Microstructures
by George Langford, Sc.D., Massachusetts Institute of Technology, Cambridge, MA, 1966
Copyright©2005 by George Langford
Cast Irons, High Alloy Steels, and Superalloys - Lesson 2 - Second specimen
Austenitic stainless steel at 500X etched electrolytically The specimen at left is an austenitic stainless steel containing 18% chromium, 8% nickel, and a trace of almost unavoidable carbon as an impurity.  The magnification is 500X, and the microstructure has been made visible by etching electrolytically in an oxalic acid solution.  Stainless steels are those that have more than 12% chromium, which permits a passive layer of either chromium oxide or adsorbed oxygen to form, preventing access of further oxygen to the iron atoms.  The rate of corrosion in aqueous solutions and the rate of oxidation at elevated temperatures in oxidizing atmospheres are thereby greatly reduced.  Since chromium stabilizes BCC ferrite, which has marginal mechanical properties, nickel is usually added to make FCC austenite the preferred phase. 

Inadvertent slow cooling after an annealing treatment permitted chromium carbide (Cr4C) to precipitate in this specimen on the austenite grain boundaries, where diffusion of carbon is very fast.  The austenite next to these grain boundaries has been depleted of the chromium that went into the Cr4C, and so the metal next to those grain boundaries is no longer stainless.  The etching effect seen above in the first photomicrograph is called grooving.
The precipitation process described here is called sensitization and is a serious problem in massive welded structures ... such as the boilers and the piping of nuclear reactors ... which cannot be adequately heat treated in their entirety after welding ... somewhere in the heat affected zones of each weld, the cooling rate will fall in the range that produces the unwelcome carbide precipitation.  The only solution is to remove all the carbon by careful melt practice (known as argon - oxygen decarburization) or by gettering with niobium or vanadium, whose carbides are more stable than Cr4C.  Quenching after annealing suffices to retain corrosion resistance in smaller pieces.
Specimen 3 is a failed stainless steel.