Firebox Corrosion Analysis with Pictures

[Rockcrawl]

A lot of you may not find this new or interesting, but it has changed what I thought I knew about how these things rot out. I'm in the middle of rebuilding/replacing the firebox in an old outdoor hydronic wood furnace. It's a Pro-Fab Industries Empyre Elite XT 200 downdraft gasifier. I don't know the history of this particular furnace, but it's obvious that it was poorly maintained at least in it's final days. It has had several repairs in the form of welds and patches inside the firebox. It is at least 11 years old, but not more than 13 based on the short time period that this model was made. The firebox is made from 10ga (.135") mild steel, and the lower half of the firebox has a 14ga (.075") 304 stainless steel liner and firebrick lining. The water jacket does not cover the entire firebox. Most of the top and a few inches of one side are not covered by water.

I removed the outer water jacket and found minimal corrosion inside, just some light surface rust. I noticed something immediately when I started cutting the firebox apart. Every mild steel surface that is in contact with water on one side and fire on the other was uniformly worn down to around .06". Parts that were not in contact with water still had their original thickness. Everything below and including the stainless liner still had it's original thickness, even though it is jacketed with water. All of the thinning is on the fire side of the firebox, and only where there is water on the opposite side. I hesitate to call it corrosion because it maintains a smooth/shiny appearance, I think acid erosion is a better term for it. It appears as though this firebox was eroded from the combination of creosote and condensation. What's really amazing is the uniformity of the erosion and how it stops almost immediately where the water jacket stops. Prior to cutting this apart I was under the impression that these things rust through from the water side, but that is not the case at least for this one.

Cross sectional view of firebox wall:

Inside wall of firebox appears to be in very good condition, but is eroded down to 1/16" thick.

Very minimal rust/pitting on the water side:

 

[cumminstinkerer]

I have seen exactly what you are seeing on a much newer boiler, it is very interesting to say the least, I replaced some of that with a metal that is made for boiler fireboxes, as in gas fired, we shall see how it holds up.

 

[salecker]

I have seen exactly what you are seeing on a much newer boiler, it is very interesting to say the least, I replaced some of that with a metal that is made for boiler fireboxes, as in gas fired, we shall see how it holds up.

Just for my curiosity...
Was the one you replaced some of the firebox on a pressurized unit?
Thanks

 

[cumminstinkerer]

Just for my curiosity...
Was the one you replaced some of the firebox on a pressurized unit?
Thanks

no it was an open system, saw exactly the same things you are though, it was all mild steel though, water side is like new, very light surface dusting of rust, fire side ate clear away to paper thin in small spot but appeared to be glass smooth until you cut it out

 

[cumminstinkerer]

@salecker I'll text some more info and pictures later

 

[Rockcrawl]

water side is like new, very light surface dusting of rust, fire side ate clear away to paper thin in small spot but appeared to be glass smooth until you cut it out

I was blown away when I started cutting it apart. For a moment I actually thought the firebox was made of 16ga sheet metal. It took me a while to figure out what had actually happened. What's really shocking is that the metal that's been eaten away still looks like new, you can't see that it's getting thin until it's too late. I guess that's why a lot of people think they rust from the water side, even people who have seen and repaired them. If the fire side looks new and you can't see the other side, safe to assume it must have rusted from the other side.

I assumed I'd be replacing the firebox on this one when I bought it, so not a big deal. But if I were looking at a good/used boiler/furnace that's "ready to go", I think I'd invest in an ultrasonic thickness meter to actually check the condition before buying. They are actually surprisingly affordable.

Just for my curiosity...
Was the one you replaced some of the firebox on a pressurized unit?
Thanks

I would think a closed/pressurized unit would be far less likely to rust on the water side, but I doubt it has any effect on corrosion rate on the fire side. What are your thoughts?

 

[hobbyheater]

I wondering if the corrosion you're seeing is just poor quality steel! When our 37 year old Jetstream was removed. there was no sign of corrosion and this was a non pressurized system!

 

[cumminstinkerer]

@hobbyheater from my experience you cant tell by looking, the fire side still looks like new as @Rockcrawl said, when in reality it is paper thin, pressure wont help the firebox side at all, I agree it will help on the water side. The one I did was mind blowing, I tend to agree with you about maybe lesser quality steel though too! I can't get pics to upload to the site from my phone but will gladly share via text if you pm me.

 

[Rockcrawl]

I think the amount of firebox erosion is primarily based on how often and for how long it's allowed to condense. Cold starts, cold return water, and wet wood all contribute. A consistent hot fire seems to be the ticket, but some erosion is probably unavoidable. Humid air in the summer months will likely condense on the walls and react with the creosote. I think there's always some amount of acid etching happening during shutdown since it's impossible to remove 100% of the soot, creosote, and moisture from the air.
I have no idea how this unit was used, or if it failed after one year or ten. I'm rebuilding it with 3/8" A36 plate steel, hopefully that will last until I'm too old to be chucking firewood. Stainless is probably the way to go, but I got scared away from it due to the stress corrosion cracking and weldability issues.