Re: Antifreeze and phosphate

[rmyers@olie.wvitcoe.wvnet.edu (Robert Myers)]

>Jokes about tasting antifreeze aside, my understanding is that phosphates
>in the antifreeze eventually tend to build up deposits in the cooling
>system. This was the explanation given to me by technical support
>people at Prestone and also at Mercedes Benz. MB also specifies a 
>phosphate-free coolant for all their cars. Seems like this phosphate-free
>requirement comes from German car manufacturers only. I haven't seen
>this requirement from US or Japanese auto makers. I don't know if the
>alloys used in the manufacture of Audi/MB engine head/block react adversely
>with phosphates or what. Or it may be that phosphates combined with
>hard tap water used to dilute the coolant will cause deposits in any engine,
>but this is an issue only in high mileage cars (and we know Audi and MB
>engines easily last 200 to 300K miles).

Hi Y'all,

Right on, Zafer.

Chemistry lesson for the day:  (He sez with an embarrassed grin on his
homely face)  Aluminum blocks, etc., will tend to corrode, to some extent,
no matter what you do to your car.  This corrosion will result in the
formation of Al+3 ions in the cooling system.  (OK - no big deal - it's just
a normal long-term, slow, and not very sigificant, process which will
happen.  Period.)  

Al+3, in the presence of PO4-3 (phosphate), however, forms AlPO4, aluminum
phosphate, which has solubility properties similar to granite.  If phosphate
is present in the cooling system, AlPO4 will be continually forming as the
Al block corrodes.  Deposits of AlPO4 will form whereever the reaction takes
place.  If left to go on long enough these deposits can block passages in
the radiator and limit the surface area through which heat can be
dissipated.  The deposits are also less conductive than the radiator itself
which further reduces the effectiveness of the radiator by reducing the rate
of heat flow even through portions of the radiator which are still open
enough for collant to pass through.  Eventually, the radiator will need to
be reoved and either replaced with a new one or cleaned and replaced after
cleaning.  I don't have information on cleaning AlPO4 deposits from
radiators.  It may work OK and then again...

An additional effect of the phosphate will be to reduce the concentration of
Al+3 ions in the coolant.  The presence of Al+3 ions will tend to reduce the
rate of the corrosion.  This is a benefit in that this will prolong the life
of aluminum parts.  But if Al+3 ions are removed as AlPO4 this corrosion
rate will be accelerated somewhat.  This will decrease (a bit) the life of
the aluminum block.  This probably will not be a major consideration.

Wby do Audi and MB specify phosphate free antifreeze while Murican
manufactures don't.  Well, duh.  I don't know fer sure, but does the phrase
"planned obsolescence" mean anything to you?  Perhaps the concept of
hi-mileage needs examination.  I remember in the long ago days of my
misspent youth that, in reference to Murican autos, high mileage meant
anything over 50K miles.  In that time, the AlPO4 deposits won't even get a
good start.

Here ends the chemistry lesson for the day.  You may carry on carrying on.  :-)

Bob
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