OT: Gas furnace flue why not galvanized

My house has a old gravity natural gas floor furnace. It has a exhaust gas collector box on the side with a flex connector leading to a transite vertical flue.
The furnace itself is made of galvanized iron but the collector box and the flex flue line is bare iron. The flex burned out and has been replaced and the box really needs to be remade. Is there some reason not to use galvanized?
This box is open on the bottom to allow extra air to be drawn up the chimney so as to reduce the flue gas temperature to keep things safer.
I noticed the wood above the flex line was a little black so good thing I have been down in the crawl space. Of course it lasted 60 years or so. I put some thin aluminum baking pans above the piping to spread out the heat and protect the wood.
Bill D

Bill D said:

Is there some reason not to use galvanized?

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Historically, supply-line side, Sulfur in the fuel gas + Zinc + reaction, and pilot orifices could be too easily plugged, hence black-iron preferred, rust being less aerosol-mobile.

OUTLET side? I've seen plenty of Galvanized flues, PA & WBGVA, and yazz, many with the Zinc gone in a pattern, and LOTS of rust, so... if in doubt, perhaps Stainless might be better yet?

Not likely they'll let us have our old asbestos cement flues back..

We call them draft boxes or draft diverters. If there's a high wind it won't suck the flame off the burners and kill the pilot. Many of them were plain old sheet metal painted black. Go to your local tin knocker and have them fab one.

Because it would cause cancer in the state of Kalifornia....

But no-where else for that matter....

Monarchist said:

OUTLET side? I've seen plenty of Galvanized flues, PA & WBGVA, and yazz, many with the Zinc gone in a pattern, and LOTS of rust, so... if in doubt, perhaps Stainless might be better yet?

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In order of cost: You can use aluminized sheet - it's more temp resistant than galvanized, 409SS straight chrome stainless or 304SS a chrome nickel grade.

Bill D said:

leading to a transite vertical flue.

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Monarchist said:

Not likely they'll let us have our old asbestos cement flues back..

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Me thinks he still has asbestos....