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Science Forum Index » Materials Forum » Reactivity of Aluminium
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| Nima |
 Posted: Sun Sep 21, 2003 6:34 am |
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Guest
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Dear All,
I was wondering if anyone here can describe the difference between the
reactivity of Aluminium and Iron when placed in dilute hydrochloric
acid. I understand that Al is higher on the reactivity series than Fe
and it should theoretically react more vigorously. I also know that
the inert oxide layer that aluminium forms is the factor that governs
this but I need a more substantial answer to the question. Can anyone
help please.
Many thanks
NJ |
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| Uncle Al |
| Posted: Sun Sep 21, 2003 10:41 am |
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Guest
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Nima wrote:
Quote:
Dear All,
I was wondering if anyone here can describe the difference between the
reactivity of Aluminium and Iron when placed in dilute hydrochloric
acid. I understand that Al is higher on the reactivity series than Fe
and it should theoretically react more vigorously. I also know that
the inert oxide layer that aluminium forms is the factor that governs
this but I need a more substantial answer to the question. Can anyone
help please.
Aluminum will disintegrate from rapid air oxidation if it is not
allowed to form a protective oxide surface barrier (e.g., by
amalgamation of a clean surface). Iron is stable to oxygen but not to
wet oxygen. Iron as such does not form a protective surface oxide
barrier, and it is very clever about setting up voltaic couples that
cause local pitting.
When yo put iron into dilute HCl it reacts nicely, reducing free
protons to H2 and itself being oxidized. When you put aluminum into
dilute HCl is mostly doesn't do anything until the surface oxide is
breached. After that, it reacts vigorously as strongly reducing active
metal
Not that alloying can drastically change chemistry (e.g., stainless
steels with their protective adherent spinel oxide surface coats).
Aluminum foil contains silicon, manganese, and iron among other
things.
--
Uncle Al
http://www.mazepath.com/uncleal/
(Toxic URL! Unsafe for children and most mammals)
"Quis custodiet ipsos custodes?" The Net! |
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| Brendan Hall |
| Posted: Mon Sep 22, 2003 7:59 pm |
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Guest
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"Uncle Al" <UncleAl0@hate.spam.net> wrote in message
news:3F6DD4AC.A5E0C713@hate.spam.net...
Quote: Nima wrote:
Dear All,
I was wondering if anyone here can describe the difference between the
reactivity of Aluminium and Iron when placed in dilute hydrochloric
acid. I understand that Al is higher on the reactivity series than Fe
and it should theoretically react more vigorously. I also know that
the inert oxide layer that aluminium forms is the factor that governs
this but I need a more substantial answer to the question. Can anyone
help please.
Aluminum will disintegrate from rapid air oxidation if it is not
allowed to form a protective oxide surface barrier (e.g., by
amalgamation of a clean surface). Iron is stable to oxygen but not to
wet oxygen. Iron as such does not form a protective surface oxide
barrier, and it is very clever about setting up voltaic couples that
cause local pitting.
When yo put iron into dilute HCl it reacts nicely, reducing free
protons to H2 and itself being oxidized. When you put aluminum into
dilute HCl is mostly doesn't do anything until the surface oxide is
breached. After that, it reacts vigorously as strongly reducing active
metal
Not that alloying can drastically change chemistry (e.g., stainless
steels with their protective adherent spinel oxide surface coats).
Aluminum foil contains silicon, manganese, and iron among other
things.
More things to add, as suggested by the literature:
Aluminium releases a hugh amount of energy when reacting with oxygen. This
may raise the local temperature and therefore increase reaction rates.
The oxide of aluminium is passivating at low temperature. Its formation is
primarily electro-chemical, with the thickness of the amorphous layer
limited by the tunnelling distance of electrons. Neither oxygen nor
aluminium can diffuse through the layer in any quantity, preventing further
oxide reaction from occurring. At temperatures greater than ~450C, a gamma
oxide forms, which grows to a considerably larger thickness, and the oxygen
and aluminium have to have reacted somehow. The literature is unsure as to
how the amorphous layer is breached in this instance.
The amorphous oxide layer is susceptible to sodium, which is why sacrificial
annodes are typically used in marine applications. It is also susceptible
to forming hydroxide layers, especially when metallic impurities are present
in the alloy. Hydroxide layers are not as protective as oxide layers. I am
not entirely sure how HCl will affect a hydroxide layer.
Brendan |
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