Etching is the selective attack by a chemical reagent that reveals the microstructural detail of the polished mount. The etched mount is examined under the reflected light in a metallurgical microscope.
Etching is usually performed by immersion. The mount is immersed with the polished face upward in a small dish of etching solution, which is gently swirled. The mount is removed when a bloom appears. A bloom is a slight haze that appears and is evidence of the first appearance of the microstructure. If necessary, further etching may be performed after examination under a microscope to strenghen any details. After etching, the mount is throughly rinsed in running water. Then alcohol is sprayed over the surface. And the mount is finally dried in a stream of hot air.
Plain Carbon Steel: 1% Nital, immersion for 25 sec.
Tool Steel: 1% Nital, immersion for 25 sec.
Stainless Steel: 10% aqueous oxalic acid solution, electrolytic etching for 90 sec. at 6V dc.
Aluminum: Keller's reagent, immersion for 10 -20 sec.
Brass: 25ml NH4OH, 25 ml H2O, 50ml (NH4)2S2O8; immersion for 25 sec.