Criterion for High Temperature Oxidation

Thermodynamics of Oxidation

An oxidation reaction between a metal (M) and the oxygen gas (O2) canbe written as:

M(s) + O2(g) = MO2(s) At Equilibrium ∆G = 0

∆Go = - RT ln ( aMO2 /aMp(O2)

Activity of pure metal and solid oxide is usually 1 so

∆Go = RT ln p(O2)

p(O2) = exp (∆Go /RT)Above Equation permits the determination of the partial pressure of oxygen in equilibrium

Formation of oxides as a function of temperature is determined by Ellingham/Richardson diagrams, which are used to get information about the partial pressure of oxygen required for any metal to form oxide at any temperature.

Generalized Treatment of Ellingham Diagram

Let us assume ∆Go is a complex function of Temperature

∆Go= A + BT + C/T + DT2

Assuming a straight line behvaiour of ∆Go with temperature terms like 1/T and T2 will be removed and ∆Go will appear as:

∆Go = A + BT

This equation if compared with the standard free energy change of a reaction in terms of

its enthalpy and entropy

∆Go = ∆Ho– T ∆So

6/21/2013 4

Salient Features of Ellingham Diagram

• The free energy of most of the metals shows linear relationship with temperature with a positive slope.

4Ag(s) + O2(g) = 2Ag2O(s)

4/3 Al(s) + O2(g) = 2/3 Al2O3(s)

Only in the case of the following two reactions is the entropy not negative:

C(s) + O2(g) = CO2(g)

2C(s) + O2(g) = 2CO(g)

• The change in the slope of the lines at certain points indicates a phase change in the metal. This could be melting, boiling or a change in structure.

• The most stable oxide has the largest negative value of ∆Go

and is represented by the lowest line in the diagram.

• Determination of oxidation potential values

Construction of Ellingham Diagram

∆Go= - RT ln ( p2CO2/p2CO.p(O2 )

How to Achieve Such a low partial pressure of Oxygen

∆Go = -511kJ/mole ∆Go = -564.8kJ/mole

ExampleFor Al the p(O2) at 1000oC is 10-14 atm. Thus using H2/H2O relation

10-14 = exp ( -511 kJ/m / 8.3kJ/m x 1273 ) ( pH2 / pH2O )2

-14 = -511 / 8.3 x 1273 ( pH2 / pH2O )2

pH2 / pH2O = 17Thus a partial pressure of Oxygen of 10-14 will achieve by making a gasmixture of H2 and H2) in the ratio of 1:17.

Ellingham Diagram for Sulphides

Ellingham Diagram for Carbides

Kinetics of Oxidation

Logarithmic Kinetics

Parabolic Law

dx/dt 1/x

x – oxide thickness

dx/dt = k/x

k – proportionality constant

xdx = k dt

x2 = kt

x = kt1/2

Linear LawParalinear LawCubic Law

Types of Oxidation

• Isothermal Oxidation

• Cyclic Oxidation

• Oxidation Rate measurement

– Gravimetric Methods

– Thermogravimetric Methods

( based upon Principle of Thermal Analysis)