Tarnish
Tarnish is a thin layer of corrosion that forms over copper, brass, silver, aluminum, 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; it does not tarnish with only oxygen. 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.
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Famous quotes containing the word tarnish:
“Whether they forget
What they wanted first or not
They tarnish at quiet anchor.”
—Philip Larkin (1922–1986)