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.
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.
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 (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 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.
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) | 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 |