Chimney and hood rust

[Itsnick]

Hello,

I noticed today my side draft hood and chimney have rotted out. It’s too bad, because they’re both only about 1 1/2 years old!

My original plan was to replace with stainless, but I have some concerns. I’ve read in other forums that even stainless (316T being the preferred grade) don’t hold up particularly well to the acid from burning coal? 12” flue pipe is crazy expensive. I’d rather not spend the money if it won’t last. 
 

The other option I’m considering is building a new hood with much thicker plate. (the current one is 16 gauge) but even the 1/4” flat bar around the opening looks closer to 3/16 now…so I’m not sure how long that would last either. I was also considering using thicker walled pipe for the chimney (something like schedule 40) if I could source a piece reasonably. 
 

I’m starting to think I need to treat them both as consumables. Unfortunate because id love an option that’ll last more than a few years. 
 

what would you guys do? 
 

oh, here’s my set up. It’s under cover but only a lean to. Burning bituminous…Quality stuff. I do have a cap (simple U shape bent over the pipe and screwed in place)

 

thanks! 
 

[George N. M.]

OK, this is kind of speculative but the rust and erosion in a coal forge is caused by sulphuric acid which is created when the sulphur impurities in the coal and water combine.  So, the way to minimize the problem is to keep them apart.  If you quench your fire when done use enough water to dilute the acid and flush it cown the ash dump and out.  I suspect that in your case in a fairly humid environment and that your forge is not inside that the sulphur deposited in the soot in the chimney is combining with water in rain or humidity which blows into your lean to.  Dealing with ths is going to be harder.  You could periodically scrape and clean off the soot and gruk inside the chimney and smoke box but that is going to be a dirty and difficult job.  Maybe you could hose it out.  Still messy but maybe easier than doing it dry.  You could also wrap your forge in a tarp when not in use.  And you do have a rain cap on your chimney to keep rain from coming down it?  Right?  You could also seal the chimney when not in use to keep water out.

You could also use a lower sulphur coal but most of the low sulphur coals are crappy at coking and make poor forge fuel.  I know. Most of the coal out here in the West is low sulphur and that is why it is mined for power plants, to reduce sulphur emmissions and acid rain.  You might try to get and burn coke because the sulphur has been driven off in the coking process.  I have burned coke for years and it takes different fire management and can be fussy but it works well.  The biggest difference is that it will go out if you don't keep some blast going to it.  You can go into the house for a bathroom break and get back in time to blow the fire up again but not long enough for lunch.  If I'm working at the bench I will have to turn around and give the blower a couple turns every few minutes.  And I try to do all my forging first and then move onto the bench work rather than going back and forth if I can.

I get my coke at a farrier supply place.

I think I'd try this before trying to address the issue with more expensive and rust resistant materials.

"By hammer and hand all arts do stand."

[Itsnick]

I do have a cap, but being under just a roof as I am, there’s plenty of moisture that still gets to things. I keep my coal in covered storage. 

in my area, coal is FAR less expensive to buy than propane or coke. So much so that replacing my hood/chimney system every few years would still work out financially. 

I’ve read that even stainless class A chimney pipe would also suffer from burning coal as we do. To what extent I am unsure. Do you find this to be true? If it’s a battle between 50$ in corrugated pipe that lasts 2 years vs 1000$ in class A that lasts 10 it still works out in favor of replacing the cheap stuff more often. 

same problem with the hood. at this rate, if I build it out of 1/4” plate I’m only looking at it lasting 6-8 years. Will stainless fair better? 

if the stainless isn’t a permanent solution, it simply becomes a cost per year analysis at that point. 
 

 

[PHDforge]

I have a portable forge setup that I bought from Centaur with the sheet metal hood. The hood was rather thin sheet metal and rusted through pretty quickly and I patched it here and there. It only lasted a couple of years. Instead of buying another one, I decided to make my own with thicker sheet metal. This second hood I also painted both inside and outside with a heat resistant paint. I work outside under a pop up during the warmer months in NH. After breaking down my setup for winter, I inspected the hood and it doesn't have any rust that I can see yet (two years old now). I was surprised that the paint has held up too. I will probably give it another coat when I setup in the spring.

 

[George N. M.]

Yes! As PhDforge says, paint the inside of the somke box and chimney with a good high temperature paint, maybe several coats.  That would be the easiest way to protect the metal from acid.

GNM

[Itsnick]

That’s a great idea. I think I’ll rebuild it out of some thicker stuff and paint it. Thanks!

[Frosty]

Check the automotive refinishing supply, there are very durable epoxy "header" paints available. Some are even acid etching so they REALLY stick. They're kind of expensive but you'll probably never have to make another hood.

Frosty The Lucky.

 

[bluerooster]

My side draft is made from an old water heater tank. It's glass lined. Less chance for acid to attack it. But the flue, on the other hand, is just cheap 10" duct. I expect to change it every other year or so.