There are several phenomena involved when cast iron objects have been in the ocean for a while.
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 percent 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.
Old cannon balls brought up from the sea sometimes explode after being handled. This happens under special circumstances, when sulphate-reducing bacteria that are common in ocean sediments colonize the minute cracks and crevices in the iron.
The bacteria use sulphates in the seawater as a source of oxygen and excrete the resulting reduced sulphur species. In the presence of iron, the soluble sulphur species react to form iron disulphide (pyrite) or iron mono-sulphide minerals.
Iron sulphides, thermodynamically stable under the reducing conditions on the seafloor, commence oxidation as soon as they are brought to the surface. This reaction is highly exothermic, produces acid, and involves a considerable increase in volume.
Substantial oxidation can occur within hours, perhaps even faster.
Within confined spaces, the rapid volume change of brittle objects during oxidation can result in potentially explosive break-up.