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  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
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  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D6/00—Heat treatment of ferrous alloys
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  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/005—Heat treatment of ferrous alloys containing Mn
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  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/007—Heat treatment of ferrous alloys containing Co
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/008—Heat treatment of ferrous alloys containing Si
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  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0226—Hot rolling
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/30—Ferrous alloys, e.g. steel alloys containing chromium with cobalt
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/34—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of silicon
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C30/00—Coating with metallic material characterised only by the composition of the metallic material, i.e. not characterised by the coating process

Definitions

• compositions, methods, and systems are directed to novel corrosion- resistant metal alloys, making and testing such alloys, and use of such alloys in biomass cookstoves and biomass pyrolysis and combustion environments for energy production.
• State-of-the-art metal combustors for clean biomass cookstoves are type 310 stainless steel (Fe-25Cr-20Ni wt.% base) and the FeCrAI family of alloys, which are generally in the range of Fe-(10-20)Cr-(2-8)AI wt.% base.
• 310 stainless steel and the FeCrAI alloys are generally in the range of Fe-(10-20)Cr-(2-8)AI wt.% base.
• These metals generally form oxide base layers to aid in corrosion protection.
• Type 310 and related stainless steels form chromium-oxide base layers to achieve corrosion protection at high temperatures
• FeCrAI alloys form aluminum-oxide base layers for protection.
• a third option for protecting metal alloys from high temperature corrosion is the addition of silicon to the formulation. The silicon helps to form silicon-oxide base layers and/or mixed iron-silicon oxides base layers to protect the alloy from corrosion.
• the temperatures, pressures, combustion environments, biomass fuel sources, corrosive species concentrations and deposit tendencies, alloy mechanical property requirements, and component lifetimes encountered in these power generation applications can differ considerably from those of low-cost ( ⁇ $10-50 range), individual cookstoves that burn local biomass fuels.
• the typical biomass cookstove has a targeted lifetime on the order of about 3000- 5000 hot hours.
• Disclosed herein is the development of accelerated corrosion test screening protocols employing highly corrosive salt and water vapor species, specifically designed to evaluate alloys for clean biomass cookstove combustors. Also disclosed are corrosion analysis for a range of commercial alloys and newly developed corrosion resistant alloys for use in cookstove.
• a new Fe (iron)-Cr (chromium)-Si (silicon) base alloy is disclosed that provides for surprisingly improved corrosion resistance at lower cost than state-of the art FeCrAI-based alloys and stainless steel alloys.
• the disclosed methods and alloys are useful in the production and use of low-cost structural alloys, for example with biomass cookstove metal combustors.
• the disclosed methods and compositions may also find use in other biomass combustion environments such as pyrolysis equipment for bio-oil production, and biomass boilers and gasification systems for energy production.
• the disclosed alloys may be ferritic-based matrix structural alloys.
• the composition of the disclosed alloys are in a range of about 13-17 wt% chromium (Cr), about 2-3.5 wt% silicon (Si), about 0.2-1 wt% manganese (Mn), about 0.3-0.7 wt% titanium (Ti), about 0.1 -0.6 wt% carbon (C), with the remainder iron (Fe).
• the alloy may be substantially free of nickel, or may include from about 0.1 to 1 .0 % nickel.
• the disclosed alloy is about 2.4-3.0 wt% Si, about 14.5-15.5 wt% Cr, and about 0.2-0.6 wt% C. In another embodiment, the disclosed alloy is about 2.4-3.0 wt% Si, about 14.5-15.5 wt% Cr, and about 0.30-0.6 wt% C. In yet another embodiment, the disclosed alloy is about 2.4-3.0 wt% Si, about 14.5-15.5 wt% Cr, and about and 0.4-0.6 wt% C. [0012] Also disclosed is a ferritic-alloy, further comprising boron (B) or nickel (Ni).
• the alloy comprises boron in the range of about 0.01 to 0.5 wt% and/or nickel in the range of about 0.1 to 1 .0 wt%. In many embodiments, boron may help to optimize the behavior of the Si-rich and/or Fe-Si rich oxide for corrosion resistance. In one embodiment, the alloy comprises boron from about 0.05 to 0.15 wt % B. In many embodiments, the addition of 0.1 to 1 wt.% Ni may help further improve alloy ductility.
• the disclosed alloys may be useful in devices that may be exposed to corrosive environments.
• the alloy is used in constructing the combustor for a biomass fuel.
• the combustor is in a biomass-fueled cookstove.
• the combustor is in a biomass-fueled energy or heat generator.
• the disclosed alloy may be hot rolled at a temperature above about 700 °C and below about 1250 °C. In some embodiments, the hot rolling may be at about 1 100 °C. In many embodiments, the disclosed alloys may be cold rolled, for example at room temperature. In many embodiments, the disclosed alloy is first hot rolled and then cold rolled. In some embodiments, cold rolling may be preceded or followed by annealing at a temperature from about 600 °C to about 1250 °C, for example about 850 °C, 950 °C, or 1000°C. In many embodiments, an annealing step may help to increase the ductility of the disclosed alloy and may further enhance its corrosion resistance.
• the disclosed alloys may resist corrosion when exposed to a corrosive environment.
• the disclosed alloys may resist corrosion when biomass fuel having greater than about 800 ⁇ g of halogen per gram of fuel is burned.
• the disclosed alloys show less than about 10% mass change over about 500 hours in a corrosive environment.
• the disclosed alloys may lose less than about 10% of the thickness of the alloy when exposed to a corrosive environment for about 500 hours.
• Fig. 1 shows computational equilibrium thermodynamic predictions for Fe-15Cr-0.5Ti as a function of Si and C content (Fig. 1 a, Fe-15Cr-5Si-0.5Ti-0.08C wt.%; Fig. 1 b, Fe-15Cr-3Si- 0.5Ti-0.08C wt.%; and Fig. 1 c, Fe-15Cr-3Si-0.5Ti-0.5C wt.%) using JMatPro_v6 and the Stainless Steel (Fe) database (Sente Software Ltd. Surrey Technology Centre 40 Occam Road GU2 7YG United Kingdom).
• Fig. 2 Top, left, photograph of lab furnace testing alumina tray with samples, right, in-situ cookstove testing. Bottom, temperature profiles for the well-controlled portion of the burn cycle with 3 wood sticks for the two cookstove test beds.
• Fig. 3 shows graph of corrosion data for the commercial FeCrAIY alloy from in-situ cookstove testing using as-received clear Pine wood and salted clear Pine wood.
• Figs. 4a-d Specific mass change (4a, 4c) and metal loss (4b, 4d) corrosion data from 600°C lab furnace testing of commercial (4a, 4b) and developmental alloys (4c, 4d).
• the metal loss data is plotted as the average ⁇ 1 standard deviation of the 3 locations of greatest attack.
• the environment was air + 10% H 2 0, with salt added at the start of testing and re-applied after every 100 h cycle.
• Figs. 5a-d shows plots of specific mass change (5a, 5c) and metal loss (5b, 5d) corrosion data from 800 °C lab furnace testing of commercial (5a, 5b) and developmental alloys (5c, 5d).
• the metal loss data is plotted as the average ⁇ 1 standard deviation of the 3 locations of greatest attack.
• the environment was air + 10% H 2 0, with salt added at the start of testing and re-applied after every 100 h cycle.
• Figs. 6a-d shows graphs of specific mass change (6a, 6c) and metal loss (6b, 6d) corrosion data from in-situ cookstove testing of commercial (6a, 6b) and developmental alloys (6c, 6d).
• the metal loss data is plotted as the average ⁇ 1 standard deviation of the 3 locations of greatest attack.
• FeCrAIY A and FeCrAIY B are duplicate samples run in each of the two cookstove test beds utilized. Salted clear Pine wood was burned to induce accelerated corrosion conditions.
• Figs. 7a-c depicts cross-section backscattered electron mode SEM images of 31 OS stainless steel after 500 h exposure in (7a) 600°C lab furnace testing, (7b) 800°C lab furnace testing, and (7c) in-situ cookstove testing.
• a cross-section of a 31 OS combustor from a field- operated cookstove is shown in (7d) for comparative purposes.
• Figs. 8a-c depicts cross-section backscattered electron mode EPMA images and corresponding elemental maps for (8a) a 31 OS field operated cookstove combustor, (8b) 31 OS after 1000 h of in-situ cookstove testing, and (8c) 31 OS after 500 h of 800°C lab furnace testing.
• Figs. 9a-d shows cross-section backscattered electron mode SEM images for pure Ni after (9a) 500 h, 600°C lab furnace testing, and (9b) after 500h, 800°C lab furnace testing.
• a cross-section backscattered electron mode EPMA image and elemental maps are shown in (9c, 9d) for pure Ni after 500 h of in-situ cookstove testing.
• 10a-f shows cross-section backscattered electron mode SEM images after 1000 h of in-situ cookstove testing for (10a) 316L, (10b) 446, (10c) FeCrAIY, (10d) 25CrFeCrAI, (10e) FeCrAISi, and (1 Of) FeCrSi.
• Figs. 1 1 a-c shows cross-section light microscopy images after 1000 h of lab furnace testing at 800°C for (1 1 a) 31 OS, (1 1 b) FeCrAIY, and (1 1 c) FeCrSi .
• Figs. 12a-c shows cross-section backscattered electron mode EPMA images and elemental maps for (12a) FeCrAIY after 1000 h of in-situ cookstove testing, (12b) FeCrSi after 1000 h of in-situ cookstove testing, and (12c) FeCrSi after 500 h of 800 °C lab furnace testing.
• Fig. 13 is a graph showing predicted phase equilibrium, re-calculated for alloy having Fe-15Cr-2.4Si-0.56C-0.5Mn-0.5Ti (generated by JMatPro v.9).
• Fig. 14 shows micrographs showing microstructure of disclosed Fe-15Cr-2.4Si- 0.5Mn-0.5Ti-0.57C wt.% alloy with and without post rolling annealing.
• Fig. 15 is a graph of Vickers Hardness vs. temperature for the disclosed Fe-15Cr- 2.4Si-0.5Mn-0.5Ti-0.57C wt.% alloy.
• Fig. 16 shows micrographs of the edge surfaces of two annealed samples of the Fe- 15Cr-2.4Si-0.5Mn-0.5Ti-0.57C wt.% alloy after 90% cold rolling.
• Fig. 17 show ductility studies on various disclosed alloys.
• alloys useful in a broad range applications where corrosion resistance is useful and methods of making, using, and working the disclosed alloys.
• the disclosed alloys may be useful in applications where corrosion may be present, for example applications related to energy conversion and combustion, in addition to applications involving alloy, chemical, and petrochemical process environments.
• the environment may include aggressive oxidizing species such as O, S, C, H 2 0, salts, heavy metals, etc. and temperatures may be in excess of about 400-500 °C.
• Resistance to high-temperature (greater than about 600 °C) corrosion is achieved by the formation of continuous, protective, slow-growing oxide surface layers (scales), which are typically based on Al 2 0 3 , Cr 2 0 3 , and/or Si0 2 , and (occasionally) NiO.
• scales typically based on Al 2 0 3 , Cr 2 0 3 , and/or Si0 2 , and (occasionally) NiO.
• novel ferritic alloys comprising high concentrations of carbon and moderate concentrations of silicon.
• the disclosed alloys possess superior corrosion resistance, without suffering from typical drawbacks seen with higher and lower concentrations of silicon.
• the disclosed alloys are more readily workable, for example by cold rolling, than other ferritic alloys.
• the disclosed alloys were compared against existing formulations for resistance to corrosion in environments with, for example, high halogen content.
• the disclosed novel alloys have surprisingly good manufacturability.
• high levels of Si result in poor manufacturability, poor mechanical properties, and weldability issues. These problems are due, in part, to promoting brittle alpha prime and sigma Cr-rich phases.
• high levels of Si increase the tendency for oxide scale spallation due to the large coefficient of thermal expansion mismatch between Si0 2 and Fe-Cr base alloys. For this reason, commercial ferritic stainless steels limit the amount of silicon to ⁇ 0.5 to 1 wt.%.
• Austenitic stainless steels can generally tolerate higher levels of Si due to their high Ni content, which helps resist alpha prime and sigma formation, typically up to ⁇ 2 wt.% Si range, although some grades as high as 3 to 3.5 wt.% Si are available. Reports in salt-containing, high-temperature corrosion conditions also indicate a benefit of Si at 3 wt.% and higher in austenitic alloys, but not at 1 .6 wt.%. Such high Si austenitics are certainly of interest for improved biomass cookstove combustors, although their relatively high cost due to their Ni content, and increased manufacturability challenges from the high Si content, are drawbacks for improved biomass cookstoves.
• the disclosed ferritic FeCrSi alloys were designed in consideration of the detrimental effects of Si on manufacturability, mechanical properties, and weldability.
• the disclosed FeCrSi alloy is readily manufacturable by both hot and cold rolling despite the high Si and C content.
• the amenability to hot rolling was likely aided by austenite stabilization in the 1 100 °C hot-rolling condition employed (Fig. 1 ), resulting from the high levels of C additions.
• the FeCrSi alloy also exhibited excellent high-temperature corrosion resistance under biomass-cookstove relevant conditions. This enhanced corrosion resistance was achieved despite the high levels of C and associated M23C6 formation, and at half the Si level of the literature reported Fe-15Cr- 5Si alloy.
• FeCrSi alloy compositions in the range of about 13-17 wt% Cr, about 2-3.5 wt% Si, about 0.2-1 wt% Mn, about 0.3-0.7 wt% Ti, about 0.1 -0.6 wt% C, and the remainder Fe show potential for use in biomass cookstove combustor components and are of strong interest for further study and scale-up development.
• Higher C levels may help to improve high-temperature strength, which, with ferritic alloys, is at the borderline for use in improved biomass cookstoves (depending on combustor/cookstove design and operation temperature).
• the annealing temperature may be after a hot rolling and/or after a cold rolling.
• the annealing temperature is between about 600 °C and 1200 °C, for example greater than about 600 °C, 650 °C, 700 °C, 750 °C, 800 °C, 850 °C, 900 °C, 950 °C, 1000 °C, 1050 °C, 1 100 °C, or 1 150 °C, and less than about 1200 °C, 1 150 ⁇ , 1 100 °C, 1050 °C, 1000 °C, 950 °C, 900 °C, 850 °C, 800 °C, 750 °C, 700 °C, or 650 °C.
• the annealing temperature is selected from about 850 °C, 900 °C, 950 °C, and
• the disclosed alloys may be subjected to an annealing temperature for various lengths of time.
• the annealing time may be between about 1 min and 120 min.
• the annealing time is greater than about 1 min, 5min, 10min, 20min, 30min, 60min, 70min, 80min, 90min, 10Omin, or 1 10min, and less than about 120min, 1 10min, 100min, 90min, 80min, 70min, 60min, 50min, 40min, 30min, 20min, 10min, 5min, 4min, 3min, 2min, or 1 min.
• the preferred annealing time may be sufficient to allow the average Vickers Hardness of the disclosed alloy to be less than about 500 HV, 450 HV, 400 HV, 350 HV, 300 HV, 250 HV, or 200 HV, and greater than about 150 HV, 200 HV, 250 HV, 300 HV, 350 HV, 400 HV, or 450 HV.
• the disclosed alloys may be resistant to a corrosive environment.
• the disclosed corrosive environment may be an environment wherein the average temperature is greater than about 400 °C, 450 °C, 500 °C, 550 °C, 600 °C, 650 °C, 700 °C, 750 °C, 800 °C, 850 °C, or 900 °C and less than about 950 °C, 900 °C, 850°C, 800°C, 750 °C, 700 °C, 650 °C, 600 °C, 550 °C, 500 °C, or 450 °C, for greater than 50h, 100 h, 200h, 300 h, 400 h, 500 h, 600 h, 700 h, 800 h, 900 h, 1000 h, 2000 h, or 3000 h, and less than about 3500 h, 3000 h, 2500 h, 2000 h, 1500 h, 1000
• the corrosive environment is one wherein the combusted fuel contains one or more halogen or halogen salt, such as sodium chloride, NaCI, chloride, CI, fluoride, Fl, or iodide, I.
• the average halogen content of one gram of fuel in a corrosive environment is greater than about 800 ⁇ g, 850 ⁇ g, 900 ⁇ g, 950 ⁇ g, 1000 ⁇ g, 1050 ⁇ g, 1 100 ⁇ g, or 1200 ⁇ g, and less than about 1250 ⁇ g, 1200 ⁇ g, 1 100 ⁇ g, 1000 ⁇ g, 900 ⁇ g, or 850 ⁇ g.
• the disclosed alloys may resist losing mass or thickness in the corrosive
• the disclosed alloys may lose less than about 50% of their mass per cm 2 , over greater than 50h, 100 h, 200h, 300 h, 400 h, 500 h, 600 h, 700 h, 800 h, 900 h, 1000 h, 2000 h, or 3000 h, and less than about 3500 h, 3000 h, 2500 h, 2000 h, 1500 h, 1000 h, 900 h, 800 h, 700 h, 600 h, 500 h, 400 h, 300 h, 200 h, or 100h.
• the average mass loss or average loss of thickness is less than about 50%, 45%, 40%, 35%, 30%, 25%, 20%, 15%, 10%, or 5% and more than about 2%, 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, or 45%.
• the disclosed alloys may be processed in a variety of ways well known in the art. As described above, the disclosed alloys may be hot or cold rolled to produce a product with a reduced thickness. In some embodiments, the thickness of the disclosed alloys may be reduced by more than 50% by rolling, for example by more than 55%, 60%, 65%, 70%, 75%, 80%, 85%, 86%, 87%, 88%, 89%, 90%, 91 %, 92%, 93%, 94%, 95%, 96%, 97%, 98%, or 99%, and less than about 99.5%, 99%, 98%, 97%, 96%, 95%, 94%, 93%, 92%, 91 %, 90%, 85%, 80%, 75%, 70%, 65%, or 60%.
• the disclosed alloys may be processed to various thicknesses, for example less than about 3 mm.
• the thickness of the product produced from the alloy is greater than about 0.1 mm and less than about 5 mm, for example greater than about 0.10 mm, 0.15 mm, 0.20 mm, 0.25 mm, 0.30 mm, 0.35 mm, 0.40 mm, 0.45 mm, 50 mm, 0.60 mm, 0.70 mm, 0.80 mm, 0.90 mm, 1 .0 mm, 1 .5 mm, 2.0 mm, or 2.5 mm and less than about 3.0 mm, 2.0 mm, 1 .5 mm, 1 .0 mm, 0.9 mm, 0.8 mm, 0.7 mm, 0.6 mm, 0.5 mm, 0.4 mm, 0.3 mm, 0.2 mm, or 1 .5 mm.
• the alloy is processed into a sheet or foil with an average thickness of about 0.3 to 0.6 mm.
• Biomass may refer to any organic fuel that may be burned, for example, without limitation, coal, charcoal, wood, dung, leaves, sticks, oil.
• Alloys for study are shown in Table 1 (compositions are given in weight percent, wt.%).
• the alloys were selected to provide information on a range of protective oxide-scale forming types and a range of base alloy compositions that may be useful in cookstove combustors.
• the tested alloys include ferritic alloys, based on body-centered-cubic (BCC) Fe, and austenitic alloys, based on face-centered cubic (FCC) Fe, which offer better high- temperature strength than ferritics but are also more costly due to their high Ni contents (Ni stabilizes FCC Fe). Commercial alloys were also evaluated.
• These alloys included ferritic, alumina-forming FeCrAIY (Fe-20Cr-5AI base) and austenitic, chromia-forming type 31 OS stainless steel (Fe-25Cr-20Ni base), which are considered state-of-the-art cookstove combustor alloys for their balance of relatively low cost and good high-temperature corrosion resistance.
• Lower-cost commercial austenitic type 316L (Fe-17Cr-10Ni base), austenitic type 201 (Fe-18Cr- 7Mn-5Ni base) and ferritic type 446 (Fe-25Cr base) stainless steels were also evaluated.
• Austenitic stainless steels can generally tolerate higher levels of Si due to their high Ni content which helps resist alpha prime and sigma formation, typically up to ⁇ 2 wt.% Si range, although some grades as high as 3 to 3.5 wt.% Si are available. Reports in salt- containing, high-temperature corrosion conditions also indicate a benefit of Si at 3 wt.% and higher in austenitic alloys, but not at 1 .6 wt.%. Such high Si austenitics are certainly of interest for improved biomass cookstove combustors, although their relatively high cost due to their Ni content, and increased manufacturability challenges from the high Si content, are drawbacks for improved biomass cookstoves.
• the disclosed FeCrSi alloy was readily manufacturable by both hot and cold rolling despite the high Si and C content.
• the amenability to hot rolling was likely aided by austenite stabilization in the 1 100 °C hot-rolling condition employed (Fig. 1 ), resulting from the high levels of C additions.
• the FeCrSi alloy also exhibited excellent high-temperature corrosion resistance under biomass-cookstove relevant conditions. This enhanced corrosion resistance was achieved despite the high levels of C and associated M23C6 formation, and at half the Si level of the literature reported Fe-15Cr-5Si alloy.
• Test samples measuring 20 mm ⁇ 10 mm ⁇ 0.75 to 1 .2 mm for lab furnace evaluation and 25 mm ⁇ 12.5 mm ⁇ 0.75 to 1 .2 mm with a 4 mm diameter hole for in-situ cookstove evaluation were electro-discharged machine (EDM) cut.
• EDM electro-discharged machine
• the as-processed surface finish was retained for corrosion testing.
• the EDM-cut surface finish of the test sample faces was polished to 600 grit finish by standard metallographic techniques in water using SiC grinding papers.
• Test sample corrosion was evaluated by two approaches: specific mass change (balance accuracy of ⁇ ⁇ 0.04 mg or 0.01 mg/cm 2 ), which can be measured nondestructively with the sample returned to testing for additional exposure time, and metal loss measurements, which required removal of a given sample from testing for destructive, cross-section analysis to determine the thickness of intact metal remaining after a given exposure period.
• specific mass change balance accuracy of ⁇ ⁇ 0.04 mg or 0.01 mg/cm 2
• metal loss measurements which required removal of a given sample from testing for destructive, cross-section analysis to determine the thickness of intact metal remaining after a given exposure period.
• the test sample mass increases from incorporation of oxygen into the alloy from the environment and the subsequent formation of oxides on and/or within the sample (also holds true for nitrogen, sulfur, etc. species). Volatilization and/or flaking off of the oxide, etc. results in mass losses. In this manner the kinetics of corrosion can be followed by mass change measurements. Combined with cross-section metal loss measurements, a more complete picture of corrosion kinetics
• the tests were conducted in a ⁇ 8.1 cm diameter alumina tube furnace using flow rates of ⁇ 925 or 760 cm 3 /min (air) and ⁇ 4-5 cm 3 /h (water) at 600 and 800 °C, respectively.
• a 3.5 wt.% salt solution was made using distilled water and Instant Ocean® Sea Salt (Blacksburg, VA 24060-6671 USA), a product which simulates the salt species (Na, K, Mg, etc. chlorides) present in natural seawater.
• the test samples were placed across alumina rods in a flat alumina tray (see Fig. 1 a).
• the top surfaces of the test samples were sprayed with salt water and allowed to air dry prior to initial exposure in the furnace, and salt water was re-applied after every 100 h cycle. (Mass change measurements for corrosion assessment were taken after every cycle prior to adding salt).
• the salt solution was sprayed only on the top-oriented test sample face (exposed surface), and the same top face was maintained for salting across all test cycles. (Orientation of the samples was maintained to salt the same top face across all test cycles.) Salt additions after drying were typically in the range of ⁇ 0.5 to 1 .5 mg/cm 2 of salt per top-exposed sample face.
• the "lab wood” burned in the in-situ test was ⁇ 20 mm x by 20 mm cross-section by 305 mm length clear Pine trim.
• a procedure was developed to controllably introduce salt (halogen) content into the lab wood to a level similar to that found in a biomass fuel known to cause highly corrosive conditions.
• Two types of Haitian charcoal known to produce highly corrosive combustor conditions were analyzed for halogen content: Mangrove lump charcoal and Chabon Ticadaie briquette charcoal.
• the fuels produced 760 ⁇ g and 1390 ⁇ g of halogen per gram of charcoal,
• the average 1075 ⁇ g/g was used as a target value for the halogen content of the lab clear Pine wood.
• the target halogen content was achieved by soaking the Pine wood in a tank of synthetic sea water solution for several days. Water was continuously circulated by a pump to promote even mixing of the salt. After soaking, the wood was air-dried for at least 2 days in ambient air and then dried in an oven at 104 °C for an additional 2 days before burning. Instant Ocean® Sea Salt was again used to create the salting solution.
• An initial target value for salt concentration in the water was determined by analyzing the halogen content of processed wood that was soaked in 1 .9 liter jars with different salt concentrations. Based on this initial salt concentration target value, several small batches of salted Pine wood were produced and analyzed for halogen content. Because the halogen content of the initial salted fuel was lower than desired (Table 2), the salt quantities were increased in subsequent batches in order to more closely match the halogen content of the Haitian charcoals. A total of ten batches of wood were processed and burned in the in-situ cookstove testing. An average value of ⁇ 1070 ⁇ g of halogen per gram of wood was achieved (Table 2).
• the average gas temperature range during steady state in-situ testing was 663 °C ⁇ 85 °C. If transient operation events such as startup, shutdown, and char/ash removal are included in the chamber temperature calculations, the average exposure temperature decreases to an average of 609 °C ⁇ 188 °C. As the majority of the testing was performed in the steady state configuration, under controlled conditions, the average exposure temperature and variability was likely closer to 663 °C ⁇ 85 °C than the 609 °C ⁇ 188 °C over the course of the 1000 h of testing.
• Fig. 3 shows in-situ cookstove corrosion specific mass change data for the state-of- the-art FeCrAIY alloy (Table 1 ) when the stove was burned with as-received clear Pine wood vs. salted clear Pine wood.
• Parabolic-like (rate decreasing with time) corrosion kinetics with relatively low specific mass gains were observed for the FeCrAIY when as-received clear Pine wood was burned, consistent with protective oxide scale formation.
• much more rapid mass gains and a period of mass loss resulting from oxide scale spallation were observed when the salted clear Pine wood was burned.
• the salting procedure adopted therefore clearly induced a more corrosive test environment to serve as an accelerated test method for evaluation of candidate combustor materials.
• Exposed test samples were cross-sectioned by low-speed diamond saw and prepared by standard metallographic techniques (oil-based rather than aqueous polishing media was used to avoid dissolution of possible chloride and related corrosion products during sample preparation).
• the cross-sections were analyzed by light microscopy, scanning electron microscopy (SEM) with energy dispersive x-ray spectroscopy (EDS), and electron probe microanalysis (EPMA) using both EDS and wavelength dispersive spectroscopy (WDS).
• Initial sample thickness was measured with a micrometer.
• Cross-section thickness measurements of corroded samples were made using an optical measurescope (light microscope with digitized micrometer stage measurement attachment).
• test sample cross-sections along the 10-12.5 mm sample width were divided into 3 regions, not including 1 mm from the sample end corners.
• the area of greatest corrosion attack was selected for measurement of intact metal remaining in cross-section.
• the boundary of the intact metal remaining was defined as the point at which the underlying metal was free of oxide scale and internal attack.
• the average of the three locations of greatest attack was used to plot metal loss vs. time in Fig. 4, Fig. 5; Fig. 6 (reported in the plots as ⁇ 1 standard deviation).
• the type 201 stainless steel exhibited extensive cross-sectional metal loss, ⁇ - 240 ⁇ after 1000 h of exposure in the 600 °C lab furnace test.
• significant cross-section metal loss was also observed for the three AFA alloys, with ⁇ - 170 ⁇ , - 210 ⁇ , and -330 ⁇ metal loss for AFA-25Ni, AFA-20Ni, and AFA- 12Ni, respectively.
• the commercial stainless steels exhibited lower corrosion rates, with metal losses of about -60 ⁇ , -70 ⁇ , and -90 ⁇ after 1000 h for type 446, type 31 OS, and type 316L, respectively.
• the lowest metal losses for the Fe-base alloys in the 600 °C lab furnace test were registered for the commercial FeCrAIY, which had metal losses of about -30 ⁇ after 500 h, and - 20 ⁇ after 1000 h.
• the 500 and 1000 h data are from different FeCrAIY test samples and are in the range expected for sample-to-sample variation and initial test sample thickness measurement scatter, estimated to be about -10 to - 20 ⁇ .
• Pre-oxidation of the FeCrAIY resulted in an even lower metal loss measurement, - 10 ⁇ after 1000 h.
• the developmental FeCrSi, FeCrSiAI, and Fe25CrAI alloys also exhibited low metal losses, in the range of -25 to -35 ⁇ (Fig.
• the FeCrAIY and 31 OS alloy samples were consumed through-thickness in some cross-section locations (starting sample thickness of ⁇ 1 mm and ⁇ 0.75 mm, respectively).
• the pre-oxidized FeCrAIY and the Fe25CrAI samples exhibited metal losses in excess of about - 500 ⁇ at 1000 h, and the FeCrAISi sample about -370 ⁇ .
• Fig. 6 shows corrosion data for the in-situ cookstove testing.
• Fig. 5 shows that the corrosion rates were essentially intermediate between the 600 °C and 800 °C lab furnace testing, consistent with the about 663 °C average temperature of the in-situ cookstove testing (Fig. 2).
• Relative alloy corrosion resistance trends were generally similar to the lab furnace testing, indicating the utility of the lab furnace protocol as a screening tool to down -select candidate alloys for the more costly and time intensive in-situ cookstove testing.
• Fig. 7 shows SEM backscattered electron mode cross-section images of type 31 OS stainless steel after 500 h of exposure in 600 °C lab furnace, 800 °C lab furnace, and in-situ cookstove testing. Also shown for comparison is a cross-section of a 31 OS cookstove combustor component (Fig. 7d) from field operation of a cookstove (information on time of operation was not available; the 31 OS combustor analyzed was not from the same 31 OS alloy batch used in the other experiments). The corrosion features were quite similar across the lab furnace, in-situ cookstove, and field-used 31 OS, with loosely adherent external oxide scales tens of microns thick overlying a zone of internal attack.
• Fig. 10 shows a compilation of SEM backscattered electron mode cross-section images for several key alloys after 1000 h of in-situ cookstove testing: commercial stainless steels types 316L and 446, commercial FeCrAIY, and the developmental model alloys
• the 316L cross-section (Fig. 10a) was similar to the 31 OS (Fig. 8b), with an external Fe-rich oxide scale overlying an internal attack zone preferentially along alloy grain boundaries.
• the depths of the internal attack zones for the 316L and 31 OS samples were similar, in the range of about -80 ⁇ , suggesting that the better corrosion resistance of 31 OS (Fig. 6, about -190 ⁇ metal loss for 31 OS after 1000 h of in-situ cookstove testing vs about -370 ⁇ for 316L) is due to slower oxide scaling from the higher Cr and Ni levels in type 31 OS (Table 1 ).
• the resistance of the FeCrSi alloy to internal attack was the source of its superior corrosion resistance in the 800 °C lab furnace testing (Fig. 5). Low magnification, light microscopy cross-sections of the test sample ends for 31 OS, FeCrAIY, and FeCrSi after 1000 h of lab furnace testing are shown in Fig. 1 1 . Both the type 31 OS and FeCrAIY alloys suffered from a transition to extensive internal attack, with the entire sample thickness consumed in some locations. In contrast, no internal attack was observed for the FeCrSi alloy.
• Fig. 12 shows backscattered electron mode cross-sections and corresponding EPMA elemental mapping for FeCrAIY after 1000 h of in-situ cookstove testing, FeCrSi after 1000 h of in-situ cookstove testing, and FeCrSi after 500 h of 800 °C lab furnace testing.
• the scales formed on both FeCrAIY and FeCrSi were Fe-rich, with fine local porosity associated with Na ingress (CI also detected in these regions).
• Chromium and trace Si (Si not shown in maps) were also detected in the external Fe-rich scale formed on FeCrAIY.
• At the alloy-scale interface in the FeCrAIY Fig.
• a qualitative ranking of alloys in order from best to worst corrosion resistance based primarily on the cross-section metal loss measurements in conjunction with cross-section microstructure features is provided in Table 3 for each of the 3 test conditions: in-situ cookstove, 600 °C lab furnace, 800 °C lab furnace.
• the alumina-forming FeCrAI-class alloys exhibited better corrosion resistance than the chromia-forming austenitic 200 and 300 series and ferritic 400 series stainless steels examined under biomass cookstove relevant conditions, although a discrete, thin, and protective alumina-based scale was not formed.
• the beneficial effects of Si have been postulated to be related to both increased Cr diffusivity in the alloy to favor formation of a more dense and protective Cr 2 0 3 scale and formation of an inner layer rich in Si0 2 at the alloy-scale interface (Si0 2 does not form at the alloy-scale interface in FeCrAISi alloys because of the greater thermodynamic stability with oxygen of Al/Al 2 0 3 vs Si/Si0 2 ). Enrichment of Si near the alloy-scale interface was observed for the FeCrSi alloy in the present work (Fig. 12), and both local Si0 2 formation and increased Cr diffusivity effects are consistent with the observed resistance to internal attack by the FeCrSi alloy.
• Fig. 14 shows microstructure analysis of the Fe-15Cr-2.4Si-0.5Mn-0.5Ti-0.57C wt.% alloy and the effect of annealing after hot rolling.
• Panels of Fig. 14, left to right show the microstructures at increasing magnification: 50X, 200X, and 1000X.
• the top row of panels show microstructure of alloy hot rolled at 1 100 °C, while the middle and bottom rows depict samples annealed at 950 °C and 850 °C, respectively, for one hour post hot rolling.
• Fig. 15 is a graph of Vickers vs. temperature (hot rolling and annealing) for the Fe- 15Cr-2.4Si-0.5Mn-0.5Ti-0.57C wt.% alloy.
• the as hot-rolled (HR) consisted of not only pearlite with -283HV but also a large volume of retained austenite with -480HV. These are a source of deformation resistance at room temperature (RT) for cold rolling.
• Annealing (or tempering) at 950 °C or 850 °C resulted in eliminating the retained austenite species, with a concomitant drop in Vickers Hardness, which greatly increased amenability to cold rolling.
• Fig. 16 shows micrographs of the edge surfaces of two annealed samples of the Fe- 15Cr-2.4Si-0.5Mn-0.5Ti-0.57C wt.% alloy after 90% cold rolling.
• the top micrograph shows the alloy annealed at 950 °C for 1 h followed by water quenching, and cold rolling to reduce the sample thickness by about 90%.
• the bottom micrograph shows the second annealed alloy, treated at 850 °C for 1 h followed by water quenching, and cold rolling. Only minor edge cracking was observed after the 850 °C annealing and 90% cold rolling, with better edge quality and even less local cracking for the 950°C annealed and cold rolled alloy.
• Additional testing was performed to analyze ductility of disclosed alloys. Specifically, three alloys were studied after 5 min of annealing at 800 °C and air cooling (AC). The three alloys were labeled Cook2 (Fe-15Cr-2.4Si-0.5Mn-0.5Ti-0.57C wt.%), Cook4 (Fe-15Cr-2Si- 0.5Mn-0.5Ti-0.1 C wt.%), and Cook7 (Fe-15Cr-3Si-0.5Mn-0.5Ti-0.3C wt.%). Two samples of each alloy were tested for ductility: 1 - an as-cold rolled sample, and an annealed sample.
• a first step may include a homogenization and hot-deformation step (e.g. forging or rolling) at or above 1 100 °C, for example from about 600 to about 1200 °C. This step may aid in reducing or eliminating any segregation issues that may result from solidification. This step may also help to break coarse grain structure in the as-cast materials. In many embodiments, this step may be followed by a cooling step, where the temperature of the alloy is cooled, (e.g. by air-cooling) to about room temperature (i.e. about 20 °C).
• This cooling may help avoid or reduce issues, such as crack formation during cooling and/or during the following thermo-mechanical treatment.
• additional annealing within a temperature range of about 700 °C to about 1200 °C, for example from about 850-950 °C, may be useful. This step may be followed by water quenching, before proceeding to additional thermo and/or mechanical treatments.
• Annealing the disclosed alloys within this temperature range may help to (a) soften the martensitically transformed grains formed during cooling after homogenization and hot- deformation, and (b) avoid or lessen sensitization through the decomposition of excess amounts of carbides (e.g. M23C6).
• the maximum hardness of the alloys after annealing may be less than about 500 HV, 450 HV, 400 HV, 350 HV, 3000 HV, 250HV, or 20 HV (or equivalent hardness). In a preferred embodiment the maximum hardness will be about 250 HV.
• the disclosed alloys may then be subjected to additional hot-rolling (for example in a temperature range of about 750-1200 °C, and preferably between about 850-950 °C).
• the disclosed alloy may be cold-rolled after obtaining the desired hardness.
• a final annealing may be useful after cold or hot rolling.
• an annealing may help to release work hardening.
• this annealing may be at about 700-1200 °C, for example 800 °C, and may be from about 1 to about 30 min, for example about 5 min.
• This final annealing may be followed by cooling, for example air cooling or water quenching. Air cooling after an optional final annealing may help to restore the tensile ductility (elongation to fracture) by from about 1 % to about 40%, and preferably about 5-8% for as-rolled sheets and about 30% for annealed sheets.
• the tensile ductility may be improved by greater than about 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 15%, 20%, 25% or 30%, and less than about 40%, 35%, 30%, 25%, 20%, 15%, 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, or 2%.
• a post rolling annealing step may be above about 800 °C, for example about 1000 °C to 1 100°C. This high temperature annealing may help to put carbon back into supersaturated solution, which may then aid in enhancing corrosion resistance further.

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