Refractories high-temperature corrosion


Ruthenium and osmium have hep crystal stmetures. These metals have properties similar to the refractory metals, ie, they are hard, britde, and have relatively poor oxidation resistance (see Refractories). Platinum and palladium have fee stmetures and properties akin to gold, ie, they are soft, ductile, and have excellent resistance to oxidation and high temperature corrosion.  [c.163]

Finally, glass-ceramics will play a key role in the growing arsenal of advanced materials, both alone and in combinations with other materials. For example, glass-ceramic composites incorporating ceramic fibers yield high temperature strengths superior to metal alloys, and also exhibit gradual failure behavior similar to that of metals and plastics, as opposed to the normal catastrophic behavior common to brittie ceramics (5) (see Composite materials, CERAMiC-MATRix). Low dielectric-constant glass-ceramics are projected as the best candidates for high performance multilayer packaging materials. Other potential products include superconducting glass-ceramics, bioceramics for bone implants and prostheses, durable glass-ceramics for waste disposal, and refractory, corrosion-resistant glass-ceramic coatings for superalloys.  [c.326]

The furnace is constmcted with a steel shell lined with high temperature refractory (see Refractories). Refractory type and thickness are deterrnined by the particular need. Where combustion products include corrosive gases such as sulfur dioxide or hydrogen chloride, furnace shell temperatures are maintained above about 150—180°C to prevent condensation and corrosion on the inside carbon steel surfaces. Where corrosive gases are not present, insulation is sized to maintain a shell temperature below 60°C to protect personnel.  [c.54]

Metals. Aircraft and space vehicles, turbine generators, and other such appHcations require high strength at high temperature along with exceUent oxidation resistance. Superalloys, ie, complex nickel and cobalt-based alloys, and refractory metals, eg, niobium, tungsten, molybdenum, tantalum, and their alloys, are used for appHcations at temperatures above 1000°C. In many cases the coatings must be resistant both to oxidation and to hot corrosion by sulfidation from sulfur-bearing gases. Testing must be done to assure compatibiHty between the coatings and the substrate and to avoid undesirable soHd-state reactions and interdiffusion, which can produce voids, cracks, and weak layers. Refractory metal coatings must be sufficiently ductile for the anticipated service and environmental pressures and stresses. AH coatings must resist thermal cycling and mechanical forces without cracking (see also Refractory coatings) (22,40).  [c.136]

Nickel-Base Superalloys. Superalloys, which are critical to gas-turbine engines because of their high temperature strength and superior creep and stress mpture-resistance, basically are nickel—chromium alloyed with a host of other elements. The alloying elements include the refractory metals tungsten, molybdenum, or niobium for additional soHd-solution strengthening, especially at higher temperatures and aluminum in appropriate amounts for the precipitation of for coherent particle strengthening (see Refractories). Titanium is added to provide stronger y, and niobium reacts with nickel in the sohd state to precipitate the y -phase y is the main strengthening precipitate in the 718-type alloys. Cobalt, generally present in many superalloys in large (>10 wt%) amounts, enhances strength, oxidation, and hot-corrosion resistance which is also provided by the chromium in the alloy. Small excess amounts of carbon usually are present in superalloys for intentional carbide precipitation at grain-boundaries which, as discrete and equiaxed particles, can provide obstacles for grain-boundary sliding and motion, thus suppressing creep at high temperatures. Small or trace amounts of elements, eg, zirconium, boron, and hafnium, may be present and these enhance grain-boundary strength and improve ductiUty. The strength and elevated-temperature properties of a superalloy are dependent on the volume fraction of the fine y -precipitates, which can be increased to ca 60 wt %, depending on the aluminum and titanium content. Besides precipitation control at the grain boundaries, improved heat resistance can result from either the elimination of grain boundaries or through the growth of aligned grains with minimum grain boundaries perpendicular to the principal appHed stress direction, eg, in turbine-blade apphcations.  [c.7]

Burned brick may be impregnated with tar or pitch to improve corrosion resistance. The heated bricks are placed in the impregnation unit which is sealed and evacuated to remove air from the pores of the refractory. The evacuated chamber is then filled with hot pitch and the vacuum is released. The treated product is allowed to drain free of excess pitch and is ready for shipment. Although this treatment is primarily used on basic refractories, it can be extended to other classes. The benefits derived from impregnation are lost if the pitch is burned off at high temperature therefore, a reducing atmosphere is required such as is encountered in a basic oxygen steelmaking furnace.  [c.32]

L. D. Pye, ed.. Introduction to Glass Science, Proceedings of a Tutorial Symposium, Plenum Press, New York, 1972. Review of glass melting refractory corrosion. liigh Temperature Material, Proceedings of the 3rd Symposium on Material Science Kesearch, Dept, of Atomic Energy, Bombay, India, 1972. Papers cover a variety of oxide and nonoxide high temperature materials like spiael, alumina, and siUcon nitride.  [c.39]

Refractory coatings denote metallic, refractory compound, ie, oxides, carbides (qv), and nitrides (qv), and metal-ceramic coatings associated with high temperature service as contrasted to coatings (qv) used for decorative or corrosion-resistant appHcations. Coatings of high melting materials that are used in other than high temperature appHcations are also considered to be refractory. A coating may be defined as a near-surface region having properties that differ significantly from the bulk of the substrate (see Ceramcs Metallic coatings Metal surface tteati nts).  [c.40]

Chemically Functional. Refractory coatings are used for corrosion-resistant high temperature service in gas turbine and diesel engines, components such as cmcibles, thermocouple protection tubing, valve parts, etc.  [c.50]

Traditionally, the metallurgical, abrasive, and refractory industries are the largest users of sihcon carbide (see Abrasives Metallurgy Refractories). SiC is also used for heating elements in electric furnaces (see Furnaces, ELECTRIC), in electronic devices, and in appHcations where its resistance to nuclear radiation damage is advantageous. The development of advanced pressureless sintering and complex shape-forming technologies has led to siUcon carbide becoming one of the most important stmctural ceramics (see Advanced ceramics, structural ceramics). Sihcon carbide has found wide acceptance in wear-, erosion-, and corrosion-resistant appHcations it has demonstrated excellent performance as a heat-exchanger material it is also being evaluated for prototype high temperature gas turbine engine component appHcations.  [c.463]

It is not economically or technically feasible to use chromium in a fabricated form, but the high resistance of the metal to corrosion can be utilised by applying a thin coating of chromium to less resistant metals. Although the metal is base ( °cr /Cr = -0.74 V (SHE)) it is protected by a thin, stable, tenacious, refractory, self-sealing film of CrjOj. This is preserved by oxidising conditions, and the metal is very resistant to high-temperature oxidation and to atmospheric exposure in most natural environments. Unlike silver and copper, it is not tarnished by hydrogen sulphide, nor is it fogged like nickel by atmospheres containing sulphur dioxide.  [c.545]

Furfural and furfuryl alcohol are specialty solvents. They also are reactive solvents and contribute low viscosity to resin formulations. Thermosetting resins containing furfural and/or furfuryl alcohol. "Furan resins" demonstrate specialty properties including corrosion resistance, high carbon yield, stability at elevated temperature, low fire hazard, and excellent physical strength. These properties are of industrial importance in making foundry molds and cores, fiberglass composites, mortars, cements, plastic insulation foams, refractory mixes, high carbon composites, and aggregate binders, among others.  [c.79]

Refractory failures resulting from erosion and corrosion from hot particulate laden gases can result in incinerator downtime and high maintenance costs. Of particular concern are flourine, sodium, potassium, and sulfate salts, which penetrate brick surfaces when hot. Upon cooldown, salt hardens and expands, causing the surface which has been penetrated to fail. In addition, organically bound alkaH metals in wastes can react chemically with the refractory to form new compounds with lower melt points (eutectics) than furnace operating temperatures. Continued operation at elevated furnace temperatures and close attention to the design and operation of the furnace to keep wastes from impinging on refractory walls, along with controlling the amount of alkaH metals fed, help prolong refractory life.  [c.54]

A simple direct-flame incinerator may be a refractory-lined furnace arranged for good mixing and fitted with a burner. Such an incinerator is low in capital cost and suitable for periodic or batch burning of a process purge gas during plant shutdown. However, such an incinerator in continuous service on other than a very small vent would have extremely high fuel operating cost. In these cases, sufficient auxiHary fuel must be provided to completely heat the incinerator gas to the ignition temperature and this heat can be further utilized. Steam can be generated if there is a need for process steam. If steam is not needed, moderate to large size incinerators have a heat exchanger between the incoming gases and the hot combustion products to preheat make-up air. Because of the temperatures involved, heat-transfer surfaces are generally of alloy constmction or ceramic materials. The latter can be damaged by thermal shock if sudden step changes in operation occur, whereas alloys may be attacked by corrosive gases such as halogens and sulfates if present.  [c.59]

At elevated temperatures and pressures, nitrogen combines with most elements to form nitrogen compounds. In the presence of metals and semimetals, it forms nitrides where nitrogen has a nominal valence of —3. Atomic nitrogen, which reacts much more readily with the elements than does molecular nitrogen, forms nitrides with elements that do not react with molecular nitrogen even at very high pressures. The biaary compounds of nitrogen are shown ia Figure 1 (1). These compounds may be classified, according to their chemical and physical properties, iato four groups salt-like, metallic, nonmetallic or diamond-like, and volatile nitrides. The nitrides of the high melting transition metals, eg, TiN, ZrN, and TaN, are characterized by high melting poiats, hardness (qv), and resistance to corrosion and are referred to as refractory hard metals (see Refractories). The nonmetallic compounds, eg, BN, Si N, and AIN, are corrosion- and heat-resistant ceramic-like iadustrial materials having semiconductor properties (see Abrasives Semiconductors).  [c.50]

In wetted-wall units, the walls of a tall circular, slightly tapered combustion chamber are protected by a high volume curtain of cooled acid flowing down inside the wall. Phosphoms is atomized by compressed air or steam into the top of the chamber and burned in additional combustion air suppHed by a forced or induced draft fan. Wetted-waU. plants use 25—50% excess combustion air to reduce the tail-gas volume, resulting in flame temperatures in excess of 2000°C. The combustion chamber maybe refractory lined or made of stainless steel. Acid sprays at the bottom of the chamber or in a subsequent, separate spraying chamber complete the hydration of phosphoms pentoxide. The sprays also cool the gas stream to below 100°C, thereby minimising corrosion to the mist-collecting equipment (typically type 316 stainless steel).  [c.327]

The outer shell of the furnace (roof, sides, and bottom) is of welded steel plate constmction employing various alloys to meet the electrical, thermal, and corrosion requirements at specific locations. Water and air cooling are used extensively to control refractory and metal temperatures. Large carbon blocks cemented together with carbon paste or ramming material are used to constmct a monolithic bottom hearth 1—1.5 m thick. Carbon and graphite bricks, rams, and pastes ate used as the primary refractory materials in the sidewall to just above the molten slag zone of the furnace. Eire brick is typically used on the upper sidewalls with castable and gunnite high alumina refractory used on the furnace roof The electrodes enter the furnace through special refractory lined sleeves that ate sometimes water-cooled. A telescopic seal is customarily used to provide a gas seal between the electrode and the furnace, which operates at about 23 cm water pressure. Refractory performance is critical in obtaining satisfactory furnace operability, and is key to getting long life between furnace rebuilds, which is a significant cost factor of manufacturing.  [c.351]

Refractories are materials that resist the action of hot environments by containing heat energy and hot or molten materials (1). There is no weU-estabhshed line of demarcation between those materials that are and those that are not refractory. The abiUty to withstand temperatures above 1100°C without softening has, however, been cited as a practical requirement of industrial refractory materials (see Ceramics). The type of refractories used in any particular apphcation depends on the critical requirements of the process. For example, processes that demand resistance to gaseous orHquid corrosion require low permeabihty, high physical strength, and abrasion resistance. Conditions that demand low thermal conductivity may require entirely different refractories. Combinations of several refractories are generally employed.  [c.22]

The metallurgy of the cyclone equipment has in recent years focused primarily on type 304 H stainless steel. The 304 H material is durable and easy to fabricate and repair, withstands the high regenerator temperatures, and is oxidation- and corrosion-resistant. Essentially all internal surfaces of the cyclone that are subject to erosion are protected with a 2 cm layer of erosion-resistant lining. When installed and cured, most refractory linings are highly resistant to erosion.  [c.218]

Generally speaking corrosion processes in liquid alkali metals are either concerned with dissolution of the component (general or selective), chemical reaction between the component and non-metallic impurities Oj, C, Nj, H2, which are soluble in liquid sodium at the ppm level, or a combination of both processes where dissolution is followed by chemical reaction in the liquid phase. Solubilities of constructional materials —refractory metals and the components of iron and nickel base alloys—in liquid alkali metals are much less than in more noble metals, mercury and bismuth, and solubilities in liquid sodium at 650°C can range from a fraction of a ppm (refractory metals) to 1-10 ppm for metals like Fe, Cr and Ni. Elements such as Ni, Cu and the precious metals have appreciable solubilities in liquid lithium and it is generally considered that alloys of high nickel content have limited use in lithium systems operating with a temperature gradient.  [c.428]

Refractories. Calcined alumina is used in the bond matrix to improve the refractoriness, high temperature strength/creep resistance, and abrasion/corrosion resistance of refractories (1,2,4,7). The normal, coarse (2 to 5 )J.m median) crystalline, nominally 100% a-Al202, calcined aluminas ground to 95% —325 mesh mesh are used to extend the particle size distribution of refractory mixes, for alumina enrichment, and for reaction with  [c.162]

Molybdenum. Molybdenum is the most readily available and widely utili2ed refractory metal. Most engineering appHcations of this metal utili2e the high melting temperature, high strength and stiffness, resistance to corrosion in many environments, or high thermal and electrical conductivity. However, molybdenum must be coated if exposed to air above 535°C. The melting point of 2610°C, over 1000°C higher than for most high temperature superaHoys, permits molybdenum to be used in inert atmosphere furnace equipment. Furnace hardware and heat shields also perform well under extreme temperature conditions. The high thermal and electrical conductivity of molybdenum, as well as its inertness to molten glasses, permits it to be used for electrical heating or heat booster electrodes in commercial continuous glass making operations. Molybdenum also is used in a wide range of electronic and thermionic devices as well as crystal growing devices, x-ray tubes, magnetism and thyristors, and resistance weld electrodes. Other characteristics of molybdenum are its low thermal expansion, high stiffness, and the abiUty to take a high surface finish. Molybdenum is usehil for high temperature laser mirror components such as those to be used in fusion power systems (see Fusionenergy Lasers).  [c.127]

Metallic Nitrides. Properties of metaUic nitrides are Hsted in Table 2. The nitrides of the transition metals of Groups 6 and 7 (IVB—VIIB) generally are termed metallic nitrides because of metallic conductivity, luster, and general metallic behavior. These compounds, characterized by a wide range of homogeneity, high hardness, high melting points, and good corrosion resistance, are grouped with the carbides (qv), borides (see Boron compounds), and siUcides (see Silicon compounds) as refractory hard metals. They crystallize ia highly symmetrical, metal-like lattices. The small nitrogen atoms occupy the interstitial voids within the metallic host lattice forming interstitial alloys similar to the generally isotypic carbides. MetaUic nitrides can be alloyed with other nitrides and carbides of the transition metals to give soHd solutions. Complete soHd solubUity has been demonstrated for a great number of combinations (2). Similarly, oxynitride and oxycarbide interstitial alloys form over wide O—N and C—N composition ranges (3). At high temperatures, aU pseudobinary systems between cubic mononitrides and monocarbides of the 4 (IVB) and 5 (VB) metals show complete miscibility, with the exception of the pairs ZrN—VN, HfN—VN, ZrN—VC, HfN—VC, and HfC—VN. TaN has a cubic high temperature modification that is completely miscible with aU other cubic monocarbides and mononitrides (4,5) (Table 3).  [c.52]

Refractories. Hexagonal boron nitride is a soft white powder and resembles graphite in crystal stmcture, in texture, and in many other properties, except that it is an electrical insulator. It is used in the refractories industry as a mold-facing and release agent. Stmctural parts made of BN are manufactured by hot pressing. Characteristics include low density, easy workabiUty, good heat resistance, and especially good thermal conductivity and stabihty to thermal shock, excellent corrosion resistance, and the abiUty to provide electrical insulation. Boron nitride-based stmctural parts are used as wall liners in plasma-arc devices, such as gas heaters, arc-jet thmsters, and high temperature magnetohydro dynamic devices (47) (see A4agnetohydrodynam ics). BN also is used as a cmcible material for reactive metal melts because of its nonwetting properties. Boron nitride composites, eg, BN—Si N and BN—SiC,  [c.57]

The highest melting refractory metals are tungsten, tantalum, molybdenum, and niobium, although titanium, hafnium, zirconium, chromium, vanadium, platinum, rhodium, mthenium, iridium, osmium, and rhenium may be included (see Refractories). Many of these metals do not resist air oxidation. Hence, very few, if any, are used in elemental form for high temperature protection. However, bulk alloys based on nickel, iron, and cobalt and alloying elements such as chromium, titanium, aluminum, vanadium, tantalum, molybdenum, siHcon, and tungsten are used extensively in high temperature service. Some modem high temperature oxidation- and corrosion-resistant coatings have compositions similar to the high temperature bulk alloys (see High TEMPERATURE alloys) and are appHed by thermal spraying, evaporation, or sputtering. The protection mechanism for these high temperature alloy coatings is based on adherent impervious surface films of AI2O2, Si02, Cr02, or a spinel-type material that grow upon high temperature exposure to air.  [c.40]

Calcium aluminate cement (81) develops very high strengths at eady ages. It attains neady its maximum strength in one day, which is much higher than the strength developed by Pordand cement in that time. At higher temperatures, however, the strength drops off rapidly. Heat is also evolved rapidly on hydration and results in high temperatures long exposures under moist warm conditions can lead to failure. Resistance to corrosion in sea or sulfate waters, as well as to weak solutions of mineral acids, is outstanding. This cement is attacked rapidly, however, by alkah carbonates. An important use of high alumina cement is in refractory concrete for withstanding temperatures up to I500°C. White calcium aluminate cements, with a fused aggregate of pure alumina, withstand temperatures up to I800°C.  [c.296]

At the opposite extreme of fossil fuel usage are single-effect evaporative systems. Conventional steam-heated single-effect evaporators have been discussed. When the BPR is extremely high (as for manufacture of anhydrous NaOH), the evaporator may be heated by molten salt or other high temperature heat-transfer fluids instead of steam. The LTV-type evaporator is normally used in this service. Wiped-film evaporators also sometimes use these high temperature, low pressure heating media to achieve high ATs without need for very heavy heat-transfer wall thicknesses. The simplest single-effect evaporative method brings combustion gases into direct contact with the material being concentrated. A spray dryer can be used in this manner to concentrate Hquids, and has been so used for high BPRHquids, such as CaCl. Less expensive in cost and more efficient in fuel consumption is the submerged combustion evaporator. In this case, the burner is immersed in the solution or slurry being concentrated and the combustion gases rise through the Hquid to release almost all but the latent heat of the water in the combustion gases. Fuel may be either natural gas or the lighter distillate oils. Such evaporators are inexpensive and well adapted to handling corrosive and severely scaling Hquids. They require no heat sink, but since the evolved vapor is mixed with large volumes of combustion gas they make it impractical to achieve much better than single-effect steam economy. These evaporators are impractical for all except very low capacities or the most refractory scale-forming or corrosive Hquids.  [c.479]

A nucleated cryst hne ceramic-metal composite form of glass has superior mechanical properties compared with conventional glassed steel. Controlled high-temperature firings chemically and physically bond the ceramic to steel, nickel-based alloys, and refractory metals. These materials resist corrosive hydrogen chloride gas, chlorine, or sulfur dioxide at 650°C (1,200°F). They resist all acids except HF up to 180°C (350°F). Their impact streuffth is 18 times that of safety glass abrasion resistance is superior to that of porcelain enamel. They have 3 to 4 times the thermal-shock resistance of glassed steel.  [c.2452]

In the roasting of molybdenum sulfide ores, any rhenium which might be present is oxidized to volatile Re207 which collects in the flue dusts and is the usual source of the metal via conversion to (NH4)Re04 and reduction by H2 at elevated temperatures. Being highly refractory and corrosion-resistant, rhenium metal would no doubt find widespread use were it not for its scarcity and consequent high cost. As it is, uses are essentially small scale. These include bimetallic Pt/Re catalysts for the production of lead-free, high octane petroleum products, high temperature superalloys for jet engine components, mass spectrometer filaments, furnace heating elements and thermocouples. World production is about 35 tonnes annually.  [c.1043]

The bath is normally at a temperature in the range 620-710°C, depending on whether the coating material is an aluminium-silicon alloy (for use in high-temperature conditions) or pure aluminium (for corrosion prevention). It is heated by inductors, by resistance heaters or by an external flame. The pot will usually be refractory lined unless cast-iron pots are needed to ensure adequate heat transfer from an external flame. As molten aluminium is extremely aggressive towards ferrous metals, replacement of cast-iron pots is fairly frequent. Refractory-lined pots obviously do not have this drawback, although the bath hardware, in particular the sinker roll and support mechanism, will still be attacked and need replacement at intervals.  [c.392]

It is used extensively by the chemical industry where corrosive agents are employed. Zirconium is used as a getter in vacuum tubes, as an alloying agent in steel, in surgical appliances, photoflash bulbs, explosive primers, rayon spinnerets, lamp filaments, etc. It is used in poison ivy lotions in the form of the carbonate as it combines with urushiol. With niobium, zirconium is superconductive at low temperatures and is used to make superconductive magnets, which offer hope of direct large-scale generation of electric power. Zirconium oxide (zircon) has a high index of refraction and is used as a gem material. The impure oxide, zirconia, is used for laboratory crucibles that will withstand heat shock, for linings of metallurgical furnaces, and by the glass and ceramic industries as a refractory material. Its use as a refractory material accounts for a large share of all zirconium consumed.  [c.56]

The industrial value of furfuryl alcohol is a consequence of its low viscosity, high reactivity, and the outstanding chemical, mechanical, and thermal properties of its polymers, corrosion resistance, nonburning, low smoke emission, and exceUent char formation. The reactivity profile of furfuryl alcohol and resins is such that final curing can take place at ambient temperature with strong acids or at elevated temperature with latent acids. Major markets for furfuryl alcohol resins include the production of cores and molds for casting metals, corrosion-resistant fiber-reinforced plastics (FRPs), binders for refractories and corrosion-resistant cements and mortars.  [c.80]

Air-cooled acid plants are characterized by a large refractory-lined combustion chamber from which waste heat is removed by radiation and convection. The combustion chamber is constmcted of graphite or of carbon steel lined with a single layer of high alumina refractory brick. Refractory units operate at cooler temperatures because of the poorer heat transfer properties of brick compared to graphite. Corrosion of the carbon steel is, surprisingly, not a serious problem as long as the combustion gas stream and refractory stay well-above the dew point of the azeotropic (92% P2O5) phosphoric acid. Air-cooled plants normally operate with about 1—200% excess combustion air to reduce the flame temperature (1000—1700°C) and carry waste heat to the hydrator—absorber, where it is removed by evaporation of water.  [c.327]

Corrosive Compounds Formed During Combustion. Combustion of fuels containing significant amounts of porphyrins and other metallic salts results in formation of low melting point vanadium pentoxide (VjOj), vanadates such as Na20-6V205 or alkali sulfates which are corrosive to metals at combustion temperatures. At high combustion temperatures, T — 2.800°F, and especially when low amounts of excess O, are present, the sub-oxides of vanadium, V2O , or VO2, are formed, and these being refractory, do not cause corrosion. At lower temperatures, T < 2,400°F, and with higher excess air levels, the low melting point V2O5 is formed. Unfortunately, the latter  [c.265]


See pages that mention the term Refractories high-temperature corrosion : [c.133]    [c.163]    [c.1204]    [c.962]    [c.295]    [c.116]    [c.7]    [c.251]    [c.230]    [c.1064]   
Corrosion, Volume 2 (2000) -- [ c.7 , c.11 ]