[Sneeck]

Hmm with my batt's it was the s-39 liquid that was rusting. When you put solder on it it will make it flow easier, all the excess stuff will go around the actual solder joint itself. That will rust, atleast on my batt's.

If it can't be brushed off easely, I would be worried. Look for sign's that the metal is very thin at some places. If it is i'd dump the pack cause like you said it isn't safe.