Oxide Film Formation on Metals and Alloys by Thermal, Electrochemical and Plasma Oxidation and Prediction of Resulting S
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oxidation process as a potential, Ad1, across the growing oxide formed by the charges at the metal-oxide (+) and the gas-oxide (-) interfaces. This potential is described as the sum of two terms: the free energy of formation of oxygen anions at the surface and a term consisting of temperature, oxygen activity and the thickness of the oxide layer. In order to fit the experiments the model had to be extended to take the alloy composition into account. Considering the electrochemical cell and the total potential as the sum of the potentials for each oxide of each metal component, Ad1(total) = AFPj + AP 1,,2+.... leads to the following expression that was proposed by Cocke and Naugle [5]:
-AGO
flu +
2be
-AGO
f"
2 de
+
kT 2be
In
kT 2 de
(2be
)b Nbab 2Ia Xb
kTcbeboaM E a
+
,
2 [1(2dedN dad ' a
I,
10
0n
Xd] M,
kT e eda "
M .
(1)
The terms in equation (1) are the free energies of oxide formation per mole of 02- (-AG~t~) for the alloy components M and M2, the stoichiometric factors from the cathode and anode reactions a, b, c and d, the oxygen, metal and metal ion activities a,, the number of surface 02- N,, the oxide layer thickness X, the absolute temperature T, the Boltzmann constant k, the relative dielectric constant 6 and the dielectric constant in vacuum c,. We are currently testing this model in thermal, plasma and anodic oxidation on binary and ternary alloys. Together with the earlier thermal oxidation work on binary alloys [6-15] these studies are providing insight into the fundamental issue in alloy oxidation -- how the chemistry and physics of one component are affected by neighboring atoms of the other components. In other words, how are the AG's of oxide formation, the intrinsic electric fields and transport mechanisms affected. It is anticipated that extension of the model and consistently more insight can be gained by comparison of the oxidation of a single alloy exposed to the different oxidizing environments of thermal, plasma and anodic methods. Equation I predicts that at low temperatures the free energy of formation of the oxide compared on a per mole of oxide ion basis will determine which alloy components will oxidize first. The oxide with the larger negative values has the kinetic preference for formation. Using the free energies has been shown to be an excellent predictive tool in the oxidation of titanium and titanium-copper alloys [5]. At higher temperatures, the second alloy component has been observed to oxidize and segregate to the surface forming an outer oxide overlayer [5]. Equation I predicts that the oxidation will be temperature dependent with increasing driving potential for oxidation at increasing temperatures. At some elevated temperature, provided the oxygen activity is sufficiently high, the potential will become sufficient to cause oxidation of the more 'noble component'. The modified Cabrera-Mott model does a good job of qualitatively describing the oxidation of binary alloys, when their components' free energies of oxide formation, -AG'f (kJ/mole 02-), are s
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