21 Nov
2009
21 Nov
'09
5:16 p.m.
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.
Extreme cases aside, though, even tap water is fit to wash PCBs, so long
as you have a little (very little!) wetting agent to ensure most of it
runs off, and/or shake it or use compressed air to ensure you don't
leave water under ICs and other components. Even quite hard water is OK
if you do that, as the impurities are in tiny concentrations. Using
solvents like alcohols is unnecessarily expensive.
--
Pete Peter Turnbull
Network Manager
University of York