Aluminum Exhaust Manifolds?

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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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.
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On 2006-04-04 11:53:11 -0400, snipped-for-privacy@computer.org (Harry Andreas) said:

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? Thanks again for any insights, Col.
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(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.
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On 2006-04-04 20:43:52 -0400, snipped-for-privacy@computer.org (Harry Andreas) said:

Harry, thank you again very much your your detailed reply. This is exactly the kind of info and expertise I was looking for.
(By the way, do you know where I might find those SAE reports you referenced?)
Apparently the aluminum manifolds do have some sort of ceramic coating ... but after reading your post, I think I'm going to back off and go with the iron, and buy Never-Seez in the economy size jar...what the heck, when a day of tuna fishing costs $260 in fuel alone, I suppose $100 per year or so for manifolds is really not that big a deal.
Thanks again.
Col.
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SAE reports can be obtained at any engineering or university library, or online from SAE.
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??? Colonel ??? wrote:

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Get stainless ones.
Aluminum is not a great choice in a marine environment. The problem is that the aluminum is connected to OTHER parts of the engine which are iron.
Yes, you can zinc it, but that's a half-solution and while it will help, it won't stop the problem.
Go to stainless. Its more expensive but it will also last longer.

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The price of stainless manifolds and risers is FUGLY. About triple the price of iron. I'm not sure I have the stomach (or the bank account) for that!
Thanks for the reply and suggestion, though.
What I'd REALLY like to see are some BRONZE manifolds and risers!

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To clear up the BS. I have a set of "Gale Banks" aluminum exhaust manifolds and risers on my larger boat. I bought the manifolds used about eight years ago and had to have one repaired before I could use them. It needed welding where water had set inside over the winter and froze cracking one of the jackets. (You can find "Gale Banks" products by doing a search on the internet).
I don't know how old the manifolds were at the time I bought then but they showed use. I am retired living on the coast, and I trailer this particular boat. It is the one I go off shore with, and it is in saltwater about 45+ days a year.
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Diamond Jim-
Thanks for your reply.
Question: Is your boat raw water cooled or fresh water cooled?
On another newsgroup, I had a guy say that the corrosion issue wouldn't bother him, but the heat issue would. He thought the flame / hot gas coming out the exhaust ports of a V-8 engine might be hot enough to cause flame erosion of the aluminum manifold. Did you see any of that?
Thanks again,
Col.
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said:

manifolds
years
welding
they
particular
45+
Engine is fresh water/antifreeze cooled by a heat exchange system. The exhaust is raw (salt) watered cooled supplied by the jet pump drive. No sign of heat erosion.
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OK, thanks again.
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