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.

 

[salecker]

All very interesting
My thoughts were it was eroding above the water line on the firebox side, but it is the opposite.
Could the difference be the steel covered in water is having a different reaction as it is transferring the heat, apposed to the steel that is in air.
We need a metallurgist on staff to explain this.
I will have to remember to clean the inside of my firebox and have a close look at the welds around the stays to see if there is any signs of erosion.

 

[stoveliker]

I did have a lot of metallurgy courses in my undergrad, and did some research in corrosion - but for aluminum.

Generally corrosion happens faster when temps are higher. So the water cooled wall would then corrode slower. Not what is seen here.

Galvanic couples with some potential at the water steel interface are a stretch too given that steel is sufficiently conductive to not have the effects be limited to only at the water-adjacent walls. (See the spacing of zinc blocks on ship hulls dictated by the limits of where the electrochemical currents can spread to - much farther apart than the <1" transition between erosion and no erosion here.)

So I'm drawing a blank. It's been a few decades tho since I thought about these things.
Interesting case though.

 

[Rockcrawl]

I think it's as simple as this: moisture inside the firebox only condenses on the colder surfaces, i.e. only where there is cold water on the other side. In the same way a glass of cold water does not condense much above the water line, the firebox will not condense where it is not jacketed by water. The condensation combines with soot or creosote and causes rapid acidic corrosion. Instead of the typical red rust we think of with steel and water, the acid (sulfuric?) leaves a nice shiny surface with no visible rust.

 

[cumminstinkerer]

I think it's as simple as this: moisture inside the firebox only condenses on the colder surfaces, i.e. only where there is cold water on the other side. In the same way a glass of cold water does not condense much above the water line, the firebox will not condense where it is not jacketed by water. The condensation combines with soot or creosote and causes rapid acidic corrosion. Instead of the typical red rust we think of with steel and water, the acid (sulfuric?) leaves a nice shiny surface with no visible rust.

I think you pretty well hit the nail on the head, I run 24-7-365 so I dont have those issues but you are hitting what I see too

 

[stoveliker]

I believe it's (rather weak, and dilute) acetic acid in creosote.
I am not sure of the finish that would leave on the steel, but I would be surprised if a weak acid like that would eat steel that efficiently.

I don't think it is water condensation driving this. When the system is not used, the temperature will quickly equilibrate along the wall due to its thermal conductivity - i.e. it won't be a sharp boundary between cold and eroded steel,.and non-eroded steel that you see. Water will condense everywhere if the dewpoint is reached because the inactive boiler will have temps that are pretty even on all surfaces.

I think it's more likely that creosote condensation makes the distinction.
Big temp difference between the water cooled wall and the non cooled wall when a fire is present,.i.e. when creosote can deposit.
Do you see that (creo deposits) only on the water cooled surface.in your boiler and not on the non cooled surfaces?

 

[Rockcrawl]

I don't like relying on AI for information, it "learns" from people like me who don't really know what they're talking about, but this is what it has to say:

AI Overview
When creosote inside a chimney mixes with water (rain or humidity), it can create a highly acidic, corrosive liquid that is often a form of sulfuric acid or sulfurous acid.

The Reaction: Creosote is a byproduct of wood combustion that contains sulfur and other compounds. When moisture enters the chimney—due to lack of a chimney cap or cold temperatures causing condensation—it mixes with the sulfur in the soot and creosote to form acid.

Corrosive Effects: This acidic mixture can damage chimney liners, corrode metal, and eat away at bricks and mortar.

Prevention: Regular chimney cleaning and using a chimney cap are necessary to prevent this, especially during humid summer months when condensation, rather than rain, can activate the acid.

End of AI response

The last part is a little concerning. Ambient air temp can easily swing 20-30 degrees in 12 hours, but I bet the stored water in an insulated boiler that's idle will not change more than 2-3 degrees in that same period. There is definitely opportunity for condensation to form on a daily basis for extended periods.

Do you see that (creo deposits) only on the water cooled surface.in your boiler and not on the non cooled surfaces?

There is definitely less creosote on the hotter un-jacketed surfaces, but not the sharp line that you might expect to see from the way the metal has corroded. I don't think you could look inside and say where the water jacket stops, it's not a well defined line. Hard to tell now since it's been cleaned and all cut into pieces, and I'm only seeing the results of the final days of burning, not the whole lifetime. You are proposing that there is a sharp cutoff for creosote condensation, why would it not be the same for humid air condensation? They happen at different temperatures, but the principals that make it happen are the same.

I'm not sure how relevant this is, but I have an outdoor fuel oil storage tank. It's not uncommon to see condesation just up to the fuel level with a sharp cutoff. Lets say I left the door open on the boiler, why wouldn't the same thing happen?

In any case it doesn't matter if it only condenses on the jacketed parts or across the whole surface. It's bad either way.

 

[stoveliker]

Interesting.
I went by

The Creosote Problem: Chimney Fires & Chimney Cleaning - Cookstove Community

A creosote problem is more likely with wood stoves than fireplace since the exhaust gases from stoves are cooler than those from the fireplaces.

cookstoves.net

don't know the veracity of that site; piece by a professor from Idaho.
I suspect some carbolic acid will be there too.
I'm not sure there's enough sulfur in clean wood to matter, though what is "enough" is not clear to me. Sulfur concentrations are quite low, though. But sulfuric acid is of course aggressive.

My point about the sharp line is that when you have a heat source (fire) and active cooling on part of the steel (the water), the heat flows are much larger and one can run into limits of temperature equilibration in the steel. I.e. one will see higher temperature differences based on location (near or far from water cooled surfaces).
Whereas in ambient, the heat flow needed to equilibrate is going to be much less, so I suspect much, much smaller temperature gradients when the boiler is not in use. So creosote can/will condense much more on the water cooled surface than on the hotter surface, whereas water condensation might not. But the latter depends quite much on the dewpoint - i.e. if the water in the system is 15 degrees cooler than the ambient temperature, whether the dewpoint is located someplace between the ambient and ambient minus 15 F. Maybe you do have that. Depends on your climate. Could be.

The fuel tank indeed suggests that that may what is happening.

Would be interesting to compare climates between folks seeing this erosion and folks not seeing it (assuming not everyone sees it, given that it's hard to see from the firebox).

 

[salecker]

It would help to know the wood burned...
I have heard that the stoves used to burn salvaged wood from the ocean will rot out fast from the salt.

 

[Rockcrawl]

It would help to know the wood burned...
I have heard that the stoves used to burn salvaged wood from the ocean will rot out fast from the salt.

That's not likely the case here, we're pretty far from the ocean.

 

[salecker]

That's not likely the case here, we're pretty far from the ocean.

That rules out the salted wood.
Is there any wood around you the has a history of burning different than others?
All i have to burn is Spruce and Alder

 

[salecker]

That's not likely the case here, we're pretty far from the ocean.

That rules out the salted wood.
Is there any wood around you the has a history of burning different than others?
All i have to burn is Spruce and Alder

 

[sloeffle]

Why I'm not a fan of cold starting my hydronic heater (hopefully I used the correct terminology!) more than a few times a season. At water temperatures <140 the inside of the boiler (generic term for anything surrounded by water that burns something) sweats, and mixes with the creosote to cause what was talked about above. Most OWBs (hydronic heaters) don't require thermostatic loading valves because they are generally only cold started a few times a season. Batch boilers and batch style hydronic heaters (hope I got that right) should have them because they are constantly cold starting their boilers. When I bought my hydronic heater, I called Tarm and asked them about getting one, and they said the same thing I just stated.

 

[cumminstinkerer]

Why I'm not a fan of cold starting my hydronic heater (hopefully I used the correct terminology!) more than a few times a season. At water temperatures <140 the inside of the boiler (generic term for anything surrounded by water that burns something) sweats, and mixes with the creosote to cause what was talked about above. Most OWBs (hydronic heaters) don't require thermostatic loading valves because they are generally only cold started a few times a season. Batch boilers and batch style hydronic heaters (hope I got that right) should have them because they are constantly cold starting their boilers. When I bought my hydronic heater, I called Tarm and asked them about getting one, and they said the same thing I just stated.

Scott, I cheat, I never let my polar get cold and the storage never gets below 140, Mine has been going solid since before thanksgiving, I let it go out right in there so I could clean the tubes and lower chamber without burning my skin off.

 

[hobbyheater]

@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.

I went inside with a camera and all looked good!

 

[salecker]

Why I'm not a fan of cold starting my hydronic heater (hopefully I used the correct terminology!) more than a few times a season. At water temperatures <140 the inside of the boiler (generic term for anything surrounded by water that burns something) sweats, and mixes with the creosote to cause what was talked about above. Most OWBs (hydronic heaters) don't require thermostatic loading valves because they are generally only cold started a few times a season. Batch boilers and batch style hydronic heaters (hope I got that right) should have them because they are constantly cold starting their boilers. When I bought my hydronic heater, I called Tarm and asked them about getting one, and they said the same thing I just stated.

My boiler gets started everyday at around 110-120. Some days i don't get home till storage is at 100 , sometimes the backup oil boiler fires up as i am starting a fire.
18 years and i have yet to see any condensation on the inside of my firebox, including during the summer when it's cold.
Than might be our climate, we have a very dry climate here.
Like i mentioned before, once i stop firing in the spring i don't do anything to the boiler till i start the next fire in the fall.
Anything i own had better be OK with being ignored when not in use as i have a lot of stuff.
And i don't have a thermostatic loading valve, but i have a near boiler pump system for bringing up the initial temp.