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I recently attended the Titanic exhibition at London's Science Museum. One of the exhibits informed me that great care had to be taken when bringing cast-iron objects to the surface from 4 kilometres down on the seabed, because when they emerge from the water they can explode. Why do these objects do this and how is the problem counteracted?
Thomas Theakston , Warlingham, Surrey, UK
There are several phenomena involved. One is that cast iron invariably contains small gas cavities or blowholes that are formed well beneath its surface. Another is that it has quite low ductility, and will fracture rather than deform. Thirdly, it is a very heterogeneous material, containing about 4.5 per cent carbon and significant amounts of silicon and manganese, together with phosphorus and sulphur. The principal phases that are present are graphite, argentite and ferrite.
When immersed in an electrolyte such as seawater, electrolytic corrosion starts up at the surface of the casting. One of the products of this corrosion is hydrogen in an ionic or atomic state. In this state it can diffuse through the ferrite lattice and find its way to the gas cavities. There it re-forms as molecular hydrogen, increasing the pressure in the cavities.
Because this electrolytic process takes place at great depth and pressure, the pressure build-up in the gas cavities reaches equilibrium with the external water pressure. Raising the cast-iron object from the deep seabed removes the external pressure on the iron, so the gas in the cavities creates very high stresses. At best, the iron will develop cracks. At worst, the casting will shatter.
C. C. Hanson , Farnham St Martin, Suffolk, UK