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. 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.

etching in metallography

Material Etching reagents Etching time
0.8% carbon steel 2% nitric in alcohol (nital) 10 to 15 secs
60/40 Brass Alcoholic ferric chloride 30 secs
Aluminium 4% copper alloy Mixed acids (HCL, HNO3, HF) 15 to 20 secs
Wrought iron 2% Nital 60 to 90 secs
Copper Alcoholic ferric chloride 60 secs

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

Electrolytic Sample Preparation

Preparing Electrolyte - Step 1

Each specimen may require different types of electrolytes. Proper electrolyte should be selected for specimen. Please see instruction manual of ELOPREP to see 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 - Step 2

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 - Step 3

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 - Step 4

Surface of specimen should not be rough for electrolyic preparation. Specimen should be grinded with 600 grit SiC grinding paper before electrolytic preparation.

preparing specimen for etching

Determining Correct Voltage - Step 5

Voltage is one of the most important parameter for electrolytic polishing. ELOPREP has scanning function for easy and exact determination of the parameters. With the scanning function of ELOPREP, correct voltage of polishing can be defined automatically. If you do not have the knowledge for the polishing voltage to be set for your sample, you can use the scanning function to determine exact voltage. You do not have to make time consuming trials and error testing for the correct 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, ideal polishing voltage for your specimen will be seen on the screen.

ideal current density curve

Electrolytic Polishing & Etching - Step 6

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 Etching

Etching Reagents for Microscopic Examination of Metals

Metals Etching Reagent Composition Remarks
Iron and steel No 1 Nital 1 to 5% Nitric Acid Carbon steels darkens pearlite, reveals ferrite boundaries; general use for high speed steels; Time: 1 to 60 secs.
95 to 99 Alcohol
No 2 Picral 4g Picric Acid Carbon and Low alloy steels, heat treated or not.Time: 5 to 120 secs.
100ml Alcohol
No 3 Ferric Chloride& Hydrochloric Acid 5g FeCl1 Reveals structures of austenite and stainless steels
50g HCl
100ml H20
No 4 Heat Treating Heat specimen on hot-plate Pearlite first to pass through a given colour followed by ferrite; cementite less affected. Useful for cast iron. Time: 10 to 60 mins
face up, 400 to 700°F
Copper and its alloys No 5 AmmoniumHydroxide-HydrogenPeroxideMake fresh daily 5 Parts NH2OH (sp.gr 0.88) General etchant for copper and many of its alloys.Time: 1 min
5 Parts H2O
2 to 5 Parts H2O2 (3%)
No 6 Chromic acid Saturated aqueous solution Copper, Brass Bronze and Nickel Silver
(CrO3)
No 7 Alcoholic FerricChloride 5g FeCl3 Copper, Aluminium, Magnesium, Nickel and Zinc alloys.Time: 1 sec to several mins.
96ml Ethyl Alcohol
2ml HCl
Stainless Steels No 8 Alcoholic FerricChloride 5g FeCl3 Quite useful, does not always show full details.
95ml Alcohol
2ml HCl
No 9 15ml Good for revealing the full structure, but with long etching times.
5ml HNO3
100ml H20
Aluminium & its Alloys No 10 HydrofluoricAcid 0.5ml HF (Conc) General etchant; apply by swabbing. Time: 15 secs
99.5ml H20
No 11 Sodium Hydroxide 10g NaOH General etchant: can be used for both micro and macro etching. Time: 5 sec
90ml H20
Magnesium &its Alloys No 12 Glycol 75ml Ethylene Glycol For most all magnesium alloys. Time: 3 to 60 secs
24ml H20
1ml HNO3 (Conc)
Nickel & itsAlloys No 13 Flat SolutionMake fresh daily 50ml HNO3(Conc) Nickel, Monel and other Nickel-Copper alloys. Time: 5 to 20 secs
50ml Glacial Acetic acid
No 14 Aqua Regia 5ml HNO3 (Conc)
25ml HCl (Conc)
30ml H20
Titanium Alloys No 15 Freshly Prepared Etching Solution 1 Etching rather difficult, two etching solutions are recommended
1ml HF (40%)
5ml HNO3
94ml H20 (Distilled)
Etching Solution 2
18.5g Benzalkonium
Chloride
35ml Alcohol
40ml Glycerol
10ml HF (40%)