I am not against the idea of electrolysis but please don't use baking soda on them. Baking soda is a mild abrasive and will harshly clean the coins. After electrolysis, you might want to try an Ultrasonic Jewelry Cleaner! These work fairly well in removing corrosion and dirt from coins and other items as well. I would hope that some of the folks that have actually salvaged these types of Spanish coins from shipwrecks, would chime if with the method that their' Conservation personnel use!
I just found this information at the link below that I believe you will find helpful in conserving the coins that you have found. Check out the information under the heading "
FATHOMS DEEP".
http://coinagemag.com/wp-content/uploads/2012/02/Conservation-vs.-Restorationreduced_size.pdf
Also, I found this information: Charles Garrett offers a simple electrolysis method for cleaning coins in his book, TREASURE RECOVERY FROM SAND & SEA. His method uses an electrolyte solution of one teaspoon of citric acid and a half teaspoon of salt dissolved in one cup of water.The positive side of a three to six volt power supply is attached to a stainless steel anode on one side of the glass, and an alligator clip is used to attach the negative side to the coin which is placed on the other side of the glass. The current that flows through the solution will loosen any encrustation.
Frank