OK,well we are beating this horse to death
My point is that there is no "acidity" to distilled water. Yes, it can dissolve carbon dioxide from the atmosphere if exposed, but this is a very small amount of free H+ unless the partial pressure of the dissolved CO2 is very high. It gets back to the statement you made about your pH measurement showing it as acidic. Measuring the pH of a near-neutral solution (especially distilled water) is nearly impossible and also nearly meaningless. Like a quantum measurement, the very act of attempting to measure the value disturbs the system more than the thing you are trying to measure. The amount of dissociation in distilled water is vanishingly small.
Now carbonated beverages is another thing. Here like you mention, the main aggressor is the phosphoric acid. The carbon dioxide in the solution is under pressure too and contributes, but a whole lot less than that phosphoric acid.
Off topic, I always chuckle at those Facebook posts where people show Coca-Cola cleaning a toilet or something due to it's acidity. They always make the statement about how introducing this horribly acidic substance into your body is somehow near lethal. No doubt it is bad for your teeth (some people more than others), especially if it is allowed to linger in the mouth. But your esophagus is not particularly vulnerable, and once it enters the much more acidic environment of the stomach it quickly becomes a non-issue. Drinking soda through a straw helps a lot to get it past the teeth BTW.
Originally Posted By: y_p_w
Originally Posted By: kschachn
Well any corrosion isn't caused by the water per se. The solubility of metals in water isn't that high in an absolute sense, especially in a closed system. Once the water is saturated it won't dissolve any more. I don't know what the solubility of iron or aluminum is in water, but it isn't very high when compared to the mass of the engine.
Corrosion in a cooling system is due to other factors that I'm not very familiar with. Things like dissimilar (Galvanic) corrosion. Something is acting as an anode and something else as a cathode. The coolant is going to inhibit this corrosion by buffering, that is what you miss out by using straight water (that and the boiling point/freezing point elevation/suppression). There are other methods of corrosion that I'm not very familiar with that I'm sure occur in cooling systems, especially at elevated temperatures.
The point is that it's not the water. The water isn't going to corrode the metals by leaching.
I was never referring to corrosion as a matter of using distilled water in a properly mixed coolant/water mixture. However, there are reports that when poured into an aluminum radiator that the slight acidity is enough to cause a bit of quick precipitation of stuff. The whole point of a radiator flush is to rinse out as much of the original coolant as possible. So by the point it's flushed away there's only going to be bare distilled water. In any case, I never said it was anything severe enough to worry about, but it would still be advisable to mix with coolant first.
Aluminum cans have to be coated to resist the acidity of most carbonated beverages as well as phosphoric acid.