Metals 137
Figure 2Concretion and mud on a split ring gun recovered from the Mary Rose
As stated in the corrosion section, metals immersed in seawater have all the
requirements for a corrosion cell to be set up on the surface. Why is there this
enormous difference in corrosion rate? The reason is due to the exclusion or
reduction in dissolved oxygen reaching the metal surface. The oxygen supply
could be restricted due to the artefact being deeply buried in the sea-bed. The
large wrought iron split ring guns from the Mar y Roseare a good example of
this and this is well illustrated in Figure 2, e.g.where the bore of the gun
recovered from the Solent is full of mud from the sea-bed. Another reason for
dissolved oxygen not being able to diffuse to the metal surface is that the arte-
fact is covered with a thick layer of concretion (calcium carbonate). These
concretions can be from 0.5 to 15 cm thick. These arise from hardness salts in
the seawater changing to solid carbonate during the initial corrosion reactions on
the metal surface and/or the settling of barnacles, mussels, etc., on the metal
surface. These die to form a thick carbonate scale. With some metals and
alloys, the corrosion products themselves may limit the arrival of dissolved
oxygen to the metal surface. The lead artefacts recovered from the Mar y Rose
are a good example of this where, even after immersion for hundreds of years in