Corrosion. That is something that happens to many of us. Here in Sweden they use salt on the winter roads so corrode parts are there when it comes to a car with some years of age. When I have a spare part and would like to clean it from corrosion, there is a slow but effective method that I use. I take my standard 12V batteri charger and hook the Plus ( + ) up to Zinc. I use the type that you have on outboard engines for boats as it is the cheepest. Then I take the Minus ( - ) and have on the corroded part. Have to clean off some just enough to get good connection. The area, not the weight, the area of the Zinc should be bigger than the area of the corroded part. Then I place it all in a plastic bucket or something. Then I add water to cover it all and some basic chemicals like NaOH or something. We call it "Lut" or "Kausticsoda" in Sweden, but I do not knoe the english names. The chemicals is just to let the electric connections between them. Remember that the parts from the Plus and Minus CANNOT tuch each other. They are just connected via the water mixture and therefor the tray must be of something that do not give electric connection. When you plug in and it starts to bubble just a little then it is corrct. If it bubbels hard, then I put in some bulbs between the chargen and the corroded part. They just reduce the bubbles and not so hard effect. If you check it after lets say 24h you will have some black pieces in the water. Take them up and clean them with water and soap. If you still have corrosion, just continue. It functions great on brake calipers, special bolts or whatever part. What happens is that the part that was corroded and has been clean has somewhat harder to corrod again. You do not restore anything or any surface as it was once were, just take away the corrosion. The Zinc is more noble that the corrosion, but less noble that the iron in the part that is not corroded.