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Galvanic Corrosion Question
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– Colonel –
science forum beginner


Joined: 04 Apr 2006
Posts: 4

PostPosted: Tue Apr 04, 2006 8:16 pm    Post subject: Galvanic Corrosion Question Reply with quote

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 lower 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 140F
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.
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Oscar Lanzi III
science forum Guru Wannabe


Joined: 30 Apr 2005
Posts: 176

PostPosted: Wed Apr 05, 2006 12:26 am    Post subject: Re: Galvanic Corrosion Question Reply with quote

Your storage environment could still be active, at least towards the
zinc. A little moisture from the air goes a long way on a
nonpassivating metal like zinc. Thus, although the aluminum itself
won't corrode during storage, you will slowly lose your sacrificial zinc
anodes. This is not to say that I can think of anything better, only
that nothing is perfect unless, perhaps, it's ridiculously expensive.

I would assume that it's cheaper to maintain the zinc than to replace
the cast iron. And you're on the right track getting rid of the
chloride-rich solution, which is aggressive towards zinc, after each
use. But even so, check your anodes periodically. You may find that a
periodic replacement of the anodes is still necessary.

--OL
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– Colonel –
science forum beginner


Joined: 04 Apr 2006
Posts: 4

PostPosted: Wed Apr 05, 2006 11:00 am    Post subject: Re: Galvanic Corrosion Question Reply with quote

On 2006-04-04 20:26:33 -0400, ol3@webtv.net (Oscar Lanzi III) said:

Quote:
Your storage environment could still be active, at least towards the
zinc. A little moisture from the air goes a long way on a
nonpassivating metal like zinc. Thus, although the aluminum itself
won't corrode during storage, you will slowly lose your sacrificial zinc
anodes. This is not to say that I can think of anything better, only
that nothing is perfect unless, perhaps, it's ridiculously expensive.

I would assume that it's cheaper to maintain the zinc than to replace
the cast iron. And you're on the right track getting rid of the
chloride-rich solution, which is aggressive towards zinc, after each
use. But even so, check your anodes periodically. You may find that a
periodic replacement of the anodes is still necessary.

--OL

Thank you for your reply, Oscar.

I should have cross-posted this question in the first place to avoid
all this cutting and pasting, but in the newsgroup sci.engr.metallurgy
I found someone who agreed with you on the corrosion issue, but who was
concerned about flame erosion of the aluminum.

I'll re-post his reply below simply for the sake of Deja News archival
purposes, in case anyone else in the future is researching the same
question.

Thanks again.

-Col.

-- PASTED TEXT FOLLOWS --

From: andreas@computer.org (Harry Andreas)
Newsgroups: sci.engr.metallurgy
Subject: Re: Aluminum Exhaust Manifolds?
Date: Tue, 04 Apr 2006 17:43:52 -0700

In article <2006040416073116807-nobody@verizonnet>,
=?UTF-8?B?4oCTIENvbG9uZWwg4oCT?= <nobody@verizon.net> wrote:

Quote:
On 2006-04-04 11:53:11 -0400, andreas@computer.org (Harry Andreas) said:

In article <2006040312342216807-nobody@verizonnet>,
=?UTF-8?B?4oCTIENvbG9uZWwg4oCT?= <nobody@verizon.net> wrote:

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 lower 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 140F
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.

Using aluminum as an exhaust manifold?
Corrosion-wise I wouldn't have a problem with the plan, but I question
whether aluminum will take the heat.

Henry,
Thanks for your reply.
My engine runs very cool, just 140F. I guess 140F is also the
temperature of the exhaust manifolds since you can put your hands on
the exhaust manifolds and hold them there for quite a while without
pain, unlike with a car exhaust manifold which would burn you instantly.
I know they use aluminum manifolds in freshwater-cooled marine engines,
which run quite a bit hotter (160-190F) than mine, yet they have no
problem with that application.
In other words, galvanic corrosion and/or chloride pitting (is that
different?) seem(s) to be the main objection(s) from the naysayers.
Having heard that, are you still concerned about the heat (and I
suspect, flame erosion) in this application?

(Note: it's Harry, not Henry)
The 140F you quote is the coolant temperature as measured by in in situ
thermometer, unless you have a different setup than I've seen before.
(I have a good friend who has a Chevy 454 V8 with a V drive)

The surface temperature of the manifold is lower due to heat transfer
gradients, which is why you can keep you hand in place.

However, the last time I checked the Exhaust Gas Temp (EGT) of a V8
was between 1100 and 1200 degrees F depending on the compression
ratio, which makes flame cutting of an aluminum manifold a real concern
to me. Melt temp of A356 aluminum (a common casting alloy) is around
1050F, and the annealing temperature is only 650F.
Maybe this manifold maker is using a ceramic coating inside the exhaust
passages. That would help.

There is not much of a galvanic potential difference between aluminum
and cast iron (the usual head material of a chevy small block), about -
..7 volts for cast iron and - .8 volts for aluminum, so it would take a
long time for any galvanic issues to develop in sea water. Especially
with a
cathodic protection plug.
The bolts holding the whole thing together (alloy steel) probably have
a higher galvanic impact than the aluminum/cast iron interfaces.

Moderate (sea)water flow will prevent aluminum pitting for the most
part. I would not worry about that. Pitting usually occurs in stagnant
conditions.

Due to the temperature question, I'd personally feel safer with cast
iron, unless the manifold maker is offering a warantee for at least 6
years (which I doubt), or he has a good engineering story.

BTW, I suggest reading SAE report J1781/HIR 1694 MATERIALS FOR FLUID
SYSTEMS OF MARINE VEHICLES and maybe
SAE HIR 1528A SEAWATER SYSTEM DESIGN CRITERIA FOR ADVANCED MARINE
VEHICLES
and
SAE J1777/HIR1063 GENERAL ENVIRONMENTAL CONSIDERATIONS FOR MARINE VEHICLES

All are good reading.

--
Harry Andreas
Engineering raconteur
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