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CHAPTER 11
High - Temperature
Metal – Gas Reactions
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Scaling, Dry Corrosion
At temp. increase Metal
oxidation increase gas turbines,
rocket engines, furnaces and high-
temp. petrochemical process.
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Mechanisms and Kinetics
• Pilling-Bedworth Ratio
• “Oxidation resistance should be
related to the volumn ratio of oxide
and metal”
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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
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• 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 .
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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.
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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.
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Electrochemical and Morphological Aspects of Oxidation
Oxidation by gaseous oxygen is an
electrochemical process.
M M+2 + 2e
(at the metal – scale interface)
1/2O2 + 2e O-2
(at the scale-gas interface)
M + 1/2O2 MO (Overall)
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• Metal ions are formed at the metal-scale
interface and oxygen is reduce to
oxygen ions at the scale-gas interface.
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• 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.
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* 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.
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The oxidation rate is most effectively
retarded in practice by reducing the
flux of ions diffusing through the scale.
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• Many metal-oxygen phase diagrams
indicate several stable binary oxides.
For ex., iron may from the compounds
FeO, Fe3O4 and Fe2O3; copper may
form Cu2O and CuO; etc.
• Fe above 5600C.
• Fe/FeO/Fe3O4/Fe2O3/O2
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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.
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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.
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• By this dissociative mechanism, such
scales are believed to grow on both
sides.
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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
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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.
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Morphological occurrences often
cause the oxidation mechanism to
deviate from the simple ideal
electrochemical model.
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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.
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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).
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• Other n-type – CdO, TiO2, Ta2O5, Al2O3,
SiO2, Cb2O5 and PbO2
Metal-difficient oxide
Electron hole (or
absence of an
electron)
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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, Cu2O, Cr2O3 and CoO
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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.
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Oxidation Kinetics
Fig 11-6 Oxidation rate laws.
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W = kLt
kL = linear rate const.
• porous or cracked scale is formed
• Na, K R < 1
• Ta, Cb R = 2.5
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W2 = kpt + C
• Ideal ionic diffusion-controlled
oxidation of pure metals paraboric
oxidation rate law.
kp = parabolic rate const.
C = const.
• Non-steady-state diffusion-controlled
reactions.
oxide layer thickness increase.
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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.
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W = ke log (Ct + A)
Where ke, C and A are const.
1/w = C – ki log t
Where ki and C are const.
• Logarithmic oxidation behavior is
generally observed with thin oxide
layers (e.g., less than 1000 angstroms)
at low temp.
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• Aluminum, Copper, Iron.
• The exact mechanism is not
completely understood.
• Under specific conditions.
W3 = kct + C
kc and C are const.
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• 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.
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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)
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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.
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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)
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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.
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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)
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Table 11-2 Oxidation of zinc and zinc alloys
3900C, 1 atm O2
MaterialParabolic oxidation
constant Kp, g2/cm2-hr
Zn
Zn + 1.0 atomic %Al
Zn + 0.4 atomic %Li
8x10-10
1x10-11
2x10-7
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Table 11-3 Oxidation of nickel and nickel alloysNickel and chromium-nikel alloys at 10000C in pure oxygen
Wt. %CrParabolic oxidation
constant Kp, g2/cm2-hr
0
0.3
1.0
3.0
10.0
3.8x10-10
15x10-10
28x10-10
36x10-10
5.0x10-10 NiCr2O4
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Table 11-3 Oxidation of nickel and nickel alloysEffect of lithium oxide vapor on the oxidation of nickel at
10000C in oxygen
AtmosphereParabolic oxidation
constant Kp, g2/cm2-hr
O2
O2 Li2O
2.5x10-10
5.8x10-11
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Catastrophic Oxidation
Metal-oxygen systems which react
at continuously increasing rates.
linear oxidation kinetics rapid,
exothermic reaction at their surfaces.
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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.
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Ex. Columbium (Niobium), ignition
of the metal occurs. Mo, tungsten,
osmium and vanadium volatile
oxides may oxidize catastrophically.
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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.
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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.
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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
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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.
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Hydrogen and Hydrocarbon Gases
C(Fe) + 2H2 = CH4
The equilibrium between carbon
steel and hydrogen methane gas
mixtures can be obtained from
thermodynamic data.
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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.
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Hydrogen and water VaporC(Fe) + H2O = H2 + CO
• Carbides and carbon react with water
vapor to form hydrogen
carbonmonoxide.
Fe + H2O = FeO + H2
• Thus. In hydrogen-water vapor
environments both decarburization
and oxidation are possible.
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Equilibriums in the Fe-O-H system.
Fig. 11-14 Equilibrium diagram of the Fe-H2-H2O system.
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Carbon Monoxide-Carbon Dioxide
Mixtures.
C(Fe) + CO2 = 2CO
Fe + CO2 = FeO + CO
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Hydrogen Sulfide and
Sulfur-containing Gases.
H2S - a frequent component of high-
temperature gases.
- act as an oxidizing agent in the
formation of sulfide scales on
metal substances at high temp.
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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.
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Iron-base alloys are often used to
contain hydrogen sulfide environment
because of their low cost and good
chemical resistance.