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 = MnOk
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.
The scales type may be determined by the Pilling-Bedworth rule:
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.
The structure of an oxide scale determines the low, according to which the scale weight increases:
dW/dt = kL
The weight-time dependence is linear:
W = kL*t
W - weight of the scale per unit area;
t - time;
kL - constant dependent on the metal and the temperature.
W2 = kP*t
kP - constant dependent on the metal and the temperature.
dW/dt = ke/t
W = kelog(at+1)
ke, a - constants dependent on the metal and the temperature.
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:
Most of oxides are not ideal. Their compositions differ from the stoichiometric ratios. The oxide structure may be divided into two groups:
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: