Micro Etchants

Micro Etching is used to highlight, and sometimes identify, microstructural features or phases present. Microscopic examination of a properly polished, unetched specimen will reveal only a few structural features. Only the non-metallic inclusions, porosities and cracks may be easily seen from the surface. Thus, it is beneficial to investigate the sample structure under the microscope after the polishing step. However the polished sample surface reflects the light equally, it would be too difficult to detect any differentiation on the sample surface; hence a generation of a contrast is highly required in that case. Thus the crystalline structure (grains and grain boundaries) of the polished surface is revealed by etching with a proper etchant. This process is named as “chemical etching” or “etching” in brief. It is based on the rate difference in chemical attack depending upon chemical composition, energy content, and grain orientation of the sample. Since the grain boundaries are attacked at a greater rate than the proper grain due to higher energy content of the grain boundaries. In addition, the presence of chemically different phases results in variations in the rate of chemical attack. These changes in the rate of chemical attack produce deviations both in angle and depth of certain portions of the surface. Thus the light is reflected in varying amounts depending on the angle and depressions of the portion of the surfaces resulting in light and dark regions. In this manner the crystalline microstructure of the specimens are revealed. Etchants are usually dilute acid or dilute alkalis in a water, alcohol or some other solvent. Etching occurs when the acid or base is contacted with specimen surface because of the difference in rate of attack of the various phases present and their orientation. The etching process is usually accomplished by merely applying the appropriate solution to the specimen surface for several seconds to several minutes. Nital, a Nitric Acid - Alcohol mixture, is the etchant commonly utilized with common irons and steels. Nital is dripped onto the specimen using an eye-dropper or cotton swab. Ten seconds to one minute is usually sufficient for proper etching depending on sample and nital concentration. The sample is immediately washed under running water, rinsed with alcohol and dried in an air blast. Do not touch, wipe or swab the specimen following etching; dry off the rinsing alcohol on the specimen with the air blast and then move on to the microscopic examination stage.

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