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 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:
- 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.
- 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.
- 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.