The most common tint etchants are those that deposit a sulfide-based interference film on the specimen. These are the best-known tint etches and are usually the easiest to use. Klemm and Beraha have developed the most widely used sulfide-based tint etchants using sodium thiosulfate (Na2S2O3) and potassium metabisulfite (K2S2O5) using Klemm’s I, II, III. Beraha developed two different complex reagents that deposit CdS or PbS films plus five reagents with a range of HCl concentrations and potassium metabisulfite (other additions are made to some) for etching a variety of Fe-, Ni- and Co-based alloys. The HCl-based reagents vary widely in concentration and can be used to color the grain structures of stainless steels, while higher HCl-concentration versions are used for Ni-based and Co-based alloys. Sodium metabisulfite has been used in a number of concentrations, from about 1-20 g per 100 mL water, and is a safe, reliable and useful color etch for irons and steels. Beraha also developed two variants of an etchant that preferentially colors cementite using sodium molybdate. If ammonium bifluoride is added, the ferrite grains are also colored.
Beraha also developed four etchants based upon sulfamic acid (a weak organic acid) that have not been used much, even though they are quite useful, reliable and easy to employ. The sulfamic acid-based reagents are applicable to cast iron, low-carbon and alloy steels, tool steels and martensitic stainless steels. Beraha also developed tint etchants that deposit elemental selenium on the surface of steels and cast iron, nickel-based alloys and copper-based alloys.
The examples shown have demonstrated the great value of color and tint etching for examining microstructures of metals. Reagents exist to develop color with most commercial alloy systems. The examples clearly demonstrate the value of these reagents in fully revealing the grain structure, even for the most difficult-to-etch specimens. Further, they are selective in nature, which can be quite useful for quantitative metallographic studies. Tint etchants reveal segregation very clearly, and either EDS or WDS can be performed on a tint-etched surface without any problems from the interference surface layer.
My full article on color metallography, with figures, can be seen here.