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Topic: rust and corrosion  (Read 3906 times)

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Offline kevinnn

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rust and corrosion
« on: April 13, 2011, 10:12:59 PM »
is ALL RUST iron oxide, FeO, Fe 2+ and O 2-, right???    if so does rust only happen with copper or materals contaning copper??  if so then why do ather metals rust??  or.. are these metals not rusting are they really just suppering corrosion from elements present in the air?? if so what elements or compounds and why does the metals react, does it provide a more relaxed state for the molecules and when they go into this relaxed state the resuld is damaging and looks like rust??  all help apprecated, i really want to learn chemestry because i find it great and i love it and i want to have a job involving it.  but i have many questons that i am pretty much on my own for, so for all the posts i put up thanks for any help at all :)

Offline Nobby

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Re: rust and corrosion
« Reply #1 on: April 14, 2011, 02:34:00 AM »
Rust is a mixture of Fe2O3 and Fe(OH)3. Iron will be oxidised in a wet enviorement.

Copper will enhance the corrosion, because copper is a more noble element as iron.

Other elements also can be oxidised. Copper itself gives the green patina. A  copperhydroxide/carbonate compound.

Offline Vidya

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Re: rust and corrosion
« Reply #2 on: April 14, 2011, 08:27:20 AM »
We know that metals are reactive and have tendency to react with oxygen in the air to form oxides which are more stable than free metal atoms .
Rusting of iron is catalyzed by water .Fe combines with O2 in the presence of water to form rust

Offline DevaDevil

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Re: rust and corrosion
« Reply #3 on: April 14, 2011, 10:46:53 AM »
...does rust only happen with copper or materals contaning copper??  if so then why do ather metals rust?? 

As Nobby indicated, copper is not required, but can enhance rusting.

More properly, you should be talking about the oxidation of metals. Oxidation is the process where a compound (in this case the metal) donates an (or multiple) electron to form a positively charged state (in the case of Iron, this is Fe --> Fen+ + n e-, where n = 2 or 3).
In order to complete the reaction -remember, a positive charge cannot remain stable unless counterbalanced by a negative charge- another compound (in the case of iron rusting, oxygen from the air) must take up the electrons that the metal is donating. In case of oxygen reduction, this goes according to: O2 + 4e- --> 2 O2-.

Now when water is present, this reaction turns initially to O2 + 2H2O + 4e- --> 4 OH-. Why? Because this reaction requires less energy. This is why water facilitates the formation of iron rust outside.
In acidic environment, the reaction becomes: O2 + 4H+ + 4e- --> 2 H2O, this reaction is even more preferred than merely the reduction of oxygen in water. That is why acid rain is so bad for metal objects outside and will cause them to rust faster.

Now said that ALL metals oxidize, that is the truth. However, there are metals more stable -more commonly said to be more "noble"- than others. The most stable of the common metals is gold, which is thermodynamically stable, even in acid environment.

The reason copper enhances the iron oxidation has something to do with this stability. In time, copper will also oxidize, but when it is in contact with iron, another reaction ("contact corrosion") will happen where the copper (hydr-)oxide thus formed will take up the electrons from the iron to reduce back to metallic copper. The iron will then of course oxidize even faster as it loses its electrons now to both the air and the copper it is in contact with.
To prevent contact corrosion, merely place an insulating layer between the copper and the iron.

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