Just cleaned a bunch of diamond plating on one of our older fire engines at work. We used a diluted solution of muriatic acid. You can buy it at Home Depot. Dilute it down to 15:1 and don't do it over cement. It took off all the oxidation and then we used Mothers aluminum polish to finish it off. Cuts out a hell of a lot of elbow grease! Wear gloves and eye protection or you will look like Fireman Bill.