Date:         Sat, 1 May 2004 15:50:06 +1200
Reply-To:     Andrew Grebneff <andrew.grebneff@STONEBOW.OTAGO.AC.NZ>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Andrew Grebneff <andrew.grebneff@STONEBOW.OTAGO.AC.NZ>
Subject:      Re: for the record water is not corrosive
In-Reply-To:  <Pine.BSF.4.58.0404301035390.9892@gull.us>
Content-type: text/plain; format=flowed; charset=us-ascii

>On Sat, 1 May 2004, Andrew Grebneff wrote:
>
>>  Cavitation produces, I think, vacuum "bubbles". The water surface can
>>  then return at speed & hit the impeller, thereby eroding (not
>>  corroding) it. This also pits boat props.
>
>If I remember right, cavitation bubbles are bubbles of water vapor.  It
>happens when the local pressure drops so far that the water "boils" at
>room temperature.  You're correct that the corrosion is caused by the
>mechanical impact when the bubbles suddenly collapse.

OK, they will contain water vapor, but the actual open space must be
produced by acceleration of the leading water exceeding the water's
ability to hang togethrer (cohesion), causing it to separate,
producing pockets containing nothing ie a vacuum... which will cause
water to rapidly evaporate from the pocket walls, thereby filling the
spaces with vapor.

The mechanical wear caused by this is corrasion (little-known term
for this physical process) rather than corrosion (chemical process).
--
Andrew Grebneff
Dunedin
New Zealand
<andrew.grebneff@stonebow.otago.ac.nz>
Seashell, Macintosh, VW/Toyota van nut