Metallographic etching is one of a number of processes that takes place during sample preparation for metallographic examination. The steps involved in preparing a sample include:
Why is Metallographic Etching Used?
Metallographic etching is the process of revealing microstructural details that would otherwise not be evident on the as-polished sample. Etching is not always required as some features are visible in the as-polished condition such as porosity, cracks and inclusions.
A properly prepared specimen will reveal properties such as grain size, segregation, and the shape, size, and distribution of the phases and inclusions that are present, while other aspects such as mechanical deformation and thermal treatments may also be able to be determined.
Metallographic Etching Processes
There main etching processes used in metallographic sample preparation are:
• Chemical etching
• Electrolytic etching
• Heat tinting
This typically involves immersing the sample in an etchant such or swabbing the surface with an etchant. The etchant selectively corrodes microstructural features. Immersion time or etching time is highly dependent on the system and in most cases requires experience. The selection of the optimum etchant is also very important in sample production.
Deeper etches are preferred for low magnification examinations, while shallow etches are preferred for higher magnification etches.
Electrolytic etching and electropolishing are in effect the same process, except that electrolytic etching uses lower voltages and current densities. Most electrolytic etching processes use direct current electrolysis. The process uses the specimen as the anode, with the cathode being a highly insoluble, but conductive material. Typical examples are platinum, graphite and stainless steels.
Heat tinting, sometimes called thermal etching is the process of oxidizing a sample in a furnace. This induces oxidation of surface features at different rates, to reveal various features.