What exactly is the parasitic white layer that forms after iron parts are treated with ammonia? I have little to do with chemistry and my only background is the inorganic chemistry I learned at school when I was 13-14 years old.

I need to have a basic understanding of the nitrating process (for an automation engineering application, more precisely an automated furnace used for nitrating iron or steels parts with the purpose of improving their mechanical and chemical properties).

A few things are not so clear for me:


What is the chemical formula of the iron lattice mixed with nitrogen atoms (see the picture)?
What is the unwanted white layer that I understand forms on the surface of the metal part treated with dissociated ammonia?
What is this dissociated ammonia? 




Gas nitriding
Steels are nitrided in ammonia gas at 900 to 1050 F ; It forms a very hard , very thin ( &lt; 0.01 ") hard layer of iron nitride . An addition of aluminum alloy to the steel ( 0.5 to 1.0 % ) enhances the nitriding. Nitriding is applied typically to cutting tools like drill bits and wearing surfaces. At higher temperatures "carbo-nitriding " is done where C and N are diffused into the steel surface. The "white layer" is something seen in a metallographic sample at high magnification -100 x and higher. It is undesirable and apparently unidentified as the ASM  handbooks refer to it as only "white layer" ; they give procedures to remove it if necessary . The white layer should be thin , even compared to a 0.005 " thick layer of nitride. Plain ammonia or dissociated ammonia may be used with slightly different procedures.