On 21/11/2009 16:54, dwight elvey wrote:
It isn't quite true that pure water won't
cause corrosion.
It is actually slightly acidic. I recall reading about
a large cosmic ray detector ( large pond of water ) where
the water was ultra pure. The metal pieces would often
just disolve.
That surprises me. Pure water, H2O, does partly dissociate, but in
doing so it forms an exactly equal number of OH- hydroxyl ions and H
+ hydrogen ions -- the latter being what makes an acid acidic, and
the former being what makes an alkali alkaline. Pure water, in
fact, is virtually the definition of pH 7.0, which is absolutely
neutral. I suspect what's actually happened is that dissolved
gasses made the water acidic, but then it's not truly pure. For
example, dissolving CO2 in water makes carbonic acid.
Water is often termed the ultimate solvent. H2O (the molecule) has
some interesting electrostatic properties that allows it to interact
with a lot of different materials (including most metals). It makes
it appear as if it has some acidic properties but it's not (ie it's
not what one would consider a chemical reaction...the dissolved
material hasn't chemically reacted with the water).
TTFN - Guy