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passivation (1 Viewer)
- Thread starter Farmerism
- Start date
well "passivating metals" by definition have an inert adherent coating....but im gathering that you want to know WHY metals such as Al, Cr, Ti form the inert coating, and K, NA, Fe etc do not......good question that.....???
kazan
im a herooo
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- May 26, 2005
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- HSC
- 2006
when a passivationg metal is oxidised, in this case i'll use aluminium, the product formed in AlO, which is insoluble, and doesn't react with the surrounding enviroment. because it will happen at each area where the Al comes into contact with the water/oxygen, it will completely cover the substances and prevent further corrosion
im 99% sure that you dont need to know why they do anymore that that in the syllabus.
im 99% sure that you dont need to know why they do anymore that that in the syllabus.
kazan
im a herooo
- Joined
- May 26, 2005
- Messages
- 285
- Gender
- Male
- HSC
- 2006
ahh yes
i rember that word
i rember that word
kazan
re: "when a passivationg metal is oxidised, in this case i'll use aluminium, the product formed in AlO, which is insoluble, and doesn't react with the surrounding enviroment."
as a matter of interest that isnt entirely correct in the case of Al and i would suggest a variety of other metals... you are right though, the oxide layer does certainly protect it from H2O and O2.
My point is that Al can still react with its surrounding enviroment in specific conditions, though for the HSC i doubt we would need to know this (this info may fetch an extra mark though - related relevant chemistry for shipwrecks option if you are talking about Al "tinnies"
)
Al2O3 reacts with Cl- ions, effectively "disabling" the passivating protection - this is why you get pitting in Al objects near the sea.
To prove this, get two pieces of Al(s), sand them down to remove any thick layer of Al2O3 intentionally put there by the supplier of the Al(s) (this also speeds the "start" of the reaction which is slow)
Now take your freshly sanded pieces of Al and place them into 2 seperate beakers. cover one with H2SO4 and the other with reasonably concentrated HCL (hardware store stuff should do) ... wait ten or so minutes and you will see the HCL reacting with the Al once the Cl ions have done their job. The H2SO4 will have done nothing regardless of concentration.
re: "when a passivationg metal is oxidised, in this case i'll use aluminium, the product formed in AlO, which is insoluble, and doesn't react with the surrounding enviroment."
as a matter of interest that isnt entirely correct in the case of Al and i would suggest a variety of other metals... you are right though, the oxide layer does certainly protect it from H2O and O2.
My point is that Al can still react with its surrounding enviroment in specific conditions, though for the HSC i doubt we would need to know this (this info may fetch an extra mark though - related relevant chemistry for shipwrecks option if you are talking about Al "tinnies"
) Al2O3 reacts with Cl- ions, effectively "disabling" the passivating protection - this is why you get pitting in Al objects near the sea.
To prove this, get two pieces of Al(s), sand them down to remove any thick layer of Al2O3 intentionally put there by the supplier of the Al(s) (this also speeds the "start" of the reaction which is slow)
Now take your freshly sanded pieces of Al and place them into 2 seperate beakers. cover one with H2SO4 and the other with reasonably concentrated HCL (hardware store stuff should do) ... wait ten or so minutes and you will see the HCL reacting with the Al once the Cl ions have done their job. The H2SO4 will have done nothing regardless of concentration.
rama_v
Active Member
Many substances will react with oxygen to form an oxide layer, e.g. Iron. However, the oxide layers probably have different permeability due to the different lattice parameters and the arrangement of atoms in the lattice of the metal beneath (for example, iron is body-centred cubic, while metals such as Aluminium have a face-centred cubic structure). I suspect this leads to different surface chemistry which you are asking about - I'm not sure though, I haven't (yet) done surface chem. In any case its beyond the HSC sylabus.Roobs said: