Ellingham diagrams – their use and misuse

Ellingham diagrams can be very useful in the trouble shooting of heat treatment problems related to the use of protective atmospheres especially for determining if a particular atmosphere is oxidising or reducing to the metal or alloy in question. However, use without understanding can lead to incorrect conclusions. It is important to understand that the diagram relates to the equilibrium condition and takes no account of kinetics. It also assumes that there is sufficient oxidising species present for the reaction to continue. How these properties can affect the interpretation of results is illustrated by some industrial examples.