Difference of opinion

[Lucienne]

Could anyone please settle a difference of opinion and answer the question, Which metal does not rust? Many thanks.

[New Judge]

The �Noble metals� are resistant to corrosion or oxidation.They tend to be precious metals, and include gold, silver, platinum, palladium and rhodium.

In particular Gold does not corrode or oxidise under any normal conditions. It and the other Noble metals can sometimes be dissolved with a solution of �Aqua Regia� - a mixture of concentrated nitric acid and concentrated hydrochloric acid.

[Sir Alec]

Just read an article which says that the only metals to NEVER rust are Gold and Platinum. They are the only two to be extracted as metals not ores. Interesting!

[Lucienne]

Your answer has just enabled me to sound extremely bright! I much appreciate your help on this one, many thanks.

[dave123456]

I thought iridium also came in the "noble metals" group.

[Lucienne]

That should have read 'Many thanks to all of you for your very helpful answers.' We're trying to answer a quiz question, it'll be interesting to see what answer is given when quiz answers are given.

[squarebear]

As mercury is normally a liquid, I don't see how that can rust either.

[beso]

Rust is essentially a form of oxidation. Most metals will form oxides. Other types of corrosion produce carbonates, sulphates, nitrates etc

Gold and platinum don't form compounds with any element. This is why they are always found as a native metal.

Other corrosion resistant metals and alloys are those whose oxides crystals are the same size as the parent metal allowing them to form a very thin layer of oxide on their surface, sealing it off from further corrosion.

Iron oxide swells causing blistering which exposes more metal. Metals like sodium corrode very quickly because the oxide ias actually smaller then the parent metal so it readily breaks away from the surface.

Stainless steel is protected by the nickel and chrome oxides which tightly seal the surface from oxidation.

[Lucienne]

Thank you to everyone who took the trouble to get back to me with very detailed and informative answers. I'm just wondering if th person who set the question has made an error and the question should have read 'Which is the ONLY metal to rust?' In which case the answer has to be iron...possibly!

[Muppit]

Square bear - Mercury is a liquid at room (and various other) temperatures; but like any material it has a freezing point and an evapouration point. The freezing point of mercuray, unlike most other metals, just happens to be considerably lower than many everdau temperatures you find yourself experienceing. When mercury freezes it too can rust.

[Doc_Spock]

Wow !

[joggerjayne]

Wow indeed, Doc.

Why do they "zinc coat" car things if zinc rusts?

[ChuckFickens]

zinc does rust (oxidise) but it does so at a much much slower rate than steel, the coating will eventually break down but it would take many years and until that happens it will protect the steel underneath.

[beso]

The zinc answer is slightly more complicated. Like the oxides of other rust resistant metals the zinc oxide doesn't tend to break away and forms a thin coating that prevents further oxidation. However it can eventually build up to a powdery white layer.

Much of the protective properties of zinc come from its propensity to sacrificially corrode. Like the zinc case of an ordinary dry battery an electric charge is produced between the iron and zinc. This charge protects the iron.

This effect is responsible for one if the great things about galvanising. It continues to protect even when the plating is scratched through.

[block105]

every metal except for iron do not rust as "rust" is the specific oxidation of iron.