Corrosion is the destructive attack of metal by chemical or electrochemical reaction with its environment. Reddish brown rust formation of iron/steel, tarnishing copper and silver, white product formation on zinc are some examples of corrosion. Deterioration by physical means is called erosion, galling or wear. Swelling of plastics, decay of wood, erosion of granite or leaching away of Portland cement are not included in the definition of corrosion. "Rusting" refers specifically to the corrosion of iron and iron base alloys. Non-ferrous metals corrode but do not rust.