>Date: Fri, 12 Dec 1997 05:34:39 -0400
>To:vanagon@lenti.med.umn.edu
>From:kelphoto@bright.net (mark keller)
>Subject:Head Corrosion prevention Discussion
>
>Well,
>
>I am still in search of data that explains why some heads are replaced
>because of corrosion,and other's have no leaking problems what so ever.
>This differance, suggest to me a variable exists.
>
>  I reinspected the "non-leaking" heads which my local VW dealer has. The
>corrosion and pitting are limited to the contact area of the outside
>perimeter seal. The corresion is very consistent both around the seal
>contact and "across the seal" contact. An area of corrosion was present
>exactly beneath the seal, not just inside the, where coolant and seal
>meet, but the complete width of contact area. There were about five pitted
>craters, .1mm deep x .2mm across, in random areas. On one head the
>corrosion seemed wider, but not any more advanced, in the region from the
>water hose nipple, as opposed to the side which a flange is bolted to.
>This region is essentially extended from the compression chamber edge
>closesest to the narrow end of the head.
>
>There was no corrosion whatsoever on any areas that had coolant flow. Tim
>Smith posted about "crevice corrosion" being the actual name given to the
>situation where coolant that gets trapped beneath the outer waterjacket
>seal, and then quickly becomes depleted in its anti-corrosive properties,
>and accelerated corresion occurs at the point of contact, in this case,
>beneath the flexiable outer seal.
>
>I still can't come to a conclusion about how the coolant gets beneath the seal.
>
> It would seem that if seal edge is curled up,as indicated by not being
>100% backed up, coolant flow is still presents.
>
>Another possibility, If the head contact "flexes at the seal" between cold
>and hot tempatures, ie the contact point of the head and the seal is
>"rocking" back and forth by virture of expansion, then some coolant could
>be present at the region, of shifted contacted, and there is a trapping of
>coolant.
>
> Another possibility would be in driving conditions where the motor is
>continually in a low operating eniviorment, trips of 6 miles or less and
>or not driven on a daily basis, a kind of "coolant caking" may occur.
>Coolant would cake,thicken, or otherwise get seperated from his buddies,
>and quickly lose anticorrsive strenth, and start muchin on aluminum, aka
>crevice corrosion, and  then remain at a corrosive state, until the engine
>was brought up to operatining temperature and driven for some period of
>time that in essence disolved the cake and then replenshied the
>anti-corrosive properties by allowing fresh coolant to move into the area.
>
>The tech mentioned that re-torquing the head was a precaution he
>practiced, he babbled somthing to the effect that heat cycling, ie hot &
>cold  cycles of normal operation was a factor in reducing the torque of
>the head bolt. I have no input on this. Would sombody email me about it.
>
>My current theory of "hydraulic pressure", generated by the water pump,
>which forces coolant beneath the seal before the engine warms up is still
>a possibility.
>
>At this point, I would like to suggest that anyone interested in helping
>in any way and those having experience to do multivariate regression
>analysis, contact me. The idea would be to get a response questionaire to
>"profile" listees who have had multiple corrosive failures, and those who
>have had no problems. This would move us from theory, to variables which
>correlate to corrosive head failures. I'll post this request separately,
>incase a interested party in helping doesn't read the techno theroy
>pieces.
>

Respectfully,

Mark Keller
91" Carat WE
"Lazarus"