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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. |
 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.
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