I got this off the Porsche list, and I thought it was a pretty good
disertation on rust and "proper" antifreeze, both of which are irrelevant to
me because I live in California and have only air-cooled cars!

BLenoble@aol.com coughed up:
> From porschephiles-request@tta.com Thu Jun  8 10:37:24 1995
> From: BLenoble@aol.com
> Date: Thu, 8 Jun 1995 13:16:45 -0400
> Message-Id: <950608131644_89843887@aol.com>
> To: porschephiles@tta.com
> Subject: Distilled vs. Tap H20, chemical explanation.
> 
> 
> >From my brother the Chemist:
> 
> 
> >I decided to do a little experiment. I polished a small sheet of 
> >aluminum, washed it with a solvent, and cut off 4 strips. I placed
> >each strip in identical jars which had been cleaned in a laboratory. 
> >I filled each jar as follows:
> >
> >1. 100% distilled water
> >2. 100% tap water
> >3. 100% Prestone
> >4. 50% Prestone, 50% distilled water 
> >
> >That was May 19th. Today, here are the results so far:
> >
> >1. White precipitate (flakes) coating the top surface of the
> >aluminum.   (looks almost like 1 mm specs of dandruff) There are 
> >also flakes settled  at the bottom of the jar.
> > 
> >2. About 1/4 the precipitate of jar 1. The flakes are much smaller,
> >almost  making the water look cloudy rather looking like they are in
> >suspension.
> >   Also visible are small gray specs of corrosion on the aluminum
> >(not  visible in jar 1 perhaps due to the thick coating of flakes).
> > 
> >3 & 4. No discernable precipitate or cloudiness. Aluminum surface
> >still as satiny as when first polished.
> >
> >So jars 3 & 4 proves what we all know already, that antifreeze
> >prevents corrosion. Only time will tell whether my use of an 
> >antifreeze which probably has phosphates will make any difference. 
> >Unfortunately, I don't have phosphate-free antifreeze to use for a 5th jar.
> >
> >However, jars 1 and 2 behave the opposite as I would have expected.
> >Any chemists out there to explain why using distilled water produced more
> >precipitate than tap water?
> > 
> >Joe '86 944
> >
> 
> 
> This is actually an interesting question from a chemistry
>           point of view. Being an analytical chemist and not a
>           inorganic chemist this is not my direct forte. I will give
>           my attempt at an explanation.
> 
>           Here goes; First we need to think about what corrosion is to
>           understand the chemistry involved. Corrosion, as used in
>           this E-Mail, would be the oxidation of metals to form metal
>           salts and  precipitates. Different metals will oxidize to
>           form different metal salts. Rust is the classic oxidation
>           product of Iron.
> 
>           For those who forgot their general chemistry oxidation is
>           the loss of electrons. This is promoted by what is called a
>           oxidizing agent. An oxidizing agent is something that wants
>           an electron or is electron deficient. Chlorine (Cl) is one
>           of the best oxidizing agents there is. Cl is one electron
>           short of a full electron state.  So it will take an electron
>           from anywhere it can.
> 
>           Next we must look at what metals come in contact with the
>           coolant to figure out what things may be corroded. In most
>           cars that would be Iron (Fe), Aluminum (Al), and steel
>           (which is a blend of Fe, Carbon, and other metals, like
>           Nickel (Ni) and Chromium (Cr). Any of these can be oxidized.
> 
>           Al is easily oxidized, more so then Fe or steel. The reason
>           that Al looks dull is the surface is oxidized and this
>           prevents further oxidation. If you polish Al it will oxidize
>           immediately and if you continue to polish it will continue
>           to oxidize.
> 
>           Now to the heart of the matter. Why did they Al in distilled
>           water oxidize more then in tap? The answer is that distilled
>           water is more reactive then tap water. This is because in
>           distilled water you need to think of the water as a lose
>           association of H+ and 2O- ions. These ions can react or
>           oxidize metals. Tap water has other ions like (Fe3+, Mg3+,
>           Fe2+, Ca2+, Cl-, F-).  These ions will form lose
>           associations with the H+ and the O- and make the water less
>           reactive.
> 
>           The bottom line is that you should use antifreeze and tap
>           water as your coolant. Antifreeze is designed to help
>           prevent corrosion. At some other time I can talk about what
>           is in antifreeze and how it works.
> 
>           Send your next chem question at Rod.Lenoble@Hauser.com
> 
> Barry Lenoble
> Blenoble@aol.com
> 89 944 Turbo, Alpine White
> 
> 

--
Thom Fitzpatrick
tfab@netcom.com