Etching In Metallography

Electrolytic polishing is the best way to polish very soft materials which are prone to smearing and deformation. It can be easily applied to objects of complex shape. Materials that work well for electropolishing or etching include soft austenitic stainless steels, aluminum and aluminum alloys, copper and copper alloys, among others. Shorter preparing time is needed for electrolytic polishing and etching process compared to traditional mechanical preparation. The primary requirement for electropolishing is that the specimen be conductive.

Polished metal specimens usually show no structural characteristics. Metallographic etching of the metal surface is done to make visible the crystalline structure of the metal and to produce optical contrast between the various constituents.

Electropolishing is also commonly applied to the preparation of thin metal samples for transmission electron microscopy because electropolishing does not cause mechanical deformation of surface layers usually observed when mechanical polishing is used.

Etchants are composed of organic or inorganic acids, alkalis, or other complex substances, in a solvent such as water, alcohol, glycerine or glycol.

Over the years, many different reagents have been developed for specific purposes and materials. Therefore they must be chosen carefully to reveal the desired structure. The specimen is either held by tongs and immersed, with the polished face down into a small petri dish partly filled with the etching reagent, or swabbed with cotton wool, which has been saturated with the etching reagent. When etching, progress should be observed and timed, depending on the materials and reagent, this could vary from a few seconds to a minute or more.

After etching, the specimen should be thoroughly washed in water or alcohol and then dried using a hot air blower. The specimen should be carefully stored and cared for in a desiccator, to prevent oxidation and scratching.

Electrolytic Sample Preparation

Preparing Electrolyte

Each specimen may require different types of electrolytes. Proper electrolyte should be selected for specimen. Please see below table for common electrolytes. Fill electrolyte container of ELOPREP with correct electrolyte and place it on polishing unit. If you have different specimens which required different electrolytes, you can use additional electrolyte containers. Thus, you do not have to change electrolyte for different types of specimen, you only need to change electrolyte container.

Choosing Masks

Acid resistant masks are used for determining polishing area of specimen. There are specific aperture on the masks. Electrolyte will contact the specimen surface from this aperture thus only this area will be polished. Masks are available with 0.5-1-2 and 5 cm2 aperture size. Mask should be placed on cathode and specimen should be placed on the mask.

preparing specimen for Metallographic etching

Adjusting Flowrate

Flowrate of electrolyte should be adjusted before operation. In other words, electrolyte should be reached upper level of the mask without any turbulance. Otherwise, polishing operation cannot be done properly. To adjust flowrate, press Pump button without placing sample. If the flow rate is not enough, increase the set value until the flow is sufficient.

Preparing Specimen

Surface of specimen should not be rough for electrolyic preparation. Specimen should be grinded before electrolytic preparation.

Determining Correct Voltage

Voltage is one of the most important parameter for electrolytic polishing. The ELOPREP has a scanning function for easy and exact determination of the parameters. If you do not know the polishing voltage to be set for your sample, you can use the scanning function to determine the exact voltage.

  • Place specimen on the mask. Be sure that the aperture is covered and closed completely.
  • Place the anode arm on the top of the sample to ensure good contact.
  • Enter Scan menu, set scanning voltage and flowrate parameters and press Start.
  • After a while, the ideal polishing voltage for your specimen will be seen on the screen.

ideal current density curve

Electrolytic Polishing & Metallographic Etching

ELOPREP has 4 different operation modes; only polishing, only etching, automatic etching after polishing and external etching.

  • Place specimen on the mask. Be sure that the aperture is covered and closed completely.
  • Place the anode arm on the top of the sample to ensure good contact.
  • Enter Polishing&Etching menu. Set parameters: Voltage, flowrate, mask type and time.

For only polishing: Set only polishing time and polishing voltage. Do not set etching time and etching voltage, these should be zero “0”. Press start and operation will be completed automatically. The specimen must be washed after polishing operation is completed.

Polished surface and microstructure

Polished surface and microstructure (Microstructure of Al-Cu-Steel)

For only etching: Set only polishing time and polishing voltage. Do not set etching time and etching voltage, these should be zero “0”. Press start and operation will be completed automatically. The specimen must be washed after etching operation is completed. Microstructure of etched surface can be seen as below:

Microstructure of etched surface

Microstructure of etched surface (Microstructure of Al-Cu-Steel)

For automatic etching after polishing: Set polishing voltage, polishing time, etching voltage and etching time. Do not set etching time and etching voltage. Press start. Etching will be done automatically after polishing operation is completed. The specimen must be washed after etching operation is completed. For external etching: If polishing and etching electrolytes different than each other, in this case external etching unit can be used.

  • Fill external etching unit with electrolyte.
  • Connect cables of external etching unit to control unit.
  • Enter External Etching menu. Set etching voltage and time.
  • Hold the sample with scissors.
  • Plunge the sample into the external etching unit. As soon as the sample contacts with the electrolyte the process will start automatically.
  • As soon as the set etching time is reached an audible signal notifies the operator that the process is completed. Wash your specimen after etching is completed.

Electrolytic Polishing and Metallographic Etching

Metallographic Etching Reagents for Microscopic Examination of Metals

Note: Many etching reagents are powerful and must be handled with care.

Class Use Formula Volume Cell Voltage Time Remark
Group I (Electrolytes Composed of Perchloric Acid and Alcohol With or Without Organic Additions)
I-1 Al and Al alloys with less than 2 percent Si, Steels, carbon alloys, stainless steels

Ethanol (95 percent)
Distilled water
Perchloric acid (60 percent)

800ml
140 ml
80 ml

30 to 80

15 to 60s

Nickel cathode
I-2

Stainless steel and aluminum

Ethanol (95 percent)
Perchloric acid (60 percent)
800ml
60 ml
35 to 80 15 to 60s
I-3

Stainless steel

Ethanol (95 percent)
Perchloric acid (65 percent)
940ml
60 ml

30 to 45

15 to 60s

I-4 Steel, cast iron,Al,Al alloys
Ni, Sn, Ag, Be, Ti, Zr, U Heat-resisting alloys
Ethanol (95 percent)
2-butoxy ethanol
Perchloric acid (30 percent)
700 ml
100ml
200 ml
30 to 60 15 to 60s One of the best formulas for universal use
I-5 Steels-stainless, alloy, high speed, Fe, Al, Zr, Pb Ethanol (95 percent)
Glycerin
Perchloric acid (30 percent)
700ml
100ml
200ml

15 to 50

15 to 60s

Universal electrolyte comparable to I-4
I-6

Al, Al-Si alloys

Ethanol (95 percent) Diethyl ether
Perchloric acid (30 percent)
760ml 190ml
50ml
15 to 60 15 to 60s Particularly good with Al-Si alloys
Group II (Electrolytes Composed of Perchloric Acid and Acetic Acid in Varying Proportions)
II-1 Cr, Ti, Zr, U, Fe, steel, carbon alloy, stainless steel Acetic acid (glacial)
Perchloric acid (60 percent)
940 ml
60 ml

20 to 60

60s Good general- purpose electrolyte
Group III (Electrolytes composed of Phoephoric Acid in Water Organic Solvent)
III-2

Pure cooper

Distilled water
Phosphoric acid (85 percent)
175ml
825ml
III-10

Cu and Cu-based alloys

Distilled water
Ethanol (95 percent)
Phosphoric acid (85 percent)
500ml
250ml
250ml
Group IV (Electrolytes Composed of Sulfuric Acid in Water or Organic Solvent)
IV-1

Stainless steel

Water
Sulfuric acid
250ml
750ml

1.5 to 6

1 to 2 min
Iv-6

Stainless steel, aluminum

Water
Glycerin
Sulfuric acid
220ml
200ml
580ml

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