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Topic: Why iron rusts but aluminium doesn't  (Read 19459 times)

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Offline thetada

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Why iron rusts but aluminium doesn't
« on: September 18, 2011, 06:28:19 AM »
Hello everyone,

This is my first time post. My name is Tom and I'm a highschool chemistry teacher and also a science writer. I mention this because I wasn't completely sure where to publish this post. Anyway, I have a batchelors in pure chemistry so that's the level that I'm supposed to be able to comprehend. I'm looking for help to understand something that I need to explain in an article. It needs to be explained to a highschool chemistry level audience but I want to understand it at a higher level so that my explanation is good.

Basically, the article is about aluminium and I'm interested in the contrast between aluminium and iron in the matter of corrosion. The simple explanation is that aluminium doesn't corrode because it quickly forms an oxide layer which adheres to the surface of the metal and thereby prevents further corrosion. The next simple explanation is that iron goes on rusting right through because its oxide layer does not adhere to the surface of the metal but rather flakes away, thereby exposing fresh metal to continued corrosion.

These explanations feel like an over-simplification. The process of rusting is obviously very complex and I've read in certain places that iron oxide adheres to iron with comparative strength to the bond between aluminium and its oxide. What is further suggested (by Wikipedia, which I don't hold up as a bastion of accuracy), is that in fact it is often the acidity of the water that bridges the two halves of the electrochemical cell that promotes rusting. The process is said to produce iron hydroxide ions. These are said to be soluble in water, unlike plain iron oxide, which is not. What is being suggested here is that the difference between the aluminium and iron oxides is not that they adhere with markedly different strengths to their respective metals but that iron oxide is more prone to dissolving in acid and forming the species iron hydroxide, which acts as an intermediary in the overall corrosion process.

Another thing on which I'm seeking clarity is the fact that aluminium does corrode by the galvanic or dissimilar metal process. In these cases, aluminium sets up a galvanic cell via an adjacent and less reactive metal and the standard salt water bridge. What intrigues me about this is the implication that another metal is required for the oxidation process to proceed whereas iron can set up a galvanic cell without the presence of a second, dissimilar metal. In the latter case, I have read that iron plays the anode while oxygen gas serves as the cathode.

So, my questions are:
1) Is it that iron oxide flakes away exposing fresh metal to further electrolytic corrosion or is it that ferrous oxide is more prone to dissolving in acids and producing species which facilitate further oxidation?
2) If aluminium oxide does adhere more tenaciously to aluminium than iron to its own oxide, why is that?
3) Basically, why is it that iron will go on and on rusting when aluminium stops after the formation of a thin layer?

Many thanks in advance for any help rendered.

Tom






Offline Borek

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Re: Why iron rusts but aluminium doesn't
« Reply #1 on: September 18, 2011, 07:27:39 AM »
Another thing on which I'm seeking clarity is the fact that aluminium does corrode by the galvanic or dissimilar metal process. In these cases, aluminium sets up a galvanic cell via an adjacent and less reactive metal and the standard salt water bridge. What intrigues me about this is the implication that another metal is required for the oxidation process to proceed whereas iron can set up a galvanic cell without the presence of a second, dissimilar metal. In the latter case, I have read that iron plays the anode while oxygen gas serves as the cathode.

I guess this one is easier to answer. Iron does corrode by galvanic cells as well. However, it corrodes so fast on its own (that is, in the presence of water/oxygen), that galvanic cells are not important. OTOH aluminum doesn't corrode in the presence of water/oxygen (all possible disclaimers go here), so galvanic cells are much more dangerous.
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Offline thetada

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Re: Why iron rusts but aluminium doesn't
« Reply #2 on: September 18, 2011, 07:45:52 AM »
Thanks for your reply Borek,

Quote
I guess this one is easier to answer. Iron does corrode by galvanic cells as well. However, it corrodes so fast on its own (that is, in the presence of water/oxygen), that galvanic cells are not important. OTOH aluminum doesn't corrode in the presence of water/oxygen (all possible disclaimers go here), so galvanic cells are much more dangerous.


In addition to your response, I've also found some helpful information here:

http://aluminium.matter.org.uk/content/html/eng/default.asp?catid=178&pageid=2144416644

The page includes Pourbaix diagrams and all sorts. I think ultimately to give a detailed explanation of the related phenomena is way above the level of a high school student (and probably mine too). However, from what I can tell, I think it's reasonable to say the following:

In fact, aluminium and iron both have the potential to oxidise and thereby corrode in a fairly similar fashion. The difference is that there is a far broader range of pHs and electrical potentials at which iron can experience corrosive oxidation.

Does that sound reasonable?

Offline Enthalpy

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Re: Why iron rusts but aluminium doesn't
« Reply #3 on: October 14, 2011, 06:50:57 PM »
Different point of view, as a user of alloys, not as a chemist...

Some iron alloys corrode, others don't, still others corrode very slowly despite containing no chrome at all.
Some aluminium alloys corrode very quickly, much faster than common steel. Others don't.
So: only steel rusts because rust is an iron oxide.

Examples:
  • I had put a D=20mm rod of Al-Zn8-Mg1-Cu in a normal soil in tempered Europe. After two days (!) the rod lost 1mm thick white shells.
  • Ship hulls, of banal carbon steel, lose about 20mm thickness after 10-20 years in sea water. To have an aluminium hull resist sea water for over a month, you must choose the alloy very carefully - which is quickly done, since only two alloys fit.
  • I had put aluminium wrap foil (so-called tinfoil) in water to observe the corrosion rate. It's rather pure aluminium, like 99%. The 30┬Ám were punched through within a week. One piece was in contact with a copper wire, which changed absolutely nothing. Corrosion couples is a legend as well, sorry folks.
  • But Fe-Co50 (very pure ferromagnetic material) resists rainwater for weeks, as does Fe-Ni18-Co8-Mo5-Ti, with but a faint surface coloration.

So the difference between iron and aluminium needs no explanation, because there is none.

Steel can be stainless, usually if it contains enough Cr, though this depends on many other factors.
Aluminium can resist corrosion, usually if it contains mainly Mg, or mainly Mg2Si. Pure alloys or Al-Mn alloys are less good.

Sure, people have proposed explanations, so many ones and so conditional that they're nearly useless. The most often cited one relates with an oxide layer hermetic to oxygen transport, which would require an oxide bulky enough that it's compressed when produced.

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