Re: '87 5KCSTQW Coolant Question

[Robert Myers <rmyers@wvit.wvnet.edu>]

All right you phosphate-free Audi aficionados.

Aluminum phosphate (AlPO4) is one of the most insoluble substances around.
Phosphate ion (PO4---) in your coolant can react with Aluminum ion (Al+++)
to form AlPO4.  The Al+++ comes, as suggested below, from Al2O3, aluminum
oxide, which  forms normally on the surface of all aluminum exposed to air.
Al2O3 is normally extremely insoluble in water.  Enough so, at least, that
the coolant water is kept physically separated from the aluminum metal parts
by the thin coating of Al2O3 and the metal does not react with the water.
If this oxide coating is removed from the surface of the metallic aluminum,
the aluminum metal is sufficiently reactive with water to react to form
hydrogen gas (H2) and more Al2O3 which then can react with PO4--- to...  and
the process continues.  

It will not be as spectacularly fast as doing more or less the same thing
with a lump of sodium and some water, but the end result is similar.  The
aluminum ends up dissolving in the water.  

Oooooops - there goes the aluminum block and all other aluminum parts
exposed to coolant.

Sorry folks, the retired chemistry professor in me took over.  Back to my
more normal "duh?" state.

At 01:01 PM 7/25/96 -0500, you wrote:
>> 
>> I was just reviewing some records I got with my '87 5KCSTQW a few months 
>> ago. Apparently, a dealer had recently flushed and replaced the coolant 
>> with the phosphate-free coolant as per Audi.
>> 
>> In some work done since I've owned it, I realized that one mechanic 
>> topped it up with regular Prestone and and his ok so did I when I changed 
>> the radiator thermoswitch.
>> 
>> Is there any grave danger to the cooling system in having the mix. I know 
>> it is not recommended but I was wondering if anyone out there has any 
>> first-hand experience or at least knows the science of it all.
>> 
>> Paul Levine
>> '87 5KCSTQW
>> Ft. Collins CO
>
>
>You took the question right off my fingertips.
>
>>From the tone of your post I presume the 'regular' antifreezes 
>don't qualify.  Can other followers confirm?
>
>I was reading a www or faq somewhere yesterday something about
>the phosphates reacting with existing alum oxides (alum rust)
>already present in radiator.  The gist seemed to be that this
>exposed more alum to oxidation (rusting), to which the phosphates
>would react with, exposing more alum to oxid, and so on.  The 
>point seemed to be you would eventually run out of alum.
>
>Enough from me, let's hear from someone who knows what they
>are talking about.  I'm only 5 days into my Audi audiction.
>
>
>Lowell
>
>
>
___
   Bob
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*  Robert L. Myers         rmyers@wvit.wvnet.edu>  *
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