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CHAPTER 11 High - Temperature Metal – Gas Reactions.

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Presentation on theme: "CHAPTER 11 High - Temperature Metal – Gas Reactions."— Presentation transcript:

1 CHAPTER 11 High - Temperature Metal – Gas Reactions

2 Scaling, Dry Corrosion At temp. increase  Metal oxidation increase  gas turbines, rocket engines, furnaces and high- temp. petrochemical process.

3 Mechanisms and Kinetics Pilling-Bedworth Ratio “Oxidation resistance should be related to the volumn ratio of oxide and metal”

4 R=Wd / Dw W= molecular wt. of the oxide w=atomic wt. of the metal D,d=the specific densities of the oxide and metal

5 Volumn ratio < 1  insufficient oxide to cover the metal and is unprotective. Volumn ratio >1  large compressive stresses  poor oxidation resistance  cracking and spalling. The ideal ratio = close to 1 This ratio does not accurately predict oxidation resistance.

6 To be protective an oxide must posses a coefficient of expansion nearly equal to that of the metal substrate, good adherence, a high melting pt., a low vapor pressure, good high-temp. plasticity to resist fracture, low electrical conductivity or low diffusion coeff.

7 For metal ions or oxygen, and a volume ratio close to 1 to avoid compressive stresses or lack of complete surface coverage. Thus, oxidation resistance of a metal or alloys depends on a numbers of complex factors.

8 Electrochemical and Morphological Aspects of Oxidation Oxidation by gaseous oxygen is an electrochemical process. M M +2 + 2e (at the metal – scale interface) 1/2O 2 + 2e O -2 (at the scale-gas interface) M + 1/2O 2 MO(Overall)

9

10 Metal ions are formed at the metal-scale interface and oxygen is reduce to oxygen ions at the scale-gas interface.

11 Oxide layer serves simultaneously as 1. an ionic conductor (electrolyte) 2. an electronic conductor. 3. an electrode at which oxygen is reduced. 4. a diffusion barrier through which ions and electrons must migrate.

12 * The electronic conductivities of oxides are usually one or more orders of magnitude greater than their ionic conductivities, so that the movement of either cations or oxygen ions controls the reaction rate.

13  The oxidation rate is most effectively retarded in practice by reducing the flux of ions diffusing through the scale.

14 Many metal-oxygen phase diagrams indicate several stable binary oxides. For ex., iron may from the compounds FeO, Fe 3 O 4 and Fe 2 O 3 ; copper may form Cu 2 O and CuO; etc. Fe above 560 0 C. Fe/FeO/Fe 3 O 4 /Fe 2 O 3 /O 2

15 The most oxygen-rich compound is found at the scale-gas interface 1. Scales formed on the common base metals Fe, Ni, Cu, Co and others grow principally at the scale-gas interface by outward cation diffusion.

16 However because of vacancy condensation at the metal-scale interface  some of the oxide in the middle of the scale “dissociates” sending cations outward and oxygen molecules inward through these voids.

17 By this dissociative mechanism, such scales are believed to grow on both sides.

18 2. More tradition base metals as Ta, Nb, Hf, Ti and Zr form oxides in which oxygen-ion diffusion would predominate over cation diffusion, so that simple diffusion control would result in scale formation at the metal-scale interface

19  the oxide formed at the metal- scale interface (with a large increase in volumn) is porous on a microscopic scales and is cracked on a macroscopic scale  these scales are said to be nonprotective.

20  Morphological occurrences often cause the oxidation mechanism to deviate from the simple ideal electrochemical model.

21 In addition, the significant dissolution of oxygen atoms in some metals, the high volatility of some oxides and metals, the low melting points of some oxides, and grain boundaries in the scale and in the metal often complicate the oxidation mechanisms of pure metals.

22 Oxide Defect Structure In general, all oxides are nonstoichiometric compounds Metal-excess oxide ZnO 2 extra zinc ions 4 excess electron Zinc oxide is termed an n-type semiconductor since it contains an excess of negatively charged electronic current carriers (electrons).

23 Other n-type – CdO, TiO2, Ta2O5, Al2O3, SiO2, Cb2O5 and PbO2 Metal-difficient oxide Electron hole (or absence of an electron)

24 Electronic conduction occurs by the diffusion of these positively charged electron holes  this oxide is termed a p-type semiconductor. Ionic transport occurs by the diffusion of the nickel vacancies. Other oxides of this type are FeO, Cu 2 O, Cr 2 O 3 and CoO

25 Summarizing, the oxidation of some metals is controlled by the diffusion of ionic defects through the scale. In principle, a diffusion-controlled oxidation may be retarded by decreasing the concentration of ionic defects in the scale.

26 Oxidation Kinetics Fig 11-6 Oxidation rate laws.

27 W=k L t k L =linear rate const. porous or cracked scale is formed Na, K  R < 1 Ta, Cb  R = 2.5

28 W 2 =k p t + C Ideal ionic diffusion-controlled oxidation of pure metals  paraboric oxidation rate law. k p =parabolic rate const. C=const. Non-steady-state diffusion-controlled reactions.  oxide layer thickness increase.

29 The ionic diffusion flux is inversely proportional to the thickness of the diffusion barrier, and the change in scale thickness or weight is likewise proportional to the ionic diffusion flux.

30 W=k e log (Ct + A) Wherek e, C and A are const. 1/w=C – k i log t Wherek i and C are const. Logarithmic oxidation behavior is generally observed with thin oxide layers (e.g., less than 1000 angstroms) at low temp.

31 Aluminum, Copper, Iron. The exact mechanism is not completely understood. Under specific conditions. W 3 =k c t + C k c and C are const.

32 Oxidation of Zirconium  combination of diffusion-limited scale formation and oxygen dissolution into the metal. * linear oxidation rate is the least desirable. Aluminum oxidizes in air at ambient temp. according to thelogarithmic rate law.

33 Effect of Alloying The concentration of ionic defects (interstitial cations and excess electrons, or metal ion vacancies and electron holes) may be influenced by the presence of foreign ions in the lattice (the doping effect)

34 1.n-Type Oxides (Metal Excess-eq. ZnO) a) Introduction of lower vacancy metallic ions into the lattice increase the concentration of interstitial metallic ions and decreases the number of excess electrons. A diffusion-controlled oxidation rate would be increased.

35 b) Introduction of metallic ions possessing higher valency decreases the concentration of interstitial metallic ions and increases the number of excess electrons. A diffusion-controlled oxidation rate would be decreased. 1.n-Type Oxides (Metal Excess-eq. ZnO)

36 2) p-Type Oxides (Metal Deficient-eg. NiO) a) The incorporation of lower vacancy cations decreases theconcentration of cation vacancies and increases the number of electron holes. A diffusion controlled oxidation rate would be decreased.

37 b) The addition of higher valency cations increases vacancy concentration and decreases electron hole concentration. A diffusion-controlled oxidation rate would be increased. 2) p-Type Oxides (Metal Deficient-eg. NiO)

38 Table 11-2 Oxidation of zinc and zinc alloys 390 0 C, 1 atm O 2 Material Parabolic oxidation constant K p, g 2 /cm 2 -hr Zn Zn + 1.0 atomic %Al Zn + 0.4 atomic %Li 8x10 -10 1x10 -11 2x10 -7

39 Table 11-3 Oxidation of nickel and nickel alloys Nickel and chromium-nikel alloys at 1000 0 C in pure oxygen Wt. %Cr Parabolic oxidation constant K p, g 2 /cm 2 -hr 0 0.3 1.0 3.0 10.0 3.8x10 -10 15x10 -10 28x10 -10 36x10 -10 5.0x10 -10 NiCr 2 O 4

40 Table 11-3 Oxidation of nickel and nickel alloys Effect of lithium oxide vapor on the oxidation of nickel at 1000 0 C in oxygen Atmosphere Parabolic oxidation constant K p, g 2 /cm 2 -hr O 2 O 2 Li 2 O 2.5x10 -10 5.8x10 -11

41 Catastrophic Oxidation Metal-oxygen systems which react at continuously increasing rates.  linear oxidation kinetics  rapid, exothermic reaction at their surfaces.

42 If the rate of heat transfer to the metal and surroundings is less than the heat produced by the reaction, surface temp. increases  chain-reaction characteristic-temp. and reaction rate increases.

43 Ex. Columbium (Niobium), ignition of the metal occurs. Mo, tungsten, osmium and vanadium  volatile oxides may oxidize catastrophically.

44 The formation of low-melting eutectic oxide mixtures produces a liquid beneath the scale, which is less protective. Catastrophic oxidation can also occur if vanadium oxide or lead oxide compounds are present in the gas phase.

45 Internal Oxidation In certain alloy systems, one or more dilute components which may form more stable oxides than the base metal may oxidize preferentially below the external surface of the metal, or below the metal scale interface. Dilute copper-and silver-base alloys containing Al. Zn, Cd, Be, etc. show this kind of oxidation.

46 Other Metal-Gas Reactions Decarburization and Hydrogen attack At elevated temp. hydrogen can influence the mechanical properties of metal in a variety of ways. decarburization or removal of carbon from an alloy

47  reduction of tensile strength and an increase in ductility and creep rate Reverse process, carburization, can also occur in hydrogen-hydrocarbon gas. (petroleum refining operations)  decreases its ductility and remove certain solid-solution elements through carbide precipitation.

48 Hydrogen and Hydrocarbon Gases C(Fe) + 2H 2 = CH 4 The equilibrium between carbon steel and hydrogen methane gas mixtures can be obtained from thermodynamic data.

49 Because atomic hydrogen diffused readily in steel, cracking may result from the formation of CH4 in internal voids in the metal. Chromium and Mo additions to a steel improves its resistance to cracking and decarburization in hydrogen atmospheres.

50 Hydrogen and water Vapor C(Fe) + H 2 O = H 2 + CO Carbides and carbon react with water vapor to form hydrogen carbonmonoxide. Fe + H 2 O = FeO + H 2 Thus. In hydrogen-water vapor environments both decarburization and oxidation are possible.

51 Equilibriums in the Fe-O-H system. Fig. 11-14 Equilibrium diagram of the Fe-H 2 -H 2 O system.

52 Carbon Monoxide-Carbon Dioxide Mixtures. C(Fe) + CO2 = 2CO Fe + CO2 = FeO + CO

53 Hydrogen Sulfide and Sulfur-containing Gases. H 2 S - a frequent component of high- temperature gases. - act as an oxidizing agent in the formation of sulfide scales on metal substances at high temp.

54 In general, nickel and rich alloys are usually rapidly attacked in the presence of hydrogen sulfide and other sulfur- bearing gases. Attack is frequently catastrophic with rapid intergranular penetration by a liquid sulfide product and subsequent disintegration of the metal.

55 Iron-base alloys are often used to contain hydrogen sulfide environment because of their low cost and good chemical resistance.


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