Post-oxidizing in air, steam or dinitrogen oxide (N2O, laughing gas) after nitrocarburizing, greatly improves corrosion resistance. It is applied for a short time at about 450 – 550 ºC. A 1 – 3 µm thick, black layer forms on top of the phase transformed layer. The layer can also be formed in other colours.

To provide good corrosion resistance properties, the layer must contain magnetite, Fe3O4.

Ref. Steel and its Heat Treatment – A Handbook (8.12.13)