Nitrogen (from the decomposition of the ammonia) diffuses through the forming compound layer to react with the alloying elements to form the stable nitrides. The morphology of the forming compound layer is being controlled by the amount of carbon present in the steel or the amount of carbon present in the ammonia atmosphere (FNC).
There is generally some porosity associated with the surface compound layer of a gas-nitrided component, and there is not yet a very clear understanding of the formation of this porosity. It is generally thought to be due to pressure from the atomic nitrogen in the epsilon phase recombining with other atomic nitrogen to form molecular nitrogen in the surface layer. This creates a high enough pressure within the compound layer to cause small microscopic high-pressure bursts in the compound layer.