Phosphates in antifreeze

[Robert Myers <rmyers@inetone.net>]

OK troops, lemme lay this one on ya.

Aluminum engines are made of - ta da! - aluminum.  Not so surprising, huh?

Aluminum is a very reactive metal which is well above hydrogen in the
activity series.  This means that a *clean* surface of aluminum will react
vigorously with water to produce aluminum hydroxide and hydrogen gas.
(Witness the behavior of the old-fashioned Drano drain cleaner [which is no
longer readily available].  Anyone remember that stuff?  You put it in
water and it got hot, bubbled vigorously and dissolved the grease deposit
in the drain.)  It is fortunate that aluminum hydroxide formed by the
reaction of aluminum metal with water will dehydrate to produce refractory
aluminum oxide which adheres tightly to the surface of the aluminum metal
beneath the oxide coating.  This coating prevents water and/or oxygen from
touching and reacting with the actual aluminum metal beneath the coating.
This is, in effect, much like a coat of paint for the metal.  (Have you
heard of anodized aluminum?  That's what is meant by the term.  The
aluminum metal has been surface oxidized.)

Aluminum oxide is very insoluble in water except for either strong acid or
strong base solutions.  (Drano contained strong base and aluminum metal.
The base provided the heat by dissolving in water and kept the surface of
the aluminum clean so that it would react with the water to stir the
solution.)  

Aluminum phosphate, which will be formed whenever aluminum ions and
phosphate come together, is *extremely* insoluble.  Even more so than
aluminum oxide.  Phosphate contained in coolant (antifreeze coolant or just
water containing phosphate - it makes no difference) will react _slowly_
with aluminum oxide to form aluminum phosphate which will then form a
sludgy deposit inside the cooling system.  This effectively removes the
oxide coating from the aluminum parts.  This newly cleaned aluminum surface
will now react with water to produce aluminum hydroxide which will then
spontaneously dehydrate, especially in a warm or hot engine, to form a new
aluminum oxide coating on the now slightly eroded aluminum surface.

The process will continue.  Oxide coating reacting with phosphate to form
sludge thus providing a clean aluminum surface to react with water to form
more oxide coating which will then...

The ultimate fate of the aluminum phosphate may be to form deposits in
small passageways inside the cooling system.  It may also remain partially
in suspension and be flushed away during system flushes.  I suspect that
the deposits will be formed and will result in lowered cooling system
efficiency.  How significant this might be I could only guess.

A short term exposure to phosphate should not cause much damage.  A long
term exposure can result in severe pitting and weakening of the aluminum
block and head.  This may not a problem with a car with a three year life
expectancy.  It will be a major concern for a car like an Audi which will
run (in terms of the automobile industry) forever.

This, friends, is why the cooling systems of automobiles with aluminum
engines should not be filled with antifreeze which contains phosphates.

OK, environmentalists (myself included) would prefer that phosphates not be
dumped into the environment where excessive plant growth in surface waters
may result or ground waters may be contaminated.  But, the main reason not
to use antifreeze containing phosphates is to help preserve your aluminum
engine and other water cooled aluminum auto parts.

The reactions involved are these:

Al  +  H2O  ---->  Al(OH)3  + H2		Metal reacting with water

2 Al(OH)3  ---->  Al2O3  +  3 H2O		Dehydration

Al2O3  +  2 PO4---  + 3 H2O  ---->  2 AlPO4  +  6 OH-		Dissolution of oxide
leaving a clean surface of Al

Please pardon my plenteous pontifications.  :-)


___
   Bob
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