Why the oxide layer of iron cannot protect iron from corrosion like aluminium and zinc?
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.
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
or google on "Pilling-Bedworth".
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)
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....
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.
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