Prestone Antifreeze question for 1990 200tq

[Robert Myers]

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[ Picked text/plain from multipart/alternative ]

It's an equilibrium thing, Scott.  At the risk of blowing away those not
familiar with the concepts of equilibrium constants and balancing several
chemical equilibria, Le Chatlier's Principle, and the Law of Mass Action,
here goes:

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.

So?  We are advised to use phosphate free antifreeze.  There are many
phosphate free antifreezes on the market.  Any of them will serve the
purpose of preserving your engine from "phosphate damage".  Which do I
use?   Try "el-cheapo brand".  There is absolutely no reason to demand the
expensive stuff from Audi when Prestone or the like will serve the
identical purpose and do it much less expensively.  Well, I can think of
one reason.  Does the term "anal retentive" ring any bells?  :-)

If switching from one brand of antifreeze to another it might not be a bad
idea to do a flush and refill rather than just topping things off with
another brand.


At 08:16 AM 3/4/2003 -0800, Scott Fisher wrote:


>--- jmcelroy <jmcelroy at metrocast.net> wrote:
> > My 200tq looks like it has green Prestone Antifreeze
> > in it. I called the Audi specialist that worked on
>it
> > for the previous owner, and they say that just about
>
> > all antifreeze now days is compatible with aluminum
> > in cooling systems.
>
>That much is true.  I use fairly generic coolant (and
>distilled water -- it's what they ask for) in the
>Alfas, which have aluminium blocks and steel liners
>(wet sleeved liners) plus brass radiator header tanks.
>  (The 356 just turns up its nose at the whole
>discussion of radiators and coolant and wonders how a
>grandson and nephew could have gone so wrong...:-)
>
> > Any thoughs on this subject?
>
>There's "compatible with aluminum" and there's
>"phosphate-free."  Audi specifies the latter, and I've
>assumed there's a difference, so I have always put
>phosphate-free coolant in my Audis.  I'd love to know
>(at a chemical level) why -- what specifically goes on
>with phosphates in an Audi engine that makes it a Bad
>Thing?
>
>My very-partially-educated guess involves an
>understanding of the negative effects of even fairly
>mild acids on aluminum -- you're not even supposed to
>cook tomato-based sauces in bare aluminum pans, for
>instance.  So I've always guessed that some chemical
>reaction with phosphate-based antifreeze under heat
>and pressure can result in the formation of phosphoric
>acid, which would attack the aluminum fairly rapidly.
>
>But as I say, that's only a very partially educated
>guess.  Who's our resident chemist again -- Bob Myers?
>
>Unfortunately, one thing I *can* verify: if you use
>very hard, iron-rich water in an Audi V6 for about a
>year after it develops a slow head-gasket leak, you
>can lose your water pump due to lack of lubrication
>and the presence of a reddish-colored sludge in your
>cooling system.  Even if you don't warp the heads, the
>repair costs more than most of the "interesting" cars
>I've owned.
>
>--Scott Fisher
>   Tualatin, Oregon (red iron-rich soil)
>
>
>
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____

I've been wondering... Who would Jesus bomb?

Bob
*****
  Robert L. Myers  304-574-2372
  Rt. 4, Box 57, Fayetteville, WV 25840 USA  WV tag Q SHIP
  '95 urS6 Cashmere Grey - der Wunderwagen   ICQ 22170244
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