16.1. Oxidation of metals

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16.1. Oxidation of metals

• Most metals are stable as oxides and are
  extracted from metal oxides.
• Metal oxides have higher melting temperatures
  than metals (e.g. 2054 ºC for Al2O3, 660 ºC for
  pure Al).
• Most metals will react with oxygen to form
  oxides, and will release heat.
• Oxidation occurs at metal surfaces.

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Oxidation reactions
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16.2 Why interest in oxidation?

• Property deterioration at high-T
• Turbine blades
• Engine valves
• Heating elements
• Protection against corrosion at low-T
• Stainless steels
• Al
• Cr plating

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16.3 Protective and non-protective oxide
film/scale
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• Metal reacts with oxygen to form an oxide scale
  on the surface.
• The oxide does not take the same volume as the
  metal.

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• Rlt1, oxide layer is stretched. Oxygen
  infiltrates through the porous oxide layer to
  enable further oxidation.
• Rgt2, large compressive stress breaks the oxide to
  expose fresh surface. Oxidation continues.
• 1ltRlt1.5 the oxide layer is compressed and dense.
  Oxidation is slow.

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R of protective oxides
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R of non-protective oxides
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Other factors affecting oxidation

• Difference in coefficient of thermal expansion
  cyclic oxidation.
• Bonding/adhesion between the oxide layer and the
  metal.
• Coatings

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16.4. Metals protected by oxide film

• Stainless steel protected by Cr2O3.
• Chrome plating protected by Cr2O3
• Al alloys protected by Al2O3.
• Anodized aluminum coating of Al2O3 coating in
  acid solution.
• Tarnish resistant silver Ag1Al, protected by
  Al2O3.

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Alloy design strategy to improve oxidation
resistance

• To alloy with Cr, Al or Si to form dense scales
  of Cr2O3, Al2O3, SiO2.
• Tradeoff/cost loss of ductility

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Effect of Al, Si and Al on oxidation of Fe
O Kubaschewski and B.E. Hopkins, Oxidation of
Metals and Alloys, Butterworth, 1962.
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Temperature limit for oxidation
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16.5. Rate of oxidation
ki growth constant
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Linear oxidation

• Oxidation rate remains constant with time.
• Associated with non-protective oxide film
• No barrier to oxygen atoms, hence constant
  oxidation rate

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Parabolic oxidation

• Oxidation rate decreases with time
• Associated with protective oxide film
• Oxygen atoms need to diffuse through a dense
  oxide layer the thickness of the layer increases
  with time

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Empirical formula for oxidation resistance of
Ni-based superalloys
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16.6 Diffusion

• Flow of atoms from regions of high
  concentration to regions of low concentration

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Ficks law of diffusion

• J flux. Number of atoms crossing a unit area per
  unit time
• dc/dx concentration gradient
• D diffusion coefficient, a material property

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Diffusivity or diffusion coefficient

• D is higher at high temperatures
• D is higher for smaller diffusing atoms

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Uses of diffusion

• Oxidation
• Doping of semiconductors
• Surface hardening, e.g. by carburization,
  nitriding
• Surface softening, e.g. decarburization