A friend of mine recently recovered an Elizabethan cannon from a site where there is no wreck. We've found out quite a bit about it so far but due to it's size, preservation is a problem. The cast iron has gone very soft, it'll never be as new but we can slow it's decay. A few years ago I wrote an article about preservation based around, and quoting the procedures used on the Mary Rose preservation techniques.
The article I wrote is - http://www.atlanticscuba.co.uk/index...=152&Itemid=28. Since then I have tried several things and found Bituminous Paint to be the easiest and best.
With the cannon, he's put it in a huge stainless steel bath and is monitoring the ph level. As and when the solution shows any sign of turning acidic he adds caustic soda to raise the ph again to keep it as close to neutral as possible.
A pic after the encrustation has been removed.