Etching metallographic specimens

Dr. Dmitri Kopeliovich

Etching is the operation of revealing micro-structural features (grain boundaries, phases, precipitates and other micro-structure constituents) of the polished specimen through selective chemical attack of its surface.

• The specimen must be washed and degreased in boiling alcohol and then cooled in running water before etching in order to prevent uneven attack and stains.

• Clean specimen is etched by being immersion in the etching reagent for a given time. Then the specimen is quickly removed from the reagent and immediately swilled in running water.

• The etched specimen is then visually inspected. Its surface should have slightly dull appearance (bright appearance requires additional etching).

• Then the wet specimen should be dried in alcohol.

• Etching reagents
• Electrolytic etching
• Thermal etching

• For steels and cast irons:

Nital reagent: 2 volume parts of nitric acid, 98 volume parts of alcohol

Etching time: 10-30 sec.

Reveal: ferrite grain boundaries, pearlite, martensite, trostite.

Picral : saturated solution of picric acid in alcohol

Etching time: 10-30 sec.

Reveal: pearlite

• For copper and its alloys:

- 25% aqueous solution of ammonium hydroxide with a few drops of hydrogen peroxide.

- 10% aqueous solution of ammonium persulphate.

- Saturated aqueous solution of chromic acid.

Reveal: grain boundaries.

• For aluminum and its alloys:

Keller’s reagent: 1volume part of hydrofluoric acid, 1.5 volume part of hydrochloric acid, 2.5 volume parts of nitric acid, 95 volume parts water.

Etching time: 10-20 sec.

• For nickel, monel and cobalt:

1 volume part of nitric acid, 1 volume part of glacial acetic.

• For magnesium and its alloys:

1 volume part of nitric acid, 24 parts of water, 75 parts of ethylene glycol.

• For zinc and its alloys:

1volume part of hydrofluoric acid, 99 volume parts of alcohol.

• For lead:

10 volume parts of nitric acid, 10 volume parts of acetic acid, 40 volume parts of glycerol.

• For titanium and zirconium:

1-5% aqueous solution of hydrofluoric acid

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Electrolytic etching is acid etching enhanced by electric current. The etched specimen (anode) is immersed in an acidic electrolyte.

Electrolytic etching is used when the specimen does not respond to conventional chemical etching reagents.

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Thermal etching is a method of revealing structure features by heating the specimen to a high temperature in vacuum or in an atmosphere of a certain gas.

The grain structure ofsome nickel alloys is revealed by heating the specimen in Hydrogen.

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• Metallographic examination - introduction
• Macro-examination of metallographic specimens
• Specimen polishing technique
• Metallurgical Microscope
• Scanning Electron Microscope
• Grain structure
• Solidification
• Phase transformations and phase diagrams
• Basic principles of heat treatment
• Chemical Composition Evaluation

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to Metallographic examination