CORROSION OF MOM STEAMSHIPS: LONGITUDINAL STUDIES'OF THE SS XANTHO (1872) AND THE CITY OF LA UNCESTON (1865)

摘要

The discovery of a very rare high pressure marine steam engine on the wreck of Western Australia's first coastal steamship Xantho (1872) at Port Gregory in 1979 catalysed the first systematic examination of an iron shipwreck in Australia. A full biological, chemical and corrosion survey provided a deep insight into the complex microenvironments that controlled the rate of decay of the ferrous and non-ferrous components (MacLeod, North and Beegle, 1986). Having established the unique nature of the engine it was determined that it would be recovered and so the first application of sacrificial anodes to initiate in-situ conservation began 30 years ago (McCarthy, 1988). The 1865 sinking of the City of Launceston in the middle of Port Phillip Bay, Victoria, after a collision with the steamship Penola, was a major disaster for the fledgling Launceston and Melbourne Steamship Navigation Company. Contemporary attempts to recover the ship using the patented Maquay lifting devices, involving the displacement of water by hydrogen generated from reaction of sulphuric acid on zinc, failed. Salvage operations removed the upper works and masts of the vessel to ensure it did not present a navigational hazard. Lying upright at the bottom of the bay in 21 metres, the wreck was forgotten until rediscovered in 1980. Owing to its well-preserved state and its iconic status as the only intact iron steamship in Victoria, a corrosion survey was conducted in 1991 as part of the systematic evaluation of the wreck. Six subsequent surveys occurred until 2006 and the data from the surveys helped to determine a management plan that permits supervised access to the wreck. A comparative analysis of the corrosion rates on these two iconic shipwrecks, which were wrecked within seven years of each other, was conducted on data collected over 25-30 years, which provided a unique long-term study in marine corrosion. This work has enabled the quantification of the impact of significant periods of burial and subsequent exposure on the 7 metre site of the Xantho and the outcomes of being partly buried in a mound of silt for the City of Launceston at a depth of 22 metres. The excavated Xantho engine took 25 years to conserve in the Western Australian Museum and the City of Launceston wreck is now undergoing in-situ conservation with zinc anodes.