i found a metal object on the shore...it was built up by a blackish material... on the bottom it has a few semi circular distinctions...which tell me it could possibly have or be coins...i need to find out on how to remove the minerals without ruining the integrity of what is inside the blackish materials... i read up on bronze disease, but couldn't find anything out about demineralizing the possible coins....i know they aren't gold because salt water leaves gold untarnished/ untouched. but if there is something underneath it i want to leave the value (historical or otherwise) of it.
thanks for any help you can give me.

You should start with the least damaging methods and slowly work yourself up to more rigorous cleaning if it is merited.
Step 1 should be just to get the object wet and brush it with a toothbrush. Does dirt come off? If yes then keep going. If no, then soak the object for 1 week in distilled water and try again. If still no action try soap and water. If soap doesn't work then you can try small amounts of baking soda. Baking soda + boiling water can help also.