Transcript CHAPTER 11 High - Temperature Metal – Gas Reactions

CHAPTER 11
High - Temperature
Metal – Gas Reactions
Scaling, Dry Corrosion
At
temp.
increase

Metal
oxidation increase  gas turbines,
rocket engines, furnaces and hightemp. petrochemical process.
Mechanisms and Kinetics
• Pilling-Bedworth Ratio
• “Oxidation resistance should be
related to the volumn ratio of oxide
and metal”
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
• 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 .
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.
For metal ions or oxygen, and
a volume ratio close to 1 to avoid
compressive
complete
stresses
surface
or
lack
coverage.
of
Thus,
oxidation resistance of a metal or alloys
depends on a numbers of complex
factors.
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)
• Metal ions are formed at the metal-scale
interface and oxygen is reduce to
oxygen ions at the scale-gas interface.
• 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.
* 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.
 The oxidation rate is most effectively
retarded in practice by reducing the
flux of ions diffusing through the scale.
• 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
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.
However
because
condensation
at
the
of
vacancy
metal-scale
interface  some of the oxide in the
middle
of
the
scale
“dissociates”
sending cations outward and oxygen
molecules inward through these voids.
• By this dissociative mechanism, such
scales are believed to grow on both
sides.
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
 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.
 Morphological occurrences often
cause
deviate
the oxidation
from
the
electrochemical model.
mechanism
simple
to
ideal
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
mechanisms of pure metals.
the
oxidation
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).
• Other n-type – CdO, TiO2, Ta2O5, Al2O3,
SiO2, Cb2O5 and PbO2
Metal-difficient oxide
Electron hole (or
absence of an
electron)
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
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.
Oxidation Kinetics
Fig 11-6 Oxidation rate laws.
W
=
kL =
kLt
linear rate const.
• porous or cracked scale is formed
• Na, K  R < 1
• Ta, Cb  R = 2.5
W2 =
kpt + C
• Ideal
ionic
diffusion-controlled
oxidation of pure metals  paraboric
oxidation rate law.
kp
=
parabolic rate const.
C
=
const.
• Non-steady-state
reactions.
diffusion-controlled
 oxide layer thickness increase.
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.
W
Where
=
ke log (Ct + A)
ke, C and A are const.
1/w =
Where
C – ki log t
ki and C are const.
• Logarithmic
oxidation
behavior
is
generally observed with thin oxide
layers (e.g., less than 1000 angstroms)
at low temp.
• Aluminum, Copper, Iron.
• The
exact
mechanism
completely understood.
• Under specific conditions.
W3 =
kct + C
kc and C are const.
is
not
• 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.
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)
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.
1. n-Type Oxides
(Metal Excess-eq. ZnO)
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.
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
decreased.
oxidation
rate
would
be
2) p-Type Oxides
(Metal Deficient-eg. NiO)
b) The addition of higher valency cations
increases vacancy concentration and
decreases electron hole concentration. A
diffusion-controlled oxidation rate would
be increased.
Table 11-2 Oxidation of zinc and zinc alloys
3900C, 1 atm O2
Material
Parabolic oxidation
constant Kp,
g2/cm2-hr
Zn
8x10-10
Zn + 1.0 atomic %Al
1x10-11
Zn + 0.4 atomic %Li
2x10-7
Table 11-3 Oxidation of nickel and nickel alloys
Nickel and chromium-nikel alloys at 10000C in pure oxygen
Wt. %Cr
Parabolic oxidation
constant Kp,
g2/cm2-hr
0
3.8x10-10
0.3
15x10-10
1.0
28x10-10
3.0
36x10-10
10.0
5.0x10-10
NiCr2O4
Table 11-3 Oxidation of nickel and nickel alloys
Effect of lithium oxide vapor on the oxidation of nickel at
10000C in oxygen
Atmosphere
2.5x10-10
O2
O2
Parabolic oxidation
constant Kp,
g2/cm2-hr
Li2O
5.8x10-11
Catastrophic Oxidation
Metal-oxygen systems which react
at
continuously
increasing
rates.
 linear oxidation kinetics  rapid,
exothermic reaction at their surfaces.
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.
Ex. Columbium (Niobium), ignition
of the metal occurs. Mo, tungsten,
osmium
and
vanadium

volatile
oxides may oxidize catastrophically.
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.
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.
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
 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.
Hydrogen and Hydrocarbon Gases
C(Fe) + 2H2
=
CH4
The equilibrium between carbon
steel
and
mixtures
hydrogen
can
be
thermodynamic data.
methane
obtained
gas
from
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
cracking
improves
and
its
resistance
decarburization
hydrogen atmospheres.
to
in
Hydrogen and water Vapor
C(Fe) + H2O
=
H2
+
CO
• Carbides and carbon react with water
vapor
to
form
hydrogen
carbonmonoxide.
Fe
+ H 2O
=
FeO + H2
• Thus.
In
hydrogen-water
vapor
environments both decarburization
and oxidation are possible.
Equilibriums in the Fe-O-H system.
Fig. 11-14 Equilibrium diagram of the Fe-H2-H2O system.
Carbon Monoxide-Carbon Dioxide
Mixtures.
C(Fe)
Fe +
+
CO2
CO2
=
=
2CO
FeO + CO
Hydrogen Sulfide and
Sulfur-containing Gases.
H2S - a frequent component of hightemperature gases.
- act as an oxidizing agent in the
formation of sulfide scales on
metal substances at high temp.
In general, nickel and rich alloys are
usually rapidly attacked in the presence
of hydrogen sulfide and other sulfur-
bearing
gases. Attack
catastrophic
with
rapid
is
frequently
intergranular
penetration by a liquid sulfide product
and subsequent disintegration of the
metal.
Iron-base alloys are often used to
contain hydrogen sulfide environment
because of their low cost and good
chemical resistance.