I have no idea why anyone advocates the use of salt in their electrolytic solution when we have sodium carbonate and plain old baking soda available. Years ago my first zapping efforts included salt. However, I got busy and discovered what I consider to be the consummate agent and that is sodium carbonate.
I only know downsides about salt and I know of no negatives with sodium carbonate. I rapidly discovered that SC is much kinder to our ancient coins that other agents. The coin I have posted with this article was zapped with SC and I continue to get good results with all my zapped coins. I know one thing for sure, if one is not using solid copper alligator clips the salt will eat right through the nickel plated clips available at most sources.
After the fact, I learned that museums use SC in their metal artifact restorations. If you have questions or if you have are interested in joining a zapping group please join us at the following URL: http://groups.yahoo.com/group/coinzappers You have a standing invitation from me to join CZ and I and the group will be happy to guide you as you learn the exciting hobby of cleaning ancient crusty coins.. God Bless.. Jerry..