<P> Tarnish is a thin layer of corrosion that forms over copper, brass, silver, aluminum, magnesium, neodymium and other similar metals as their outermost layer undergoes a chemical reaction . Tarnish does not always result from the sole effects of oxygen in the air . For example, silver needs hydrogen sulfide to tarnish, although it may tarnish with oxygen over time . It often appears as a dull, gray or black film or coating over metal . Tarnish is a surface phenomenon that is self - limiting, unlike rust . Only the top few layers of the metal react, and the layer of tarnish seals and protects the underlying layers from reacting . </P> <P> Tarnish actually preserves the underlying metal in outdoor use, and in this form is called patina . The formation of patina is necessary in applications such as copper roofing, and outdoor copper, bronze, and brass statues and fittings . Patina is the name given to tarnish on copper based metals . </P>

Why can a piece of aluminum be used to remove tarnish