What Is Metallographic Etching? Why Is It Used?

A Technique For Exposing Features in Metals

metallographic etching
Photo © 2006 Ryan Wojes, licensed to About.com

Metallographic etching is a chemical technique used to highlight features of metals at microscopic levels. By studying the character, quantity, and distribution of these different features, metallurgists can predict and explain physical properties and performance failures of a given sample of metal.

How Does Etching Expose Problems in Metals?

Most metallurgical features are microscopic in size -- they cannot be seen or analyzed without optical magnification of at least 50x and as much as 1000x when using light microscopes.

To analyze such features, a metallic sample must be polished to a very fine mirror-like finish. Unfortunately, under a microscope, such a finely polished surface just looks like a plain white field.

To create contrast between the elements of the metal's microstructure, chemical solutions known as etchants are used. Etchants selectively corrode some of those elements, which show up as darker regions. This is possible because differences in the composition, structure, or phase of a metal alter the relative rates of corrosion when exposed to an etchant.

Etchants are used to expose:

  • the shape and size of grain boundaries (defects in crystal structure)
  • metallic phases (different types of metal in an alloy)
  • inclusions (tiny amounts of non-metal material)

How Etching Is Used to Prevent Metal Failure

Metallurgists are scientists who specialize in the structure and chemistry of metals. When metals fail (for example, a structure collapses) it's important to understand the reasons.

Metallurgists examine samples of the metal to determine the reasons for failure.

By etching with different chemicals, metallurgists can expose a variety of possible problems in metal samples. Etching can reveal tiny cracks, pores, or inclusions in metal samples. The information provided by etching allows metallurgists to discover why the metal failed.

Once a particular problem is identified, it's possible to avoid the same issue in future.