Date:         Sun, 17 Jan 1999 19:37:43 -0500
Reply-To:     Martin Jagersand <jag@CS.YALE.EDU>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Martin Jagersand <jag@CS.YALE.EDU>
Subject:      Re: corrosion & anodes (long)

> Free copper ion can be devastating to aluminum as some saltwater
> sailors found when they painted their aluminum propeller gear casing
> with copper antifouling paint. Copper reacts withthe aluminum, sets up
> a local corrosion cell, and perforates the = aluminum in sea water in
> short order. So don't put any copper pipes inline with your coolant to
> an aluminum engine. (This is why I doubt that your radiator is
> copper.)

During the corrosive process a transport of charge takes place
(through ions). If there is no secondary current path to level
out this charge the two dissimilar metals will build up different
electrical potentials and hinder further ion transport and
corrosion. This is like a battery with no load connected to it.
(However small leak currents are always present so in practice
the corrosion doesn't stop completely)

If there is a secondary current path the condition is a
"shorted" battery, and very rapid ion transport and corrosion
can take place.

Lets look at some of the pracical situations brought up in
other posts:

Aluminum head on cast iron block (eg. VW I4 in Diesel Vanagon):
The two electrodes are shorted through the headbolts. Rapid corrosion
can take place.

Copper radiator and/or copper pipe pieces in the cooling
system: Unless explicitly grounded these are typically
connected through rubber hoses (and the radiator is
mounted with rubber mounts), so there is no direct current
path between the copper components and the aluminum head.
Hence only little/slow corrosion is likely.

At VW shows I've seen many VW Diesel Caddy's where the
expensive VW "octupus" coolant hoses have been replaced
with standard soldered copper plumbing components. The
owners claim they have had no problems. I still wouldn't
reccomend this though.

I've also worked on cars where I know the radiator was copper
(because I fixed it by soldering) and head aluminum (e.g. some
Chrysler K cars and vans).

/Martin
--
Westy 1.9l Turbo Diesel
Quantum 1.6l Turbo Diesel

Martin Jagersand                 email: jag@cs.yale.edu
Computer Science Department             jag@cs.rochester.edu
Yale University

        Slow down and visit the VW diesel Westy page:
          WWW: http://www.cs.rochester.edu/u/jag/vw
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