Oxide layer of iron cannot protect iron

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In summary, the oxide layer of iron is not as effective at protecting against corrosion as the ones on aluminum and zinc due to its porous and varied composition. Rust, which is formed on iron, is easily scraped off and not as structurally strong. In order to resist rust formation, iron must be alloyed with elements like chromium and nickel to create stainless steel. The presence of an aqueous ionic substance, which acts as an electrolyte, can speed up the rusting process by making it easier for electrons to be conducted and for the oxidation reaction to occur. This is due to the transfer of electrons between the iron and oxygen, which results in the formation of iron (III) oxide. The process of oxidation and reduction, in which
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primarygun
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Why the oxide layer of iron cannot protect iron from corrosion like aluminium and zinc?
 
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  • #2
While the corrosion products of aluminum and zinc are dense and impervious (ceramic) oxides, rust on iron, is quite permeable - both to air as well as moisture. The reason the rust does not form a coherent diffusion barrier is, in large part due to the fact that rust typically has 3 layers.

The outer layer, with easy access to oxygen and water, is usually hydrated ferric oxide or even ferric hydroxide. Then there's an intermediate layer of magnetite (Fe3O4), and finally layers of ferric and ferrous oxides. This variation in composition, and the attendant lattice mismatch makes rust quite porous, and hence ineffective as a diffusion barrier.

You may have noticed that it's quite easy to scrape off or chip off rust, but impossible to do the same with aluminum oxide.
 
  • #3
Adding to what Gokul43201 wrote, the hydroxides and oxyhydroxides of Fe are not very stable - they are soluble - nor are they structurally strong.

Iron has to be alloyed to resist rust formation - usually with Cr and Ni - to produce 'stainless steels'. The Cr (primarily) and Ni form protective oxides.

You may also want to look at the Pilling-Bedworth ratio which is a relationship between the structure of the oxide compared to the metal.

See - http://www.corrosion-doctors.org/HotCorrosion/Pilling.htm [Broken]

or google on "Pilling-Bedworth".
 
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  • #4
How does presence of aqeuous ionic substance speed up the process?
Why does rusting occur on the tip and the tail of an iron nail first?
(shown by potassium hexacyanoferrate)
 
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  • #5
well an aqeuous ionic substance will speed up the rusting process because, for the most part, they are electrolytes (at least the ones you are talking about anyway). they make it easier for electrons to be conducted from substance to substance, speeding up the oxidation reaction, afterall, all oxidation is, is an atom loosing an electron...

leo ger
loose electron OXIDATION
gain electron REDUCTION

depending on what is loosing or gaining the electron, it will gain either a positive or negative charge and then be attracted to the opposite charge.

so in the case of Iron (III) Oxide, the Iron is oxidized since it looses an electron (actually 3 in this case, since it has a positive charge of 3) and gains a positive charge, and the oxygen gain an electron since it has a negative charge, then the two are then attracted together. this whole process is sped up when the electrons can transfer easier, for example in the presence of an electrolyte.
 

1. Why can't the oxide layer of iron protect iron?

The oxide layer of iron, also known as rust, is actually a form of corrosion that weakens and breaks down the iron underneath. While it may provide a temporary barrier against further corrosion, it cannot fully protect the iron due to its porous and flaky nature.

2. How does the oxide layer form on iron?

The oxide layer on iron forms when iron is exposed to oxygen and moisture, a process known as oxidation. This causes the iron atoms to lose electrons and combine with oxygen molecules, creating the red-brown rust that we see on the surface.

3. Can the oxide layer be removed or prevented?

The oxide layer on iron can be removed through various methods such as sanding, chemical treatments, or electrolysis. It can also be prevented by keeping the iron dry and free from exposure to oxygen and moisture.

4. What happens if the oxide layer is removed?

If the oxide layer on iron is completely removed, the iron underneath will be exposed and vulnerable to corrosion. This can lead to further rusting and deterioration of the iron object.

5. Is there a way to strengthen the oxide layer?

While there are some methods that claim to strengthen the oxide layer on iron, such as applying oil or paint, these only provide temporary protection and do not actually strengthen the layer. The best way to prevent corrosion is to keep the iron object in a dry environment and regularly maintain it to remove any rust that may form.

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