Take em to a store that gives you cruddy service. Cruddy service lousy attitude from the cashier? There's your candidate! Works best on purchases over $2, and don't forget to wait until there's a long line. This way the cashier with the lousy attitude has to deal with the next ten customers behind you that now have lousy attitudes
If you roll your pennies mix in your junk clads, nine out of ten times the clerk will just take the rolls.
I pass corroded zincs on for someone else to worry about. I drop them in a short copper pipe, beat the ends down and sell them with the rest of the copper.