Philippe,
Look for a better explanation from someone who is more proficient in
this area than I, but here's my .02 worth based on my experience with
marine engines, boats in salt water, and dissimilar metals.
I think there are at least two factors operating here. One is
dissimilar metals. Each metal has a place on the electrolytic scale,
based on its reactivity to other metals. Magnesium is at one end and lead
at the other. Magnesium reacts with everything; lead reacts with almost
nothing. Aluminium is more active than steel. Copper eats aluminum. If
you can find the scale somewhere, perhaps in a HS or Jr HS science book,
you will see what I mean. In any case, you have here a mix of metals, a
copper radiator, iron block, steel pipes, copper head gasket, different
copper in the heater, different steel in the heater valve, different iron
in the head, ad infinitum. The water pump is made of still another
metal, as it its shaft, and the impeller. All these metals are making
electricity and swapping electrons around in an electrolyte (water).
There are chemicals in the antifreeze that are supposed to tame these
raging electrons, but the chemicals wear out with time. That's why they
tell us to change AF every two years-it's not because the AF protection
wears out, but because the anti corrosion stuff does.
So, with all the chemistry brewing away, there is the action of moving
water. Dissolved oxygen in the cooling water is always present in an open
system, such as cooling system. Given time, oxygen can be absorbed into
some metals, such as into the steel pipes in a fire sprinkler system,
where the same drop of water is in the same place in a pipe for 25 years.
The pipe rusts a black rust (not the red rust that we know and love in
our MGs) as the Oxygen is absorbed. But in the cooling system, it's
there and active. The rotation of the impeller tends to churn the water,
making small bubbles. THat's called cavitation, and it's a real problem
in marine propeller design. The breaking, fizzing bubbles actually tear
bits of the metal away. If the metal is already being weakened by the
water chemistry, then the rate of erosion increases significantly.
Thus the damage is probably a combination of the destructive efforts
involved, time, temperature and karma too.
Bob
On Mon, 28 Sep 1998 14:21:55 -0700 (PDT) (21:21 UT) PHILIPPE TUSLER
<TUSLER@mp050.mv.unisys.com> writes:
>>Curious ... Was it a cast-iron or alloy pump?
>
> Safety Fast! ... larry.g.unger@lmco.com
><'61 MGA 1600 MkII
>
> Alloy body with metal impellers, from Moss. Someone has already
>suggested an electrolytic effect, which is certainly what the
>deterioration
>looks like. The question is: "how?" The engine ground strap is in
>place,
>if a little frayed. Is is really routing all that many electrons?
>
>************************************************************************
>*Philippe Tusler - Mission Viejo, CA | "MILOU" '57 MGA Roadster
>*
>* | "TINTIN" '66 MG/MGB-GT
>*
>*InterNet: Philippe.Tusler@Unisys.Com | N/A '88 ISUZU
>Trooper*
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