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.
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%) |
