Date: Fri, 30 Apr 2004 17:21:20 -0400
Reply-To: David Beierl <dbeierl@ATTGLOBAL.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: David Beierl <dbeierl@ATTGLOBAL.NET>
Subject: Head Corrosion cause and cure? was Re: Cooling strategies,
color coded, etc.
In-Reply-To: <1629373.1083299145344.JavaMail.jrunberg@mac.com>
Content-Type: text/plain; charset="us-ascii"; format=flowed
At 12:25 AM 4/30/2004 -0400, John Runberg wrote:
>Something causes vanagon heads to corrode, causing pits and the resulting
>leaks
Ask a corrosion engineer to design a gasket system to promote corrosion and
he'll hand you something like the vanagon head-to-water-jacket
gasket. It's subject to large strains (about 1/64" or .5 mm) as the water
jacket grows and shrinks relative to the iron cylinders, and has a flat lip
that coolant can creep under -- that's not even under external
pressure. When coolant *does* creep under it causes crevice corrosion --
see http://www.corrosion-doctors.org/Localized/Crevice.htm -- and the
corrosion products likely lift the gasket to allow the coolant to creep
farther into the interior of the joint recursively.
>This topic comes up again and again and I haven't read a definative reason
>heads leak.
IMO...they leak because VW built them to leak, in the course of cobbling-up
an external water-cooling system for an air-cooled engine. When they
enclosed the cylinders in an aluminum fish-tank with a seal subject to
differential expansion between iron and aluminum, they signed their (and
our) own death warrants.
>Until I do, I'll follow the directions the manual gives me. Why not?
Either a) better or b) cheaper or c) less hassle with the dealer for
warranty stuff. C doesn't apply and I regard the others as Not Proven,
which in Scotland means roughly "Not Guilty and don't do it again"
(apologies to what's-her-name the mystery author).
I'm not aware that anyone has come up with a procedure demonstrably better
than the specified one (I'm also not aware of anyone who's paid a corrosion
engineer to consult on the problem). *Most* of the argument seems to be
motivated by desire to avoid paying factory ransom for the coolant. The
BASF engineer I spoke to about it years ago said "We make the stuff for
VWAG -- we don't think the no-phosphate spec is necessary but we make it
the way they want it" or words to that effect. It has been often alleged
that VW's reason is that much European water is highly mineralized and
would precipitate out with phosphates -- seems plausible. Also seems
plausible to me that the VW engineers may have specified their alloys to
meet some arcane desideratum and depended on owners to maintain the system
as specified, something more likely in Germany than here. But their gasket
design is so awful that ISTM it hardly matters.
Suggestion for a cure (are you listening, Stan?) -- develop a ceramic
coatindcēr the seal area on both head and water-jacket, so that there's
nothing there to corrode. It would have to extend far enough that its edge
would be clear of any sealant applied at installation time, and to give a
long path for any corrosion that does occur at its edges. If the coating
is impervious and isn't subject to edge-peel that should take care of it
nicely -- I think. Is there a corrosion engineer in the house?
I suspect the mechanism for head-to-cylinder pitting is also crevice
corrosion, but harder to cure. Press-fitted gaskets? Square
O-rings? Beats me. Seems like a much lesser problem though.
david
