MadSci Network: Other

Re: Which metal copper, bronze, or silver will rust first

Date: Tue Nov 23 15:34:50 2004
Posted By: John Christie, Faculty, Dept. of Chemistry,
Area of science: Other
ID: 1100817311.Ot

Damien, that is a really interesting project.

There is no simple answer to your question -- it all depends! Here are several bits of information that might help:

"Rust" is a term that is normally used for iron-based metals only. It is the name for the familiar dusty or flaky red-brown iron oxide that slowly builds up on these metals. On copper, bronze, or silver, we talk about a "tarnish" if we are thinking about the degradation of the metal surface, or a "patina" if we are thinking about the pretty and sometimes artistic effects that a tarnish can produce on the surface of these metals.

"Steel" is the name of a large number of different metal alloys, containing mostly iron, a little carbon, and varying amounts of different other metals. Different steels are designed and made for different purposes. "Tungsten steel" is designed to be very hard and strong for use in tools; your tools will rust quite badly if you leave them out in the weather for a few days. "Stainless steel" is designed to resist rusting and other sorts of attack by chemicals in a wide range of situations, but it is quite soft. So it is almost impossible to compare the rusting rate of "steel" with anything -- it depends what sort.

The process of rusting or tarnishing occurs when a metal reacts with something in the air. It is not always the same component of the air. Air varies greatly in its water content, its content of salt spray from the ocean, and the presence and amounts of various pollutants.

Iron reacts very slowly with ordinary oxygen, but the reaction is catalysed by water, and especially by acid or salt. So iron in a damp climate rusts very quickly if it is exposed to acid rain or to ocean spray. It rusts much more slowly in damp mountain climates with clean air. It hardly rusts at all in the desert.

Copper hardly reacts with oxygen at all. It does react quite slowly with carbon dioxide and water and oxygen to form a green tarnish (basic copper carbonate), more quickly with salt spray and acid to form a different green tarnish (basic copper chloride) or very quickly with sulfur-containing gases to form a black tarnish (copper sulfide).

Silver does not tarnish at all in ordinary air. But if it is used in the kitchen, where traces of sulfur containing gases are produced during cooking, it rapidly forms a black tarnish of silver sulfide.

Bronze is an alloy of copper and tin. It will react very slowly with ordinary oxygen to produce a whitish tin oxide tarnish, or more rapidly in the various types of polluted air to form the same products as copper.

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