Corrosion potential measurements were carried out on cannons and anchors. The technique requires a narrow hole to be drilled through the concretion that develops around iron objects in seawater, through which a voltage probe is inserted.
Plotting the corrosion potential and solution pH results on a Pourbaix diagram indicated, unsurprisingly, that the residual iron was actively corroding.
More interestingly, however, drilling into these artefacts revealed that very little concretion had formed on them, often less than one millimetre. This is unusually thin for iron artefacts of any age. Penetration depths of the drill bit indicated that almost all of the iron had been lost. This observation correlates with the finding in the shallow water part of the site that one anchor shaft was found to have no residual iron, - all that remained was a thin concretion cast.
The observations on the extent of deterioration of the heavy iron artefacts indicate a very aggressive environment, in direct contradiction to the corrosion rate measurements (previous slide).