The corrosion also known as tarnish affects many metals, including brass, copper and silver. There are times when tarnish on metal gives it an antique look, but generally tarnish is not attractive. Soap and water will clean dirt off metal but will not clean tarnish off metal, no matter how hard you scrub. For cleaning tarnish, you'll need products made specifically for removing tarnish--namely, metal polishes. Some severely tarnished items can be also cleaned with abrasive tools, such as abrasive sponges--but this is usually not best, as these may leave tiny scratches in the metal.
Put on a pair of vinyl or nitrile gloves. These gloves will protect your hands from chemicals you might use and will protect the metal from dirt or oils on your hands.
Place the metal item--if it is small--on a soft foam pad or folded towels. This will help protect it from scratches as you clean.
Use a soft cloth to dust off loose dust from the metal. If the surface is greasy or otherwise dirty, dampen a cloth and place a small amount (dime- to quarter-sized) drop of mild soap on the cloth. Wipe the metal clean, rinse with clean water and dry thoroughly.
Place metal polish on a microfiber polishing cloth. Gently rub the polish into the metal, using short strokes--if you can see the grain of the metal, go with the grain. Switch to a clean area of the cloth whenever the cloth discolors from the tarnish. When all tarnish is gone, buff the item with a clean polishing cloth.