Rusting of iron

[Primarygun]

Hello,

I want to have a clear mind before taking the exam.

Isn't the layer of iron can resist corrosion,.ie. rusting, like zinc and aluminium?

Also, why is rust so strange, the rust is red in colour? Shouldn't it be yellow or green in colour?

[Gilded]

In the case of iridium or such, where usually a very, very thin almost unnoticeable layer of oxide forms on the layer, it protects the pure metal underneath. In the case of iron, however, the iron not only forms oxide but also hydroxide, which lets further corrosion happen underneath. Don't know about rust's color though, that's more physics than chemistry, but probably has something to do with the spectre of an iron atom.

[TWJian]

Alright,let me explain.All metals except noble metals are thermodynamically cpable of undergoing air in room temprature.However,oxidation can result in the formation of an isulating,protective oxide layer that prevents futher reaction of the underlying metal.Example,aluminium has a standard oxidation potential of 1.66V.It readily oxidizes but the layer of oxide prevents futher oxidation.Magnesium and stainless steel is similiarly protected.

 

The corrosion of iron is believed to be electrochemical in nature.A region on the surface of iron acts as an anode where the iron undergo oxidation:

 

Fe(s)->Fe 2+(aq)+ 2e- Eo ox =0.44V

(Forgive me for the Eo ox part but i can't write it properly)

 

The electrons then migrate to another potion of the iron surface(cathode)Oxygen is then reduced:

 

O2(g) + 4H+(aq)+ 4e- -> 2 H2O(l) Eo ox=1.23V

 

Notice that H+ takes part in the redution of O2.As the concentration of H+ is lowered(ie:increase in pH),the reduction of O2 become less favorable and vice versa.

 

The Fe2+ ion is then futher oxidized to Fe3+.The Fe3+ forms the hydrated iron(III) oxide known as rust

 

4Fe2+(aq) +O2(g) +4H2O(l) +2xH2O(l)* -> 2Fe2O3.xH20(s) + 8H+(aq)

 

The water becomes acidic.The hydrogen ions futher corrode the iron later.

 

*Rust is a hydrate of ferric oxide with a variable amount of water of hydration.

 

Because the cathode is genarally the area with the largest supply of O2,rust often deposits there.(experiment:place an iron nail in water.More rust will form on the part exposed to air)

 

The enchanced corrosion caused by salts are caused by voltaic mechanism:The ions of a salt provide the electrolyte necessary for completion of the electrical circuit.

 

The ferrous salt(Fe2+) is green but the Ferric salt(Fe3+) is yellow and brown.The red colour could have been formed from complex compounds with impurities like cyanide(the coordination of Fe3+ causes it to change to a red colour)

 

here's a link that might help you if these are too complex:

http://science.howstuffworks.com/question445.htm

[TWJian]

Oops, while i was writing this gilded have posted.The changes of colur is somewhat related to chemistry(coordination of transition elements).

[Primarygun]

Thank you very much.

Is this the property that iron can be made as magnet? Which else metal can also be made as magnet?

And, how can a good electrolyte speed up the process?

Ions attract the iron atoms leave the solid?

[Silencer]

The fact that a layer of oxidised metal can protect the metal underneath from oxidation is the "blueing" and "browning" on guns, right?

[Primarygun]

blueing? You mean the rust?

Rust disintegrates when it forms, a new surface exposes again/

[TWJian]

Primarygun,I believe i have posted in one of your threads before.I seem to have a vague memory of it when I just joined.Your question about iron as a magnet seems rather vague...I presume you meant ferromagnetism.Reread my post.I have explained that a good electrolyte speeds up the transfer of electrons.Quote myself:"The enchanced corrosion caused by salts are caused by voltaic mechanism:The ions of a salt provide the electrolyte necessary for completion of the electrical circuit."

 

I quote the Encyclopedia Britinicca since i'm too lazy to type my explanation:

 

"Ferromagnetism is a kind of magnetism that is associated with iron, cobalt, nickel, and some alloys or compounds containing one or more of these elements. It also occurs in gadolinium and a few other rare-earth elements. In contrast to other substances, ferromagnetic materials are magnetized easily, and in strong magnetic fields the magnetization approaches a definite limit called saturation. When a field is applied and then removed, the magnetization does not return to its original value—this phenomenon is referred to as hysteresis (q.v.). When heated to a certain temperature called the Curie point (q.v.), which is different for each substance, ferromagnetic materials lose their characteristic properties and cease to be magnetic; however, they become ferromagnetic again on cooling.

 

The magnetism in ferromagnetic materials is caused by the alignment patterns of their constituent atoms, which act as elementary electromagnets. Ferromagnetism is explained by the concept that some species of atoms possess a magnetic moment—that is, that such an atom itself is an elementary electromagnet produced by the motion of electrons about its nucleus and by the spin of its electrons on their own axes. Below the Curie point, atoms that behave as tiny magnets in ferromagnetic materials spontaneously align themselves. They become oriented in the same direction, so that their magnetic fields reinforce each other.

 

One requirement of a ferromagnetic material is that its atoms or ions have permanent magnetic moments. The magnetic moment of an atom comes from its electrons, since the nuclear contribution is negligible. Another requirement for ferromagnetism is some kind of interatomic force that keeps the magnetic moments of many atoms parallel to each other. Without such a force the atoms would be disordered by thermal agitation, the moments of neighbouring atoms would neutralize each other, and the large magnetic moment characteristic of ferromagnetic materials would not exist.

 

There is ample evidence that some atoms or ions have a permanent magnetic moment that may be pictured as a dipole consisting of a positive, or north, pole separated from a negative, or south, pole. In ferromagnets, the large coupling between the atomic magnetic moments leads to some degree of dipole alignment and hence to a net magnetization."

[ffsjoe]

When the iron rusts and iron oxide is formed the oxygen atom is bigger than iron and causes the metal to flake, exposing more of the surface.