Dr. Dmitri Kopeliovich

High temperature oxidation of a metal is a corrosion process involving the reaction between the metal and the atmospheric Oxygen at elevated temperatures.

General equation of oxidation reaction of a metal M:
nM + 1/2kO₂ = M_nO_k

• Types of oxide layers
• Kinetics of high temperature oxidation
• High temperature oxidation of alloys
• Effect of oxide structure on oxidation
• Internal oxidation

High temperature oxidation usually results in formation of an oxide layer on the surface of the oxidizing metal.
Thin oxide layers (commonly thiner than 3000 Å) are called films. Thicker oxide layers (above 3000 Å) are called scales.
(1.0 Å = 10^-10 m)

Oxide film does not form if the partial pressure of oxygen is lower than the oxide’s dissociation pressure of a particular metal. Such conditions are characteristic for the noble metals (eg. gold, platinum, silver) oxide films of which are unstable at the temperatures below 932ºF (500ºC).

Oxides of some metals (molybdenum, osmium) are volatile. The oxide molecules evaporate from the oxide layer surface in parallel to the oxidation process. The rates of the two processes (volatilization and oxidation) equalize at a particular thickness of the oxide layer.

Oxide scale may be composed of several layers of different oxides. At the temperatures above 1050ºF (566ºC) iron scale consists of three layers: FeO (the layer adjacent to iron), Fe₃O₄ (middle layer) and Fe₂O₃ (surface layer).

Depending on their structures the scales may be categorized into two groups: protective scales and non-protective scales.

• Protective scale prevents access of oxygen to the metal surface due to non-porous continuous structure of the oxide layer.
  
• Non-protective scale has loose porous structure providing free access of oxygen to the underlaying metal.
  

The scales type may be determined by the Pilling-Bedworth rule:

• The scale is protective (adherent and non-porous) if the volume of the oxide is not less than the volume of metal, from which the oxide was formed.
  
• The scale is non-protective (porous) if the volume of the oxide is less than the volume of metal, from which the oxide was formed.
  

Oxides with volume much greater (twice and more) than the volume of metal, from which the oxide was formed cause developing compressive stresses. The stresses may lead to cracking and spalling of the scale, which result in faster penetration of oxygen to the metal surface.

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The structure of an oxide scale determines the low, according to which the scale weight increases:

• Non-protective (porous) scales are formed in the process, rate of which is independent on the scale thickness due to fast transfer of oxygen to the metal surface. The process rate is controlled by the reaction of oxidation. In this case the process rate is constant:
  

dW/dt = k_L
The weight-time dependence is linear:
W = k_L*t
Where:
W - weight of the scale per unit area;
t - time;
k_L - constant dependent on the metal and the temperature.

• Protective (adherent and non-porous) scales are formed in the process, rate of which is controlled by diffusion of oxygen through the oxide scale. The weight-time dependence obtained from the First Fick's law follows the parabolic law:
  

W² = k_P*t
Where:
k_P - constant dependent on the metal and the temperature.

• Thin protective films formed at lower temperatures are described by asymptotic logarithmic law providing more rapid decrease of rate with time:
  

dW/dt = k_e/t
W = k_elog(at+1)
Where:
k_e, a - constants dependent on the metal and the temperature.

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Oxidation of alloys is more complex process than oxidation of pure metals.
The formed oxides may either solve in each other or form separate phases.
Some elements when added to the alloy in small quantities exert disproportionate effect on the scale properties (eg. 0.1% of cerium in nickel-chromium alloys).
Contents of metals in the oxide scale differ from the alloy composition due to the following factors:

• The alloy components may have different affinities for oxygen. Some of them oxidize more rapidly than other. In some alloys only one component (most reactive) oxidizes - the process is called selective oxidation.
• Different alloy components (metal ions) may have different diffusion coefficients in the oxide and alloy, which causes preferential oxidation of the component having higher diffusivity.
• Complex (ternary and higher) oxide compounds may be formed in oxidation of alloys.
• Some alloy components may oxidize out of the scale - within the alloy below the metal-scale interface (Internal oxidation).

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Most of oxides are not ideal. Their compositions differ from the stoichiometric ratios. The oxide structure may be divided into two groups:

• n-type oxides with anion deficiency (ZnO, ZrO₂, MgO, Al₂O₃, SiO₂, SnO₂, PbO₂). Anion: negatively charged ion (oxygen).
• p-type oxides with cation deficiency (NiO, CoO, FeO, PbO, MnO, Cu₂O). Cation: positively charged ion (metal).

Additions of alloying elements having a valencies, which differ from the valency of the base metal may effect on the oxidation rate if it is controlled by diffusion:

• In n-type oxides.
  • Addition of higher valency cation (eg. addition of Al to Zn) results in lowering the oxidation rate.
  • Addition of lower valency cation (eg. addition of Li to Zn) results in increasing the oxidation rate.
• In p-type oxides.
  • Addition of higher valency cation (eg. addition of Cr to Ni) results in increasing the oxidation rate.
  • Addition of lower valency cation (eg. addition of Ni to Cr) results in lowering the oxidation rate.

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Internal oxidation is oxidation of an alloying element within the matrix of the alloy.

Internal oxidation may occur either with presence of the oxide scale or in the absence of any film or scale.
Mechanism of internal oxidation involves diffusion of Oxygen inward (below the metal surface), nucleation of oxides, growth of the oxide particles due to diffusion of both ions of oxygen and the ions of alloying element.

Internal oxidation takes place under th following conditions:

• The alloying element has a greater affinity for oxygen than the matrix metal.
• Diffusivity of oxygen in the matrix should be greater than that of the alloying element.
• Penetration of the internal oxidation inward should be faster than the rate of the scale formation.

Internal oxidation is used for surface dispersion hardening of alloys.
Internal oxidation zone cannot be removed by conventional descaling methods.

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• Electrode potentials
• Polarization
• Pourbaix diagrams
• Galvanic corrosion
• Pitting corrosion
• Crevice corrosion
• Intergranular corrosion
• Selective corrosion (dealloying)
• Stress corrosion cracking
• Corrosion fatigue
• Cathodic protection
• Corrosion protection coatings
• Corrosion inhibitors
• Volatile corrosion inhibitors (VCI)

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