Nitride Fe4N structure

The case structure of nitiided steel depends on its type, concentration of alloying elements and particular conditions of nitiiding treatment. The diffusion zone is the original core microstructure with the addition of nitride precipitates and nitrogen solid solution. The surface compoxmd zone is the region where y (Fe4N) and e (Fe2-3N) intermetallics are formed. The corrosion resistance of steel varies with nitrided layer structure. The surface "white layer" can contain e nitride, y nitride or a two phase mixture e+y, below that is the diffusion zone. In add solutions the iron nitrides corrode more slowly than iron and when the "white zone" is formed on the steel surface the improvement of the corrosion resistance is a fact. In Fig. 1 is shown the typacal structure of nitrided plain carbon steel (Minkevich, 1965). 

This structure is very similar to that of the (002) plane of (bulk) Fe4N and hence no reconstruction and the assignment of surface nitride. (It should be noted that the known bulk iron nitrides will never be thermodynamically stable under the conditions of ammonia synthesis but may be readily formed in NH3/H2 mixtures providing high virtual N2 pressures.)... 

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