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 planar features such as grain boundaries, 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 to 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 will create electrochemical potentials that alter the relative rates of corrosion when exposed to an etchant.
Applications of EtchantsChemical etchants can be designed to do most anything to a metal surface. Some common practical uses of etchants are to:
Expose grain boundaries. The boundaries of metallic grains can corrode very quickly when the appropriate etchant is used, highlighting the edges of the grains and making it possible to determine their size and shape.
Highlight metallic phases. Different phases will have different shading and texture after etching, allowing for phase fraction estimation and identification of undesirable phases.
Identify inclusions. Nonmetallic inclusions often have characteristic shapes and colorations after appropriate etching, which allows for quick size measurement and number density estimation.
See many more applications of metallographic etching and some great visuals in Buehler's etching guide, part of the Buehler Book online.
Common Metallographic Etchants
- Nital: Reveals austenite grain boundaries and other phases in plain iron and most steels. Typically mixed as 2 mL HNO3 in 98 mL methanol.
- Picral: Good for showing carbides in ferritic steels, but does not show ferritic grain boundaries. Typically mixed as 4 g picric acid in 100 mL ethanol.
- Vilella's reagent: Also good for ferritic steels, but shows contrast between prior austenite grains and can be used for tool steels and martensitic stainless steels. Mixed as 1 g picric acid and 5 mL HCl in 100 mL ethanol.
- Marble's reagent: Exposes the grain structure of nickel superalloys. Mixed as 10 g CuSO4 in 50 mL HCl and 50 mL water.
- Keller's reagent: Highlights common features in aluminum and aluminum alloys, and is sometimes effective at showing grain boundaries. Mixed as 1.0 mL HF and 1.5 mL HCl in 2.5 mL HNO3 and 95 mL water.