I use an electrolytic process in which involves a solution of Sodium Carbonate (plain washing soda) and a 12V car battery charger, running overnight. Afterward I rinse and dry the chain, grind it, and oil it.
Here's the link:
Recently, one of my regular customers, who sends in up to 30 chains at once, had a rusted chain in the mix. I quoted him a "derusting" fee equal to a sharpening charge (so effectively that chain cost twice as much to grind), and he was very happy. Since I charge about 10% of a new chain price for grinding, he effectively saved 80% over buying a new one.
Hope this helps.
Mike