Etching is done to obtain visible grain boundary. It is a surface treatment process.
Etching: Microscopic examination of a properly polished, unetched specimen will reveal only a few structural features such as inclusions and cracks or other physical imperfections. Etching is used to high- light, and sometimes identify, microstructural features or phases present. It is done to obtain visible grain boundary.
Etching occurs when the acid or base is placed on the specimen surface because of the difference in the rate of attack of the various phases present and their orientation.