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Tarnish
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{{Short description|Corrosion on outer layer of some metals}} {{For|1924 silent film|Tarnish (film)}} [[File:Wolfram evaporated crystals and 1cm3 cube.jpg|thumb|upright=1.3|[[Tungsten]] rods with evaporated crystals, partially [[Redox|oxidized]] with colorful tarnish]] '''Tarnish''' is a thin layer of [[corrosion]] that forms over [[copper]], [[brass]], [[aluminum]], [[magnesium]], [[neodymium]] and other similar [[metals]] as their outermost layer undergoes a chemical reaction.<ref>{{cite book |last1=Revie |first1=R. Winston |last2=Uhlig |first2=Herbert Henry |title=Corrosion and corrosion control: an introduction to corrosion science and engineering |date=2008 |publisher=Wiley-Interscience |location=Hoboken, N.J |isbn=978-0-471-73279-2 |edition=4th}}</ref> 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. The layer of tarnish seals and protects the underlying layers from reacting. Tarnish preserves the underlying metal in outdoor use, and in this form is called chemical [[patina]],<ref name=":0">{{Cite web |date=2019-08-14 |title=Guide to Tarnish |url=https://www.riogrande.com/knowledge-hub/guide-to-tarnish/ |access-date=2024-08-17 |website=Rio Grande}}</ref> an example of which is the green or blue-green form of [[copper(II) carbonate]] known as [[verdigris]]. Unlike patina advantageous in applications such as copper roofing and copper, bronze, and brass statues and fittings exposed to the elements, a chemical patina may be considered undesirable, as on silverware,<ref name=":0" /> or a matter of taste or convention, as in [[Toning (coin)|toning]] on coins.
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