MadSci Network: Chemistry

Re: What elements/compounds tarnish raw metals such as Cu, Mg, Ag, and Al.

Date: Mon Mar 9 22:47:03 1998
Posted By: John Christie, Faculty, School of Chemistry, La Trobe University, Bundoora, Victoria, Australia
Area of science: Chemistry
ID: 887210968.Ch

There are many different metals with different properties. Some of them, 
like iron and lead and zinc, rust and tarnish quite readily in ordinary 
environmental conditions, so perhaps it is not even interesting asking 
about them. The four metals you have mentioned are ones that do not tarnish 
much: they certainly do not rust, or anything like that. But there are two 
different types of metal, and two reasons for not tarnishing, among the 
metals you have mentioned.

Copper and silver are fairly unreactive metals, that do not react with 
ordinary acids, and do not readily react with oxygen. Both metals retain a 
fairly high polish, with no tarnishing, in many environments. But keep them 
out of the kitchen! Both copper and silver tarnish very readily in the 
presence of sulfur, or certain sulfur compounds. In the presence of rotten 
egg gas (hydrogen sulfide), or certain organic sulfur compounds, or even 
yellow elemental sulfur, copper and silver surfaces readily react to form 
black layers of copper or silver sulfide.  These layers are very hard and 
insoluble; they are also rather unsightly! Cooking eggs is a notorious 
source of sulfur compounds that tarnish silver; so are natural gas or town 
gas fires. 
Both copper and silver also react readily with chlorine to form whitish 
powdery tarnishes, so swimming pool patios are perhaps another place to 
keep copper and silver away from!
And copper (not silver) also reacts in the environment with carbon dioxide 
and rainwater, very slowly, to form a greenish grey coating of basic copper 
carbonate (verdigris). Greenish looking copperwork on building exteriors or 
statues is sometimes thought to be quite attractive, so much so that other 
chemicals are sometimes used to artificially hasten the process!

Aluminium and magnesium are quite different. They are both actually very 
reactive metals. But these two metals (most especially aluminium) are 
protected from tarnishing in a special way. Aluminium reacts very rapidly 
with oxygen in air to form a surface coating of aluminium oxide. But the 
coating is quite different to the coating of iron oxide that forms when 
iron rusts. A rust coating is powdery and flaky, and when it forms, it 
exposes deeper and deeper layers of iron to be attacked by the rusting 
process. But the aluminium oxide coating is hard, invisible, and 
impervious. And it adheres tightly to the metal below. It just will not let 
other materials through to attack the underlying aluminium. It forms in a 
few seconds, and then the reaction stops. It only ever gets to be two or 
three molecules thick. If aluminium gets scratched, a new protective 
surface layer rapidly forms. In the absence of this surface layer, 
aluminium reacts rapidly and violently with air: machining aluminium is 
very dangerous, and has led to nasty fires; powdered aluminium is used in 
fireworks, and aluminium dust has caused very destructive explosions. 

The chemicals which tarnish aluminium, then, are unexpected ones. The metal 
is not easily attacked by acids or oxygen or chlorine because of the 
protective layer; The things that do make aluminium tarnish are things that 
dissolve or otherwise disrupt that surface layer. Alkalis are quite 
effective; powders that are used in automatic dishwashing machines are 
usually quite alkaline, and they will certainly help aluminium to tarnish 
by dissolving and disrupting that surface layer, and allowing the 
underlying metal to be attacked by oxygen. Aluminium cookware that is put 
into automatic dishwashers usually gets very tarnished. 
Mercury and mercury salts (** DANGER: very poisonous **) also disrupt the 
structure of the surface layer. Aluminium that has been scratched in the 
presence of liquid mercury, or dipped in mercuric chloride solution, 
rapidly grows a set of white whiskers of aluminium oxide when exposed to 
air again!

Magnesium is also a very reactive metal that is protected by a surface 
layer in a similar way to aluminium. But the surface layer of magnesium 
dissolves in acids, and is unaffected by alkalis.


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