Ellingham Diagram

Last updated: January 13, 2014

What Does Ellingham Diagram Mean?

An Ellingham diagram is a tool widely used in the field of extraction metallurgy in order to determine conditions such as the necessary temperature needed to reduce the ores of essential metals.

The diagram is a plot of Gibbs free energy versus temperature. This form of energy reaction measures the required thermodynamic driving force to trigger a reaction. A positive value means a reaction will not occur, while a negative value indicates otherwise. This tool is very useful in various industries for monitoring and preventing corrosion.


Corrosionpedia Explains Ellingham Diagram

In an Ellingham diagram, the Gibbs free energy of a reaction is plotted against temperature, which can be illustrated in a chain of straight lines where there is a Y-intercept and a slope. The slope changes as the subjected material vaporizes or melts. The three major applications of this diagram include identification of:

  • Relative ease in reducing a particular metallic oxide into metal
  • Partial pressure of oxygen in an equilibrium state with metal oxide at a certain temperature
  • Ratio of carbon dioxide to carbon monoxide which is capable of reducing oxide into metal at a certain temperature
  • In an Ellingham diagram, the line position displays the oxide stability as a temperature. The reactions nearest to the top are the noble metals such as platinum and gold, as the oxides of these metals can be easily reduced and unstable. As the line moves toward the bottom, the metals become more reactive, making oxides more difficult to reduce.

    The right portion of the diagram has a PO2 symbol, which reflects the partial pressure of oxygen needed to achieve equilibrium with metal oxide and the metal itself at a certain temperature. If the PO2 is higher than the value of the equilibrium, the metal will undergo oxidation, and if the value is lower, the oxide will go through reduction.


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