|Here is another way of coating iron with
chromium. The photomicrograph at left shows a chromized steel nut
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.
|This 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.