@jcjeant
Concretions on the CVR pinger ..... indication of a crack ?
I think this may be galvanic action where the aluminium housing of the pinger is acting as an anode and is corroding rather than the ferrous cylinder. Further evidence of this is that there appears far less rust on the CVR cylinder than the FDR which has lost its pinger, and hence no sacrificial anode to stop it rusting. There needs to be an electrical path between pinger and cylinder for the effect to occur, which I assume is there somewhere.
Galvanic anode - Wikipedia, the free encyclopedia
Sacrificial protection on the hull of a ship using a Zinc anode.
Zinc anodes protect the steel parts of the hull | Flickr - Photo Sharing!