Corrosion and Rust: What is Corrosion & Why Do New Layers Form?

[gracy]

what corrosion really is?my teacher says it is disintegration and decaying of metals via chemical and more precisely redox reaction.so according to my teacher corrosion is the wearing away of a metal through a chemical reaction.my question is as corrosion is process of decaying,wearing away then why there is formation of new layers rather than disintegration of already existing layer? for eg.when Aluminum corrodes, forms a thin layer of aluminum oxide over the surface of the metal and also when Copper corrodes produces the green color layer known as patina

 

[Borek]

Layer of oxides is typically weaker than the metal itself, and quite often only lousily attached. As it is the metal below that is strong and capable of carrying the load, when part of the metal is removed, whatever is left is weaker than it initially was.

 

[gracy]

sorry but i didn't understand.i am asking as corrosion is process of decaying,wearing away then why there is formation of new layers rather than disintegration of already existing layer? for eg.when Aluminum corrodes, forms a thin layer of aluminum oxide over the surface of the metal and also when Copper corrodes produces the green color layer known as patina.

 

[SteamKing]

Not all metal oxides, which is what happens when metal corrodes, form in the same way.

For example, when aluminum corrodes, a thin layer of oxide forms between the aluminum metal and the ambient environment, and this layer prevents further corrosion of the aluminum from taking place, unless the oxide coating is disturbed or removed.

http://en.wikipedia.org/wiki/Aluminium_oxide

The aluminum oxide does not have a color which is that different from pure aluminum, thus weathered aluminum is a lot like pure aluminum in appearance.

Iron and many iron alloys form oxides which don't have the same physical characteristics as aluminum oxides. Most notably, the oxides of iron can range in color from orange to black. Second, the oxides of iron are much weaker than iron itself, so any protection the iron oxide layer provides to the base metal will be lost when the iron oxide coating weathers away. Once this happens, corrosion continues in cycles until all of the iron is converted to oxide form.

http://en.wikipedia.org/wiki/Corrosion

The study of corrosion and how to control it and prevent it forms an important part of material science.

 

[Bandersnatch]

sorry but i didn't understand.i am asking as corrosion is process of decaying,wearing away then why there is formation of new layers rather than disintegration of already existing layer?

The new layer doesn't grow on the metal object. It replaces the outermost part of the object. Once patina, rust or aluminium oxide forms, there's less of the original pure metal left. If somethig then removes the new layer (very easy with rust), the deeper layers can oxidize in turn, leaving even less of the original object intact.

 

[gracy]

The new layer doesn't grow on the metal object. It replaces the outermost part of the object. Once patina, rust or aluminium oxide forms, there's less of the original pure metal left. If somethig then removes the new layer (very easy with rust), the deeper layers can oxidize in turn, leaving even less of the original object intact.

hmm...now it makes sense to me.patina and aluminium oxide etc prevents further oxidation i.e corrosion of metals right ?but i think rust doesn't prevent further oxidation of metals or (original object)am i correct?

 

[Borek]

patina and aluminium oxide etc prevents further oxidation i.e corrosion of metals right ?but i think rust doesn't prevent further oxidation of metals or (original object)am i correct?

In most cases oxides produced don't protect the metal surface. Sometimes they do - which is why aluminum and copper are quite popular in some applications.

Note, that patina on the copper is not just an oxide, it is rather a basic copper carbonate.

 

[bigfooted]

hmm...now it makes sense to me.patina and aluminium oxide etc prevents further oxidation i.e corrosion of metals right ?but i think rust doesn't prevent further oxidation of metals or (original object)am i correct?

Aluminum and copper oxides are more dense, and once the oxide layer covers the entire surface, fresh oxygen cannot penetrate the oxide layer and form an oxide with the underlying metal. Iron oxide is very porous and oxygen molecules can pass through the iron oxide and reach the underlying iron and form new iron oxide.