Galvanic Corrosion Question

â€“ Colonel â€“ · science forum beginner Joined: 04 Apr 2006 Posts: 4

Hello all.

I have a question about galvanic corrosion involving aluminum, zinc,
steel and other metals in a saltwater marine environment.

I have a boat with an inboard Chevrolet 350 V-8 engine which is
raw-water-cooled. What that means is that salty seawater is drawn from
outside the boat, it is circulated through the engine, then it exits
the engine through the exhaust manifold, riser and exhaust pipes in
what is known as a "marine wet exhaust."

Raw-water-cooled engines usually have cooler thermostats than
freshwater-cooled motors (which typically have a sealed cooling system
on one side and a heat exchanger on the other), I suppose to help
prevent deposition of salt in the engine. (My thermostat opens at 140°F
and that is pretty much the normal operating temp.)

OK, the exhaust manifolds and risers in marine engines are usually made
from cast iron, and they usually must be replaced every 6 years (parts
cost: $600) to prevent a hole from rusting through between the water
jacket and the exhaust gas passage. If this happens, you can easily
ruin an engine by trying to compress water, or by seizing everything up
with rust.

I'm due to replace my manifolds and risers, and I now have an
opportunity to buy aluminum ones instead of cast-iron ones (although
they're about $750 instead of $600 for iron) and I'm thinking that it
might be a good way to end the cycle of replacing manifolds every six
years.

Some fellow boaters are saying "Don't do it, because aluminum and
saltwater don't mix!" but I'm not so sure that it's such a bad idea.

Here's my reasoning, see whether you agree:

1. Outboard engines are made of aluminum and aluminum alloys, and they
do just fine in saltwater marine environments SO LONG AS THEY HAVE
ACTIVE ZINC ANODES ATTACHED. Cast iron, in contrast, rusts out
regardless of whether or not you have zinc anodes. There's no way to
stop it that I know of.

2. There are threaded plugs in the aluminum manifolds, into which I
would install a brass plug with a zinc anode inside it. This zinc anode
would be in the "wet" part of the exhaust manifold, where immersion in
ionic saltwater would allow galvanic corrosion of the zinc to take
place. Zinc is less "noble" in the galvanic series than aluminum, so as
long as the zinc anode is actively corroding, the aluminum should not
corrode.

3. This boat is stored on a trailer, not in the water, so it wouldn't
be in an "active" galvanic environment all the time. In addition, after
every time I use the boat in saltwater, I flush the engine with
freshwater by using a garden hose. This, I believe, should lengthen the
life of my zinc anodes.

What do you metallurgy experts think? I would appreciate any thoughts
or insights, since I probably know just enough about metallurgy to be
dangerous.

Thank you.

Col.

William P.N. Smith · science forum addict Joined: 23 Nov 2005 Posts: 56

– Colonel – <nobody@verizon.net> wrote:

â€“ Colonel â€“ · science forum beginner Joined: 04 Apr 2006 Posts: 4

On 2006-04-04 16:52:02 -0400, William P.N. Smith
<news2006b@compusmiths.com> said:

William P.N. Smith · science forum addict Joined: 23 Nov 2005 Posts: 56

– Colonel – <nobody@verizon.net> wrote:

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