8.8A: Ellingham Diagrams

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It is possible to use plots of the free energy of formation of metal oxides vs. temperature to predict the temperatures at which a metal is stable and the temperatures at which it will spontaneously oxidize. For temperatures at which the free energy of formation of the oxide is positive, the reverse reaction is favored and the oxide will spontaneously decompose to the metal. From evaluation of the thermodynamic data presented in this figure, it can be seen that at 1100oC, Al will oxidize in an environment that has an oxygen partial pressure of 10-32 atm or greater, while chromium will oxidize in an oxygen partial pressure of 10-19 atm or higher. In general, a vacuum environment will be oxidizing to these elements unless a reducing species such as hydrogen is present. If inadequate oxygen is present, a nonprotective oxide film may be formed which could promote alloy depletion and loss of strength.