Microstructures
by George Langford, Sc.D., Massachusetts Institute of Technology, Cambridge, MA, 1966
Copyright©2005 by George Langford
Surface Coatings and Heat Resisting Alloys - Lesson 3 - Eleventh specimen
The results of a similar surface treatmen, siliconizing, are quite different from chromizing, as you can see below, where a medium carbon steel has been heat treated in contact with a silicaceous material in a chlorine atmosphere at 955C.
Siliconized steel at 50X etched
 The photomicrograph at left is shown at 50X magnification with a nital etch.

Silicon increases the chemical activity of carbon in iron, so the BCC alpha (ferrite) case at upper left in the photomicrograph has essentially no carbon left in it ... the carbon migrated inwards, carburizing the steel substrate.  There was an interlayer of austenite ... which has now completely transformed to pearlite.  Silicon slows down the austenite ==> ferrite plus cementite reaction much less than chromium does.  That's porosity in the alpha (siliconized ferrite) case, which is a consequence of sulfur impurities in the steel.
Iron - silicon phase diagram The iron rich side of the iron - silicon phase diagram is similar to the iron - chromium system.

SUMMARY:  The metallurgical effects of coating or otherwise treating the surface of one element with another one can often be understood by referring to their binary phase diagram.  However, third and even fourth alloying elements usually have significant influences.  Therefore, you should be able to gain a qualitative understanding of the coating process and the microstructure of the product from the elementary principles of phase diagrams plus some knowledge of the chemical activities (and activity coefficients) of the minority alloying elements that are also present.  The coatings and microstructures you have seen here are not all in commercial use or even practical for such use, but they serve to indicate the kinds of results you can expect to obtain in your own experiences.
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