Etching In Metallography

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.

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.

etching in metallography

After polishing and etching, the specimen should be carefully stored and cared for in a desiccator, to prevent oxidation and scratching.

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.

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%)