Why FireBricks / Refractories Fail: Silica+Flux, Not Heat

[stinwell]

Your generosity is astounding Forsythe. Thanks for taking the time to compile all of this awesome info here for the likes of us folks out here in internet land.

[martyn]

So we now know that ceramic fibers can change characteristics when super heated but, does this danger apply to all ceramic fiber products including Morgan's super wool?

[Forsythe]

Oct 5, 2022 8:14:08 GMT -8 martyn said:

So we now know that ceramic fibers can change characteristics when super heated but, does this danger apply to all ceramic fiber products including Morgan's super wool?

There's a post specifically about Morgan Superwool in the "Disintegration of Ceramic Fiberboard" thread here: donkey32.proboards.com/post/37560/thread

The short answer is that Morgan Superwool is an Alkaline Earth Silicate (AES) fiber, which isn't (strictly speaking) a "ceramic"... and AES wools —including Morgan Superwool— are not capable of use in the flame path without damaging changes to their fiber structure, crystallizing, becoming brittle, and posing an inhalation health risk at even lower temperatures than true "ceramic" fiber products do.

The safe-use temperature rating on Morgan Superwool is far below rocket stove flame path temps, and far below aluminosilicate ceramic fiber temperature ratings. Full data sheet and explanation are in that other thread / post.

[Forsythe]

Just a heads-up that I updated the title post at the beginning of the thread with a list of zircon-based refractory coatings available in various regions, etc.

Brand names that I'm aware of so far:

"ITC-100HT" from International Technical Ceramics
(Manufactured in USA, available on Amazon, and carried by many pottery- and metalworking- supply stores [places you'd go to buy blacksmithing or forging/smelting/welding gear.]) www.itccoatings.com/products-1

"Zircon" from Vitcas
(Manufactured in the EU, available online & in EU pottery- and metalwortking- supply stores.) shop.vitcas.com/vitcas-zircon-paint-coating.html

"HeatGaurd Refractory Coating" from Simond Refractories
(Manufactured in India, available on Amazon & online store) simondstore.com/heat-guard-refractory-coating-3270f-5-lbs.html

"Furnascote" from Consolidated Refractories
(Manufactured in Australia, available in AU and NZ) consolidatedrefractories.com.au/products/coatings/

"RTZ Washcoat" from Matthews Refractories
(Also manufactured in Australia) mathews.com.au/company-profile/products/rtz-washcoat/

"Zircoat" from Jyoti Refractories
(Manufactured in India, appears to be available worldwide via their webstore) www.jyoticeramic.com/zircoat.php

"Zircar Zirconia Coating Type ZC-2" From Zircar Refractories
(available online, apparently globally) www.zircarceramics.com/product/zc-2/

"Hi-Purity Zirconia Coating EQ-634-ZO-LD" from MTI Corporation
(available online globally with various distributers carrying it in stock) https:www.mtixtl.com/1800c3270fhi-purityzirconiacoatingquart.aspx



If anyone knows of additional products to add, let me know! (I'm sure there are at least one or two Russian-market brands out there, but I can't seem to get Cyrillic and Roman fonts to play nicely together with search engines...)

Cheers

[woodrascal]

Hi Forsythe. Thanks very much for your incredible detailed knowledge on this topic and thanks for taking the time to share it with us.

I'm suffering failure of vermiculite board, hard refractory firebrick and secondary air stainless steel components in the three rocket stoves I have built.

I'm based in the UK and have access to shop.vitcas.com/vitcas-zircon-paint-coating.html . Could I use some of this zircon coating, applied to the components that are failing (to prevent further deterioration) or will I simply have to start again and apply the coating to the brand new, rebuilt firebox and heat riser areas of my rockets before the next firing?

Thanks very much for any advice and for your postings so far.

[Forsythe]

Oct 25, 2022 3:24:52 GMT -8 woodrascal said:

I'm suffering failure of vermiculite board, hard refractory firebrick and secondary air stainless steel components in the three rocket stoves I have built.

I'm based in the UK and have access to shop.vitcas.com/vitcas-zircon-paint-coating.html . Could I use some of this zircon coating, applied to the components that are failing (to prevent further deterioration) or will I simply have to start again and apply the coating to the brand new, rebuilt firebox and heat riser areas of my rockets before the next firing?

If using the Victas product "Zircon" paint-on coating, I would advise a rebuild with new brick, stainless steel air tubes, and vermiculite board. Because the components are already failing, that's a pretty sure sign that they've already absorbed a lot of fluxing slag and will continue to expand and contract with thermal cycling, which would probably make the Vitcas-Zircon-Paint coating over the top of them crack away with stove cycling. It might work as a temporary band-aid fix to limp through a heating season, but it would be best to replace the damaged components before applying the coating.... otherwise you'd be stuck in a cycle of continually buying more "vitcas-zircon-paint" to paint over areas that crack away, throwing good money after bad. Would probably cost less over the long run to buy new brick and vermiculite which will provide a stable substrate to which the Zircon can form a durable bond.

Other option: Vitcas also offers a patching compound for damaged refractories here: shop.vitcas.com/zircon-patch.html. Replacement of the damaged brick would be preferable, but this might work better than simply coating with the paint-on product.

Same for the stainless steel secondary air tubes, except that the new metal tubes need a product designed to adhere to metal. ITC-213 can do that.... but I can't seem to find info on whether Vitcas Zircon paint coating will work on metal. If it does, I suspect it's not as durable as products like ITC-213 designed specifically for metal use, because the chemistry of bond formation to aluminosilicate refractory —vs. bond formation to metal surfaces— is different between the two. Almost all Thermal Barrier Coatings (TBCs) designed for metal substrates come in a two-part, successive application formula: the first part of the dual-product coating bonds to the metal, creating an intermediate layer — to which the second, more thermally- and chemically-resistant zirconia (meaning: zirconium oxide)-based coating can form a solid bond.

...I wish Vitcas hadn't named their entire line of products simply "Zircon"... it makes it confusing to talk about these coatings writ large. Also because Vitcas's products probably contain zirconia: zirconium oxide rather than actual "zircon" which is zirconium-silicate.

[woodrascal]

Many thanks for your help and advice Forsythe.

I'll rebuild the stoves next summer and apply the Victas Zircon product to all the new board and brickwork.

A couple more questions, if you have the patience... I have a 'Superwool' type of 5 minute riser in one of my rockets (that is so far, performing well) - could I apply the Zircon to the exposed fibres of this riser?

I'm struggling to find the ITC-213 over here in the UK - is there an equivalent product that you could suggest I use for coating the steel secondary air tubes?

Thanks very much once again for your help.

[Forsythe]

Oct 26, 2022 1:06:17 GMT -8 woodrascal said:

Many thanks for your help and advice Forsythe.

I have a 'Superwool' type of 5 minute riser in one of my rockets (that is so far, performing well) - could I apply the Zircon to the exposed fibres of this riser?

Absolutely yes. You not only can, you should.... to ensure that not only your riser continues to perform well, but to ensure that your lungs do, too.

shop.vitcas.com/vitcas-zircon-paint-coating.html#tab-label-description



(also note it says there that Vitcas's "Zircon" Paint is not bond-adherent to metal.)

I'm struggling to find the ITC-213 over here in the UK - is there an equivalent product that you could suggest I use for coating the steel secondary air tubes?

Try the Zircoat product(s?) from Jyoti Refractories... and/or Anderman Ceramics in the UK... They're apparently global partners and Anderman UK claims to have developed Zircoat in partnership with Jyoti India, but Anderman directs you to Jyoti's website to learn about it... I don't quite understand whose product it is.

www.earthwaterfire.com/products/refractory-kiln-and-furnace-ceramic-coating-cement/#

(incidentally, Anderman UK's website is variously listed as "earthwaterfire.com" and "andermanceramics.com" ... maybe the company is undergoing a merger or a restructuring or something. Several refractory companies announced big business shifts like that last year, including Saint-Gobain Refractories in France... and their website is currently an unnavigable hot-mess, too. 😂 )

Both Anderman and Jyoti websites talk about "Zircoat" as though it's only one single product, but there's apparently a "Zircoat-HT" (which I think was the original formula called simply "Zircoat" when released in 2014,) and what appears to be a much newer "Zircoat-M" with confusingly sparse info available about it online.

www.jyoticeramic.com/zircoat.php#section2

Zircoat-M says that it's specifically formulated for metal... but Zircoat-HT says that it offers excellent thermal protection to metal substrates...

Zircoat-M is good up to 1500ºC... but Zircoat-HT is good up to 1800ºC

...They both have roughly equivalent Zirconia contents...

both products are variously touted as protecting aluminosilicate refractories and both are stated to offer heat and corrosion protection to domestic woodburning (steel and cast iron) stoves and chimney flue pipes.



Zircoat-HT is the only one of the two products with application instructions published online... and it clearly instructs the process for bonding to metal. Maybe the Zircoat-M adheres to metal without needing the slow-ramping heat treatment. Who knows?

It would be awesome if you wanted to call Anderman UK and ask them what the hell is up with these two products, (or, rather: kindly ask why a person would choose one over the other when they're both stated as superior zirconia coatings for bonding to metal substrates, but the one "for metal" is rated 300ºC lower than the one not-intended-for-metal-but-also-excellent-for-metal.)



I'd certainly love to know the answer to that, myself, and I'm sure it would be a big help to other forum members, too.

[woodrascal]

Thanks again Forsythe for your swift and very detailed reply - Brilliant information!

I'll try to get in touch with Anderman in the next week or so and see what I can find out...

Thanks once again.

[Forsythe]

I figured I'd follow-up here in this thread regarding the Austrian Eco-Firebox design and the widely-misunderstood cause of that design's early brick failure, (and of firebrick cracking in general) from a discussion that began in this thread:donkey32.proboards.com/thread/3895/austrian-eco-firebox.

Oct 30, 2022 15:45:52 GMT -8 josephcrawley said:

It is my understanding that the primary cause of rebuilds is spalling due to the extreme difference in temperature between the hot face and the air supply side. Personally I have not seen wood ash slagging in masonry heater fireboxes.

This general and very-widely-held, widely-shared misunderstanding is one of the primary reasons I created this thread. I've seen it put forward and propogated by some of the people I respect the very most within the MHA-NA and its leadership.

If thermal differential or thermal shock were to blame for brick cracking, then those cracks would appear well before the first 10-20 firing cycles of the stove. If you've got brick that's cracking more than a month after installation and daily firing, then heat in the form of thermal shock or thermal differentials between refractory hot-face and cold-face sides cannot be to blame. If it were, then you'd have to assume that the stove was being fired progressively hotter over time — until the brick's thermal tolerance were exceeded. Why else would the brick have survived the exact same thermal load and cooling cycle for weeks, months, or even years on end without cracking?

Obviously, the stove's operating temp is not increasing over that time period...So what could possibly change over the course of months to make the brick crack under a given, repetitive thermal load?

The brick's thermal tolerance to that exact same thermal load and cooling cycle.

...and what would cause that slow-onset change in the brick's thermal tolerance to occur? The amount of slagging flux which has absorbed into the refractory, which takes time and many multiple firing cycles, and which is changing progressively over that time period. Only a slowly-changing brick microstructure (and thus its thermal tolerance) can be to blame for this late-onset cracking phenomena.

And because of the mode in which masonry heaters and rocketstoves fire, that slag is more-often-than-not going to be the kind which isn't clearly distinguishable from the outer brick surface, as in clinker deposits. (visible clinker slag is more often visually apparent in forced-air, fluidized-bed biomass furnaces, where the more-viscous calcium slag is forced to circulate within the flame and vapor path, and is thus deposited and stuck to the outer refractory surfaces more readily.)

In a fixed-bed biomass burner, (à la masonry heater or rocket stove) the less-viscous, low-melting alkalis like sodium and potassium — which vaporize at lower temperatures than calcium compounds — can wick into the refractory face at temperatures well below that of these fireboxes' operating temperatures. Potassium salts in biomass ash have been shown to wet and absorb into aluminosilicate refractories at temperatures as low as 600ºC.

There are a whole bunch of cited research articles at the end of this thread's first post — which explain these corrosion mechanisms in granular detail — but I've pulled some key visual aids and snippets to try to condense this down and make it a bit more easily digestible, 'cause I know it can seem a little complex and counter-intuitive when the ash / slag isn't immediately apparent on brick faces when viewed via cursory visual inspection of the superficial surface:













As the above shows, the penetration of the low-melt (potassium/sodium) portions of the ash-slag is rarely clearly visible A) from the outer surface of the refractory, and B) from the nature and orientation of the cracking it causes.... but when the refractory is cross-sectioned and viewed from a perspective perpendicular to the angle of absorption, then the wicked-in slag damage is a lot more obvious.

In very large biomass burners with very thick refractory walls, the damage can be seen to progress in stages through refractory cracking & crumbling caused initially by the uptake of those sodium and potassium glass-formers, (which create the delaminating spallation from within the deepest zones of the brick/slag interfacial melt layer,) with the accumulation of more-viscous melts at the superficial face:



...but then consider what happens when you expose your firebrick to slag (particularly the vaporized sodium and potassium fraction) from not only the front face... but also the top, bottom, and [to some extent] rear sides...as is the case with this Austrian Eco-firebox:



...now you have a scenario where vaporized and molten ash slag can attack the brick from every angle — and its penetrative ability is no longer constrained by the surface tension which would normally be created by a single, uniform front hot-face. ... At that point, you basically have a crucible cup test turned inside out... with slag absorbing into the brick from every direction.

That's why these style of fireboxes erode so quickly — and why that erosion doesn't happen immediately as the result of simple thermal differentials acting upon undamaged, un-slagged, un-altered brick— which would have been present from the very first day of their installation and operation.

[woodrascal]

HI Forsythe and other esteemed forum members

I contacted Anderman in the UK regarding 'Zircoat-M' and 'Zircoat-HT'. This was their reply:-



'Thank you for your email below and for reaching out to Anderman.



I am sorry to advise that we no longer offer ceramic coatings so we cannot assist you with this enquiry.



Best Regards'

I've also made enquiries with Victas regarding their Zircon products and asked if they can provide (or suggest) a suitable heat protective coating for metals. I'm awaiting their reply.

[Forsythe]

Nov 1, 2022 1:19:48 GMT -8 woodrascal said:

I contacted Anderman in the UK regarding 'Zircoat-M' and 'Zircoat-HT'. This was their reply:-

'Thank you for your email below and for reaching out to Anderman.


I am sorry to advise that we no longer offer ceramic coatings so we cannot assist you with this enquiry.

Ooof. That sounds like a company going through an awkward and messy divorce with their former "partner." 😂

They seriously need to get their webmaster to update their site info, because they're the top search result for zirconia ceramic coatings in the UK... (at least on my browser...maybe my search results are skewed and wonky because I'm not in the UK.)

You might also try searching on Amazon UK and Ebay UK for some of those other brands mentioned in the list. I'd at least give the Jyoti Zircoat a search, as well as the Simond Refractories HeatGuard. They appear to be available globally.

The other option might be to call around to local pottery, refractory, and metalworker supply shops and see what they carry. I tried doing that myself from here, but all my search results were peppered with hits for things in the US, and the UK hits were for large B2B companies who do plasma spray coatings... rather than offering a DIY coating which the average person could apply, themselves. There's a company called Jay's refractories that might be worth a shot. I wasn't able to learn much from their website, but they claim to be an independent stockist who appears to cater to small-business needs and home hobbyists....

[fiedia]

Forsythe,

Thanks for making us aware of soft brick weak point vs slag and ashes. I wonder now how to design my stove regarding this new information.

The last papers you gave showed experiments at 1200°C. But I measured that my HR will not work above 900°C at its hottest spots (hotter is possible but leads to thermal runaway).

I am wondering where it is worth to protect my soft bricks :

- inside the HR: definitely the lower half at 900°C, what about the upper half which is running at 650°C?

- above the HR: assuming it remains below 500°C, is slag etching rate relevant at this temperature?

- around the HR: same question for 350°C.

[Forsythe]

Nov 1, 2022 6:04:43 GMT -8 fiedia said:

I wonder now how to design my stove regarding this new information.

To be clear, none of this information is new or different from the info in the thread's first post...(about the corrosion mechanisms at hand with wood ash alkali's ability to slag, flux, and alter the crystallographic microstructure of aluminosilicate firebricks — which lowers their thermal tolerance as a result of the exposure to wood combustion conditions.)

...Here, I just framed that same information wihin a specific context, and shared snippets from 2 (of many) research papers employing the slag crucible / cup test to evaluate this broad tendency of wood ash to damage aluminosilicate refractories used in woodburning devices.

The last papers you gave showed experiments at 1200°C.

They also showed that various alkali compounds from wood ash (like potassium or sodium + Carbon monoxide, or + Chloride, etc.) are capable of melt, slag penetration, and refractory damage at 850º, 930º, 950º, 1000º, and 1180ºC.

One of the papers shown focused its study on ash slag damage at 1200ºC simply because that's their region's [Czech Republic's] standard biomass-burning, boiler-furnace operating temperature

It's important to note: their study in no way suggested that 1200ºC is the onset temperature at which ash slagging occurs. They were simply studying the problem they were encountering under their specific use-case (to explore better ways of addressing it) for their boiler-furnace needs— which require that 1200ºC operating temp.

I could have just as well cited this equally-appropriate and relevant study which shows potassium and sodium salts in ash slag melting into and severely damaging aluminosilicate refractories at 700º, 800º, and 900ºC, with melt and slagging *starting* even lower — at around 600ºC, as mentioned in the previous post.

But I measured that my HR will not work above 900°C at its hottest spots (hotter is possible but leads to thermal runaway).

I am wondering where it is worth to protect my soft bricks

I think you're getting hung up on the temperature ranges at which these various studies are conducted. They are simply the temperatures the scientists had selected for experimentation. They are not the relevant takeaway.

The key takeaway (the "problem-at-hand" here) is that:
wood ash slag damage happens to aluminosilicate refractories at wood combustion temperatures. Full stop.

These cited research papers' experiments are all attempts at understanding the thermochemical mechanisms and thereby develop methods to mitigate them.

- inside the HR: definitely the lower half at 900°C, what about the upper half which is running at 650°C?
- above the HR: assuming it remains below 500°C, is slag etching rate relevant at this temperature?
- around the HR: same question for 350°C.

Trying to parse specific temperatures of various combustion zone areas is really overcomplicating this, and isn't useful to address the problem at hand. There is far too much variation in wood ash composition and melt/flux range to warrant fretting over which parts of the burn chambers to protect and which ones to leave uncoated. There's also far too much variation in flame temperature and flame length with the type of wood fuel used. And perhaps worst of all — you'll place undue stresses on the firebox lining by coating some areas while neglecting to coat others.

One of the best ways to protect your combustion zone's aluminosilicate refractories from slagging is: coat the hot-face of the firebox with a slag-resistant zirconia coating.

👉 If an area in question is part of the combustion zone, then it will benefit from the slag-resistant coating.

Other things you can do to prolong the life of the refractory
• Don't burn slash (with lots of bark, leaves, pine needles, pinecones, etc.) in your heater's firebox. Heartwood is immensely cleaner-burning than the outer bark layers and the parts of the plant responsible for photosynthesis, seed or fruit production, etc. Even in tree species with high potassium content (like pine) the heartwood itself contains very little of that potassium. The slagging alkalis are mostly within the foliage, bark, and reproductive structures.
(For those who haven't purchased their firebox firebrick or castable refractory yet)
• the higher-alumina content aluminosilicate bricks (70%+ Al2O3, AKA "mullite" brick) withstand slagging better than lower-alumina bricks (<38-40% Al2O3)
• the more-dense the brick microstructure, the better it resists alkali slag absorption and fluxing. Zirconia-mullite is excellent in this regard (but it is very pricy if you aren't making your own bricks.)
• the porous insulating refractories responsible for clean rocketstive combustion can be given a very thin (ideally 1mm thick, but not exceeding 3mm maximum) zirconia coating on the hot-face, which will provide astronomically higher protection from alkali slagging than uncoated porous insulating refractory surfaces in the combustion chamber(s) hot-face. PLUS it will even **improve** the thermal insulation performance of that porous refractory. Win-win.
• Phosphate binders in castable refractory appear to show better slag-resistance than castable mixes bonded by calcium aluminate. The science is still up for debate on whether that difference is still present at the higher rocketstove combustion temps.
• If you're building a dedicated gasifier or TLUD which operates in an oxygen-starved "reduction" gasification mode: Silicon carbide (SiC) added to the aluminosilicate will help protect it from destruction by alkali ash reduction reactions within the refractory microstructure. (ONLY relevant to gasification stoves operating under reducing atmosphere.)

.

[foxtatic]

Regarding Zircon application to vermiculite board: I am looking to build my next core out of it. After reading this excellent thread, I contacted Vitcas to ask if Zircon coating could be applied to vermiculite board. The response was “We wouldn't advice this as we haven't tested the application ourselves.”

Clearly covering their butts, so I’m wondering if anyone has any direct experience in trying this application?

Also, I built a DSR2 core entirely out of very soft CFB. Not knowing any better at the time, I sprayed all the pieces down with sodium silicate to contain the fibers and firm up the edges. Otherwise my test pieces virtually disintegrated under flame. So if I understand correctly from this info, sodium silicate alone is not great and will cause fluxing and eventually weakening of the structure.

I see earlier in this thread where you recommend applying colloidal silica, then a refractory coating. I have only fired the core maybe 8 to 10 times. Do you believe I will eventually see issues develop, and should I apply a refractory coating now before much further use?
If so, would that be to the entire core (inside and out $$$!) or just to the hot surfaces in the firebox and afterburner?