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Metallic Corrosion
Name: Joe L.
Status: other
Age: 40s
Location: N/A
Country: N/A
Date: 2001
Question:
Some metals do not rust, like gold and aluminum. Is
there a complete list somewhere of metals that do not corrode? Further, what
is it about the metal and water that cause the metal to disintegrate.
Replies:
Corrosion does not just describe a single chemical degradation or
reaction. It is a whole family of chemical reactions. What metals corrode
and under what conditions depends upon the metal and the environment in
which it is surrounded.
In the case of iron the corrosion is a series of reactions with oxygen
and water to form a mixture of iron oxides. These reactions are catalyzed by
the presence of chloride ions.
Aluminum and magnesium react readily with oxygen, but the oxide formed
forms a protective film that inhibits further reaction.
Gold and platinum do not corrode because their oxides are less stable
than the metals themselves. The same is true of silver to a lesser extent,
because silver sometimes occurs as the metal in ores. On the other hand
silver tarnishes when exposed to air.
Copper slowly oxidizes under atmospheric conditions, but is not corroded
very much by elemental fluorine, because copper fluoride forms a protective
layer that inhibits further reaction.
Of course temperature also plays a role.
So it is not possible to just make a list of metals that do not corrode.
Vince Calder
Hi, Joe !!
I believe that you are talking about metals exposed
in air. Aluminio, in contact with a solution of NaOH
(sodium hydroxide) is corroded.
The problem of metal corrosion when exposed to air
concerns to the existence of impurities of the air, like
sulphur dioxide, chlorides, etc, and water. Such
impurities may deposit over the surface of the metal
and cause reaction with oxygen of the air.
In time, the layer that deposits over the metal surface
will lead to galvanic cells, which corrode the metal.
That is why it is necessary to keep the surface dry,
in order to avoid corrosion.
On the other side, if two different metal are kept for
a long period of time together and between them a
layer of electrolitic solution is formed, then there will
occur corrosion of one of them.
The remaining metal not corroded is called "noble".
The most easily corroded metal is Aluminium. Than
comes zinc, etc, like :
Aluminium, zinc, iron, tin, lead, copper, silver and gold.
A more complete list may be got in a book of chemistry.
By the way, aluminium is also corroded when exposed
in the air. Through a process named "anodization",
a layer of aluminium oxide is formed over its surface
and polished, so that the metal is protect ( aluminium
oxide is more resistant than the metal itself ).
About the second part of your question : " metal that
disintegrates in water". Metallic sodium disintegrates
in water, just because of its strong reaction, giving off
hydrogen : 2Na + 2H20 -> 2NaOH + H2.
This is an extremely violent example of corrosion.
Alcir Grohmann
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Update: February 2012
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