CHAPTER 11

High - Temperature

Metal – Gas Reactions

Scaling, Dry Corrosion

At temp. increase Metal

oxidation increase gas turbines,

rocket engines, furnaces and high-

temp. 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 stresses or lack of

complete surface coverage. 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 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.

• 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 the oxidation mechanism to

deviate from the simple ideal

electrochemical model.

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.

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 = kLt

kL = 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 diffusion-controlled

reactions.

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 = 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.

• Aluminum, Copper, Iron.

• The exact mechanism is not

completely understood.

• Under specific conditions.

W3 = kct + C

kc and C are const.

• 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.

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)

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.

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)

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

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

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

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 hydrogen methane gas

mixtures can be obtained from

thermodynamic data.

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.

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.

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) + CO2 = 2CO

Fe + CO2 = FeO + CO

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.

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.

Iron-base alloys are often used to

contain hydrogen sulfide environment

because of their low cost and good

chemical resistance.