Date:         Mon, 18 Jan 1999 13:49:05 -0400
Reply-To:     Tim Smith <smitht@UNB.CA>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Tim Smith <smitht@UNB.CA>
Subject:      corrosion
Content-Type: text/plain; charset="us-ascii"

Hi All,

I keep seeing speculation about corrosion mechanisms in vanagons, based on
different engine/plumbing metals setting up anodes/potential flow loops etc.

Corrosion in the vanagons is a very specific type, yup galvanic like they
all are, but induced NOT by different metals but by trapped pockets of
coolant (between gasket and head) that become individual highly localized
little galvanic cells. The anti-corrosion stuff in the coolant gets
depeleted fast then, and we get that nice white alum oxide paste growing.
The low pressure/loading of the gasket against the head allows some
vibration and micromotion too I'll bet, scuffing off the surface finish
(gold coloured cadmium on newer heads I think), and this just aggravates
the problem.  Using a thick soft gasket in a corrosive environment is a
complete no-no, and is mentioned in most books on corrosion.  Bottom line
here is that we have the same metals in our engines as millions of other
companies engines, but we have a badly designed engine. VW screwed up
royally here, take a look at a Subaru head/gasket to see why their wasser
gets 250K miles without leaking a drop, just like diesel Golfs etc.


  You also get corrosion due to such things as dissolved oxygen creating
potential gradients for galvanic action.  This is what happens in our body
seams, where O2 rich water on surface sets up a cell with current flow loop
between rich surface region and O2 depleted inner reaches of the seam.
This is called 'crevice corrosion' and can be found in any text on
corrosion.  The idea of trapped pockets of isolated coolant leading to
corrosion lead to reshaped inner wheel fenders years ago, eliminating those
old rolled and folded edges that just got eaten away in 3 years.

HTH, Tim