by George Langford, Sc.D., Massachusetts Institute of Technology, Cambridge, MA, 1966
2005 by George Langford
Surface Coatings and Heat Resisting Alloys - Lesson 3 - Tenth specimen
Chromized nut at 500X etched
Here is another way of coating iron with chromium.  The photomicrograph at left shows a chromized steel nut at 500X. 

The nut has been coated by exposing it to a CrF2 (chromous fluoride) atmosphere at 1040C.

Here, the interlayer is the result of ordinary diffusion processes at the elevated temperature.

Can you make sense of the diffusion zone ?

The answer at the bottom of this page includes a
portion of the iron - chromium phase diagram as an aid.

Iron - chromium systemThis is part of the iron - chromium phase diagram.  The surface of the specimen above is BCC alpha (ferrite) containing 80% iron and 20% chromium.  The interior is also ferrite ... what's between the two ?

Note that the manganese sulfide (MnS) stringers from the steel substrate extend into the coating.  They indicate that the coating has incorporated a large amount of the substrate into itself.

The interlayer is FCC gamma (austenite) which has partly decomposed into pearlite.

Specimen 11 is a steel surface treated with silicon.