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topic 10593

Black iron



Can someone tell me what black iron is? I have black iron marine tanks in my boat that I want to coat with an epoxy. The tanks have rusted inside a bit. So I want to clean off the rust and then epoxy to prevent future rust. Someone has told me that epoxy won't work because the tanks ( 26 years old ) have soaked in diesel fuel and therefore will not bind to the epoxy.

So my question is.. can liquid soak into a metal?!

Cheers and thanks.

David Balme
- Mississauga, On, Canada


I can help with part of your problem. "Black iron" means steel that is painted black, as in "black iron pipe" that is used for gas lines.

Liquid can not soak into a metal, but it can soak into the porosity of the surface layers of a metal. It is possible to adequately remove diesel oil or any other oil--plating shops do it a hundred times a day--but in practicality it might involve removing the tank and immersing it in a hot caustic cleaning solution.

If you are putting this epoxy on the interior of the tanks, I'd be concerned about whether it can stand up to diesel fuel, and I'd be careful to keep my fuel filters in place to catch any particles that flake off.

Ted Mooney, Teds signature
Ted Mooney, P.E.
Pine Beach, New Jersey

February 5, 2008

Q. I am restoring an old fiberglass sail boat. The previous owner says he used black iron bolts to attach the cast iron keel. I am unable to find anything about that kind of bolt and am concerned about the tensile strength. Your comments, please.

Roy Tate
Sail boater - Hampton, Virginia

February 5, 2008

A. It's just slang and as previously mentioned, just means steel that is black as opposed to plated, galvanized, or otherwise bright. You can tell nothing about the strength from this, but if you can see the heads of the bolts you may see symbols which indicate the strength.

Ted Mooney,
Teds signature
Ted Mooney, P.E.
Pine Beach, New Jersey

January 29, 2015

Q. My previous question boils down to this: I can probably use steel machine screws but what finish provides the best protection from galvanic corrosion occurring with the cast iron keel and will have some exposure to sea water. Finishes to consider are: none, zinc plating or galvanizing.

Roy Tate [returning]
Sail boater - Hampton, Virginia USA

February 2015

A. Hi again. Galvanizing is the best finish for the screws, but painting the exposed part of the screws, or the keel, or both, will increase their life because the zinc coating on the screws will be sacrificial to the presumably much larger keel. Zinc anodes would be a good idea.


Ted Mooney,
Teds signature
Ted Mooney, P.E.
Pine Beach, New Jersey

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