open later this month. Under the glass plate, the first of its kind in the world, temperature and moisture levels are controlled to keep corrosion at bay and preserve the ship’s original structure. The ss Great Britain should now be safe for the next 100 years. 

ss Great Britain was built in Bristol for the Great Western Steamship Company between 1839 and 1843 by a team led by Isambard Kingdom Brunel. Widely recognised as a forerunner of modern shipping, she exemplifies the inventiveness of the Victorian era and was the start of the modern era of international luxury passenger travel. At the time of her launch, she was the largest ship in the world and the first iron ship to use the new technology of screw propulsion to cross the Atlantic.   

At over 320 feet long and 50 feet wide, she originally weighed 1930 tons and boasted a lower hull constructed from iron plating. She was also the first ship to include the combination of waterproofed bulkheads, iron wire rigging, a balanced rudder, iron lifeboats, and a ’patent log’ for measuring the distance she travelled.   

Luxurious trans-Atlantic travel 

ss Great Britain was designed initially for the trans-Atlantic luxury passenger trade, and could carry 252 passengers and 130 crew. Re-fitted in 1852, over the next 24 years she carried over 15000 emigrants to Australia; the return journey took 120 days, a very competitive time for the mid-19th century.   

She also ferried troops to and from the Crimean War and the Indian Mutiny. In the 1880s she was converted into a three-masted sailing ship and took Welsh coal to San Francisco around Cape Horn. On her third trip she was forced to shelter in Port Stanley in the Falkland Islands where she remained as a coal and wool storage hulk. By 1937 her hull was no longer watertight, and after being towed a short distance from Port Stanley, she was beached in Sparrow Cove.   

Finally, in 1970, an epic salvage effort refloated the ship, and she was towed back home on a pontoon across the Atlantic to Bristol. Despite her age, the Great Britain was able to float up the river Avon by herself. She now lies in the grade II listed dry dock that Brunel built for her construction. 

Constant corrosion 

During her eventful life, she suffered from constant corrosion, particularly along the waterline, spray zone and drip zones from deck drains, explains Shane Casey, the ship’s curator. Although in the past much of this area had been protected with thick layers of pitch pine, there was a considerable amount of rusting, particularly in the plating around the bow and propeller aperture.   

’When the ship was re-floated, the rescuers observed layers of corrosion scale detaching in huge sheets,’ says Casey. ’Unfortunately, this process then exposed the fresh surfaces to further corrosion. Once the vessel was dry docked, the lower hull started corroding at a much faster rate, as it was now exposed to constant wet/dry cycles and wildly varying relative humidity.’   

In the late 1990s, the ss Great Britain Trust commissioned a scientific examination of the ship’s ironwork. Conservators from Eura Conservation confirmed that there was substantial corrosion inside and outside the ship, particularly in the lower parts of the hull. However, the iron in the exposed upper works of the ship was far less contaminated with chloride than the iron below the waterline.   

Eura cleaned about 20 per cent of the surface, blasting it with an air-abrasive system involving crushed garnet. The other 80 per cent was blasted with pure water at very high pressure. Then they coated the surface with a protective paint system of zinc-rich epoxy and acrylic urethane coatings.   

But the lower hull was in poor shape. Eura estimated that 17 per cent of the hull plating was unlikely to survive traditional ship-yard cleaning, while a further 45 per cent of the material would be damaged. ’They told us we had three to five years before her metal hull would be irretrievably degraded,’ recalls Casey.   

The trust had already tried conventional techniques, such as scraping, sandblasting, and applying ’anti-rust’ coatings, but they had not halted the ship’s deterioration. Modern coating systems such as tannic acids, phosphoric acids, and rust-neutralising compounds, didn’t work either, explains Casey.   

’We concluded that the only practical way to stop corrosion without losing original material was to seal off the hull in a dry, stable museum-like environment,’ he said. ’Drying the environment around the hull was a technically challenging task that involved constructing an envelope around the ship and controlling the relative humidity within the space.’   

Achieving low relative humidity 

But the conservation team needed to know how dry the atmosphere should be to halt corrosion. ’Dehumidification is a well-known technique in conservation to prevent the corrosion of iron, but it had never been done on this scale before. We needed to know whether reducing humidity by a few percentage points would be worthwhile and make running costs very different. Very low relative humidity (RH) will cost much more money to achieve and sustain due to plant requirements for dehumidification and the architectural design of the controlled space,’ says Eura’s Robert Turner   

This is where David Watkinson and his colleague Mark Lewis from the school of history and archaeology at Cardiff University stepped in. ’Desiccation (drying) has been employed for over 40 years, with varying degrees of success, to store and preserve archaeological iron,’ says Watkinson, ’But remarkably, very little investigation of this process has been carried out. There were no accurate figures on the level of desiccation necessary to prevent further corrosion of iron.’   

Analysis of the hull revealed corrosion had left significant amounts of soluble chloride and other products such as ferric oxy-hydroxides (FeOOH), including beta ferric oxyhydroxide ( b FeOOH or akageneite), and ferric oxide (Fe₃O₄ or magnetite), reports Watkinson.   

In the dry dock, the iron hull was corroding rapidly, thanks to the high RH that allowed the soluble chloride to act as an electrolyte to support corrosion. ’This would change when the hull was dried. Chloride concentrates as the moisture within the iron evaporates and anode sites become increasingly acid due to hydrolysed Fe²⁺. This provides low pH conditions in which solid ferrous chloride could form,’ Watkinson says. 

This poses a risk to the hull as ferrous chloride tetrahydrate (FeCl₂.4H₂O) can corrode iron in contact with it, explains Watkinson. However, the dihydrate (FeCl₂.2H₂O) does not cause corrosion. ’By examining the weight response of these chlorides, both alone and mixed with iron powder, to fixed relative humidity values, it was possible to determine their stability range and their influence on the corrosion of iron.’ 

But what levels of RH would prevent as much corrosion as possible without being too expensive to produce? At 21 per cent RH, the researchers showed that the ferrous chloride tetrahydrate was stable. The extra water it contains supports electrolytic corrosion and corrodes iron it comes into contact with. Consequently, the researchers found slow corrosion happened at 25 per cent RH and rapid corrosion at 30 per cent RH.   

Expensive desiccation 

When the RH was lowered to 19 per cent, the researchers found that the dihydrate took over as the most stable chloride form, and they did not detect any iron corrosion. ’To prevent the tetrahydrate from corroding iron, relative humidity will have to be lowered to 19 per cent so that the dihydrate forms,’ says Watkinson. 

Watkinson and Lewis knew that, as the hull dried, chloride-bearing   b FeOOH was also likely to form, and it too can corrode iron if it comes into contact with it. On investigation, they found that corrosion was undetectable at 12 per cent RH. However, Watkinson was pragmatic. ’Desiccating the 322 feet long hull to 12 per cent RH would be costly and technically challenging, with potentially high maintenance costs. We had to look at the overall effect of relative humidity on the rate of corrosion and consider that   b FeOOH would probably not be around in sufficient amounts to cause a significant corrosion problem.’   

After considering the Cardiff team’s results, the trust adopted a relative humidity of 20 per cent. It was able to fine-tune the dehumidification machinery to produce a RH of 20 per cent within the ship and under the glass plate. This is enough to keep moisture levels down and corrosion at bay, but also maximises efficiency and running costs, says Nicola Watt, the trust’s director of museum development. 

Watkinson and Lewis are now looking at the influence of temperature on corrosion. ’If the temperature rises higher than it should be, it could influence corrosion rates,’ Watkinson points out. The trust has installed monitors and sensors to keep track of temperature and humidity fluctuations, and to look out for signs of corrosion throughout the hull.   

When this research is completed, Watkinson plans to produce a model that the curators will be able to use to maintain the ship on a day-to-day basis. For example, if the temperature rises for some reason, then the curators know they can alter the humidity by a certain amount to prevent the onset of corrosion.   

Watkinson believes these guidelines should be applicable to all dry storage of chloride-contaminated archaeological iron in museums around the world. ’We should be able to give curators information on how to store artefacts at a specific humidity and how to care for them if conditions changed, such as a dehumidifier broke down.’   

A glass sea to encase the ship 

Visitors see the ship encased in a flat glass plate roof at the level of the ship’s waterline to ground-level at the dockside. The plate is constructed from a laminate of heat-strengthened glass to provide a sealed chamber for the dehumidification of the lower hull, says Watt.   

It is covered in 50mm of continuously flowing water, which keeps the surface clean and the temperature cool, while also giving visitors the impression that the ship is afloat. This ’glass sea’- water running over a huge plate of glass - is a world first. The interior of the ship has also been modified to produce an environment with minimum air leakage between the lower and upper decks.   

The dehumidification machinery removes moisture from the air under the glass and can cope with 85 kg per hour of water. An air-tight seal between the glass, the ship and the dry-dock allows the interior and exterior of the ship to be kept at a constant RH of 20 per cent. Visitors can descend ’beneath the water’ to view the submerged hull, which is dried by a constant flow of dry air and kept at a temperature between 16?C and 26?C, which makes it easier to control the RH tightly, explains Watt. ’Most moisture will come from visitors and the fresh air they need,’ she says.   

’The dehumidification machinery can deal with the moisture load from up to 200 people per hour in the ship and 64 in the dry dock. If visitor numbers are higher, then we can introduce timed tickets. The dehumidifiers also cater for doors opening and closing, and some water leakage into the dry dock and some through the hull.’   

From July, Brunel’s ss Great Britain will become one of the most exciting and accessible historic ships in the world, says Watt. This work will give the historic steam ship an additional 100 years of life, and is expected to heavily influence the way museums conserve important iron artefacts.   

Time will tell just how successful these new conservation techniques will be. However, conservators and curators are united in the view that this is a 21st century initiative that would have made Isambard Kingdom Brunel, the greatest of Victorian engineers, proud. 

Maria Burke is a freelance science writer from St Albans, UK