Prestone Antifreeze question for 1990 200tq

Livolsi, Stephane Stephane.Livolsi at
Wed Mar 5 09:31:57 EST 2003

You know, when I see posts like this, demonstrating the vast array of
knowledge and expertise that various listers have, I think there is nothing
we can't do if we all got together sometime....

thanks for sharing the info Robert

> Aluminum metal spontaneously reacts with water to form Aluminum hydroxide
> [Al(OH)3] and hydrogen gas.  The only reason that  a block of aluminum in
> contact with water does not do this to any great extent is that the
> Al(OH)3
> spontaneously decomposes in an equilibrium process to form aluminum oxide
> Al2O3.  This forms a quite refractory coating of aluminum oxide on the
> surface of the metal which prevents contact between the aluminum metal and
> the water in much the same way a good coat of paint can protect a piece of
> steel from rusting when exposed to moist air.
> 1.      2 Al  +  3 H2O  -->  2 Al(OH)3  +  3 H2
> 2.      2 Al(OH)3  <==>  Al2O3  +  3 H2O  (with a large Keq)
> also
> 3.      Al(OH)3  <==>  Al+3  +  3 OH-    (with a small Keq thus
> maintaining
> a very low but essentially constant amount of Al+3 ions in solution))
> Now enters the phosphate ion from phosphate containing antifreeze.
> 4.      Al+3  +  PO4-3  <==>  AlPO4   (with a VERY large Keq - aluminum
> phosphate is EXTREMELY insoluble in water)
> This last process tends to lower the Al+3 concentration and this, in turn,
> causing equation 3 above to shift to the right in order to form more Al+3
> ions to keep the system in a proper state of equilibrium.  This, in effect
> then effectively "cleans" the surface of the aluminum block and allows
> more
> of the metal to react with water in an effort to keep everything in
> balance
> as far as the complementary equilibria go.  The end result after a long
> term is that the metal of the aluminum block gets ":eaten away".  This is
> not a particularly desirable result for an aluminum engine.

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