I'm no metallurgist, but my understanding is that despite the hot side of the burn chamber and the heat riser being "low" oxygen, all metals to include the refractory will oxidize starting at around 400* C if there is any atmosphere (i.e. not a vacuum). The spalling seen in the metal chambers so far is evidence of sublimation and dissipation which reinforces the fact that there is an accelerated oxidation process occurring and it is this oxidized surface that is peeling or spalling off into the chambers. Add the fact that most fuel sources will create an acidic gaseous environment and you are essentially anodizing the metal at extreme temperatures and then spalling it off as thermal expansion occurs. This is basically like running cheese through a grater rapidly shredding it into small bits. Metal fatigue and spalling in the cores of these RMH is simply the exothermic physics of rapidly aging metal hundreds of years in mere weeks or months. The second challenge with using metal as the core of an RMH is the process of annealing which occurs in a well designed and insulated RMH. The metal gets super heated quickly and cools VERY slowly resulting in a metal with increased ductility, decreased hardness and decreased thermodynamic integrity resulting in a softer metal more susceptible to rapid oxidation and fatigue. This is a vicious loop that would appear to make the use of metal in the cores of our RMH ill advised.
Normally this is a non-issue as Insulated fire brick, refractory cements, etc. are all relatively inexpensive solutions without this oxidative drawback. Really if you are building a stationary RMH in a traditional housing situation, you're a fool to employ any other construction method than insulation wools, clay, cement or IFB. Physics, experience and anecdotal evidence has demonstrated that in the middle of the cold season your RMH will fail, at the very least leaving you with no heat, but at worst leaving you or your family dead from waste gas pollution in your abode. Every year people die from heater related fuel and gas polution. So again, why gamble with your heat source and life?
What if however, you wish to employ an RMH in a situation where the weight or fragility of an IFB or refractory cement furnace makes these solutions equally unviable? For instance, I wish to build an RMH on a city bus I'm converting into a tiny house RV. How do I then address the design weaknesses of cement and metal? My hypothesis is two pronged in its solution. We must address both oxidation and annealing, but how?
First, I propose using a nickel based alloy such as Inconel, Hasteloy, or Incaloy as our core metal as these will be highly resistant to the observed oxidation and annealing experienced with all other alloy blends.
Second, to further protect our robust metal cores, applying ceramic coating layers inside and outside our metal cores. ITC makes several products that I believe will work well for this purpose.
Description:
Alloy 625 is a nonmagnetic , corrosion - and oxidation-resistant, nickel-based alloy. Its outstanding strength and toughness in the temperature range cryogenic to 2000°F (1093°C) are derived primarily from the solid solution effects of the refractory metals, columbium and molybdenum, in a nickel-chromium matrix. The alloy has excellent fatigue strength and stress-corrosion cracking resistance to chloride ions.
INCONEL alloy 625 has good resistance to oxidation and scaling at high temperature. Its performance in an extremely sever tests is shown in comparison with that of other materials in Figure 15. In this test, periodic weight-loss determinations indicate the ability of the alloy to retain a protective oxide coating under drastic cyclic conditions. 1800°F is a temperature at which scaling resistance becomes a significant factor in service.
Some typical applications for alloy 625 have included heat shields, furnace hardware, gas turbine engine ducting, combustion liners and spray bars, chemical plant hardware, and special seawater applications.
The hardening effect that takes place in the material on exposure in the range centered around 1200°F (See Mechanical Properties section) is due to sluggish precipitation of a nickel-niobium-rich phase, gamma prime. This phase gradually transforms to orthorhombic Ni3 Nb when the alloy is heated for long times in the intermediate temperature range. Extensive investigation of the stability of alloy 625 following exposure for extended periods in the 1000° to 1800°F temperature range has shown complete absence of embrittling intermetallic phases such as sigma.
Corrosion Resistance:
Alloy 625 has withstood many corrosive environments. In alkaline, salt water, fresh water, neutral salts, and in the air, almost no attack occurs. The nickel and chromium provide resistance to oxidizing environments. Nickel and molybdenum provide for resistance to non-oxidizing atmospheres. Pitting and crevice corrosion are prevented by molybdenum. Niobium stabilizes the alloy against sensitization during welding. Chloride stress-corrosion cracking resistance is excellent. The alloy resists scaling and oxidation at high temperatures (SPALLING).
Preferred Alloy 625 heat treatment:
(1)High Solution Anneal - 2000/2200°F (1093/1204°C), air quench or faster.
Weldability:
Welding can be accomplished by the gas-shielded processes using a tungsten electrode or a consumable electrode. INCONEL alloy 625 is readily joined by conventional welding processes and procedures. INCONEL Filler Metal 625 and INCONEL Welding Electrode 112 are nickel-chromium-molybdenum products designed for
welding INCONEL alloy 625 to itself and to other materials. Postweld heat treatment of the weld are not necessary to maintain corrosion resistance. Heavy restrained sections can be welded and the weld's mechanical properties follow the same trends as base metal properties. Standard practices such as clean surfaces, good joint alignment, U-joints for thicker sections, etc., should be followed.
As for the ceramic coating, ITC 213 Ceramic Coating for Metals (3500* F operating temperature) as a base layer to 1/16-1/8 inch thick would act as both vapor and thermal barrier limiting both oxidation and annealing. Then, adding an additional 1/16-1/8 inch top layer of ITC 296A Ceramic Top Coating (5000* F operating temperature). This top coat should theoretically protect the initial coat and further prevent oxidation and annealing. This coating process would include all internal and external metal core surfaces. According to their published literature: Coated 1/8” thick plain carbon steel w/ITC 213 & 296A on both hot face and cold face results in 2500*F hot face, 1450*F cold face, with a 1050*F differential! The operating temperatures of Inconel combined with this barrier should prevent the catastrophic failures we're seeing in metal core RMHs.
I'd love to hear people's thoughts and for an adventurous tinkerer or two to test my theory.
