Etchant

Liquid chemical reagents used to make the material structure (see Structure) visible on a polished surface, usually ametallographic specimen (metallographic cut).

Besides general requirements, the surface of the specimen must meet special requirements in order to be suited for metallographic tests after its preparation. These requirements refer to the surface design and the property of the surface, to interact with the incoming light as desired. In order to improve the reflection and absorption properties of the structural elements in the material to be examined, the material must usually be etched (see Etching) to produce the required grey and color contrasts.

A huge number of acids, bases, neutral solutions, solution mixtures, salt melts, etc. can be used as etchants. The reaction rate is mainly determined by the degree of dissociation and the electrical conductivity of the etchant. Etching time is between seconds and hours. Etching time and etching temperature correlate: Etching time can be reduced by increasing the temperature.

Below is a list of the main etching techniques used in the foundry technology:

Matrix in cast iron and steel

3% alcoholic nitric acid
Solution: 97 ml ethyl alcohol, 3 ml conc. nitric acid
Etching: 5 - 7 s at room temperature
Effect: Graphite unchanged (see Graphite form), perlite with grey-brown stripes or granular, ferrite not affected, grain boundaries in case of major etching, phosphide eutectic white with dark dots, cementite not affected, framed, primary eutectic cells visible to the human eye in case of more than 0.1% P (see Eutectic grain)

Phosphide network

10% ammonium persulfate solution
Solution: 10 g ammonium persulfate, 100 mm dest. Water (to be newly prepared every three weeks)
Etching: 45 s at room temperature
Effect: Phosphide eutectics light, matrix black (see Metal matrix of cast iron)

Cementite

Alkaline sodium picrate solution
Solution: 25 g sodium hydroxide, 75 ml dest. Water, 2 g of picric acid are added to 100 ml of this solution and dissolved in a hot water bath for 30 minutes (solution must always be newly prepared)
Etching: 10 min at approx. 50 °C
Effect: Cementite colored brown to black, depending on the etching time, phosphide eutectic, ferrite and chromium carbide not affected

Eutectic cells (Klemm)

Solution: 50 ml cold saturated sodium sulfate solution, 2 g potassium metabisulfite
Etching: 20 s to 1 min, depending on phosphor and S content
Effect: Light phosphide eutectic, dark matrix, etching also on ferrite, bainitic stage (bainite), martensite (dark), cementite and austenite (light)

Proof of steatite (Fe-Fe3C-Fe3P) in cast iron (Murakami)

Solution: 100 cm3 water, 10 g NaOH or KOH, 10 g potassium ferricyanide
Etching: at 20 - 50 °C, 2 to 20 minutes (only newly prepared solutions to be used)
Fe3C with more than 10% of C sooner gets a dark color than Fe3C with low C contents, Fe3P dark colored

Austenite steels, sigma phase (V2A etchant)

Solution: 100 cm3 water, 100 cm3 HCL, 10 cm3 HNO3, 0.3 cm3 Vogel inhibitor
Etching: at 50 °C some seconds to some minutes

Chromium alloyed steels (WII)

Solution: 60 ml concentrated HCl, 16 g FeCl3 + 50 ml water, 9 g (NH4)2SO4 + 50 ml water
Etching: 3 - 7 sec at room temperature

Primary structures (Jurich)

Solution: concentrated Sulfuric acid with crystallized boric acid as precipitate (approx. 1 g/10 ml)
Etching: 20 sec at room temperature, polish, etch again and repeat this procedure until dendrites are well visible
Effect: Primary dendrites appear lighter in the dark matrix

Sulfur

Sulfur print
Solution: 100 ml dist. Water, 5 ml concentrated sulfuric acid
Procedure: Silver bromide photographic paper is immersed in the solution in the darkroom for 5 minutes. Then the paper is removed, briefly drained and quickly pressed with the layer side onto the finely sanded and polished unetched specimen beginning at one edge and making sure to avoid bubble formation. After 5 minutes, the paper is removed from the specimen, rinsed in water, normally fixed, watered for one hour and dried. In order to keep the edge of the paper purely white, the print should be made in a semi-dark room. If the print is not successful, it is not enough to polish the affected specimen. It must be sanded again, starting with the roughest sandpaper.
Effect: The paper is colored dark brown at spots with sulfide inclusions.

Phosphorous segregations (Czikel and Klemm)

Thiosulfate etching
Solution: 50 ml cold saturated sodium sulfate solution, 2 g potassium metabisulfite
Etching: 10 - 20 sec at room temperature
Effect: A thin layer of brown iron sulfide is formed on the iron which gets thinner with increasing phosphorous content.

Grain surface etching especially for pure copper

Nitric acid
Solution: 30 ml dest. water, 70 ml concentrated nitric acid, some drops of silver nitrate solution
Etching: 10 - 60 sec at room temperature
Effect: The macrostructure is visible, not suited for microscopic examination.

Grain boundaries or grain surfaces on copper and bronzes

Ammonia hydrogen peroxide
Solution: 50 ml water, 50 ml saturated ammonia solution, 20 - 50 ml 3% hydrogen peroxide solution
Etching: 10 - 30 s, room temperature
Effect: The macrostructure is visible; also suited for microscopic examination in case of weak etching.

Microstructure of all copper alloys

Iron chloride
Solution: 120 ml ethyl alcohol, 30 ml concentrated hydrochloric acid, 10 g iron chloride (FeCl3)
Etching: 5 - 10 sec at room temperature
Effect: The structural components especially in brass and bronze are made visible.

Grain boundary etching

Aluminum mixed acid
Solution: 30 ml concentrated hydrochloric acid, 10 ml concentrated Nitric acid, 60 ml water, addition of solid sodium fluoride or cryolite until sedimentation starts (approx. 5 g)
Etching: 1 - 2 sec at room temperature. Interrupt a several times during the process and rinse with water to prevent the temperature from rising too fast.
Effect: The macrostructure is visible. Not suited for microscopic examination.

Microstructures in alloys (in particular aluminum cast alloys) (Keller and Wilcox)

Mixed solution
Solution: 100 ml dist. Water, 0.5 ml concentrated hydrochloric acid, 1 ml concentrated nitric acid
Etching: 10 - 20 sec at room temperature
Effect: Silicon is not affected (violet), Mg2Si initially blue, then black, remaining structural elements mostly framed only

Structure of pure aluminum and aluminum alloys below 1% Si

Electrolytic polishing
Solution: 100 ml methyl alcohol, 6 ml concentrated perchloric acid
Procedure: Electrolyze anodically at 48 volt for 5 - 15 seconds at max. 25 °C stirring well
Effect: Silicon and Mg2Si are dissolved. All other structural elements are framed and well visible.

Grain orientation (Lacombe)

Solution: 25 ml conc. hydrochloric acid, 25 ml conc. nitric acid, 1 ml hydrofluoric acid (in a paraffined tray)
Etching: 5 - 30 s, room temperature
Effect: etching patterns occur indicating the cuts of the cube.

Zinc- and lead alloys

Until now, only 3 % alcoholic nitric acid has proven itself.
Etching: approx. 10 s, room temperature
Effect: development of the micro structure

Cobalt-chromium-molybdenum alloys (Visil)

Etching with aqua regia
Solution: Ratio of hydrochloric acid and nitric acid = 3:1
Etching: 10 - 30 s, room temperature
Effect: development of the micro structure

White metals (Vilella)

Solution: 16 ml conc. nitric acid, 16 ml pure acetic acid, 68 ml glycerin
Etching: some minutes at room temperature
Effect: antimony and antimony-containing phases are bright.

Micro etching of pure magnesium

Nital etchant
Solution: 100 ml ethanol (96 %), 1 to 8 ml nitric acid (65 %)
Etching: seconds to minutes, room temperature
Effect: grain boundaries

Acetic acid solution
Solution: 100 ml ethanol, 5.2 g citric acid
Etching: up to 5 min
Effect: grain boundaries

Solution according to Bastien
Solution:: 100 ml ethanol, 2.2 g citric acid
Etching: up to 5 minutes
Effect: development of the micro structure

Citric acid solution
Solution: 100 ml ethanol, 2 to 11 g citric acid
Etching: up to 30 s, wipe etching, rinse specimen with hot water
Effect: development of the micro structure

Micro etching of magnesium alloys

Nital etchant
Solution: 100 ml ethanol (96 %), 1 to 8 ml nitric acid (65 %)
Etching: seconds to minutes, room temperature
Etching of: suitable for all alloys
Please note: applicable for cast, forged and rolled states

Citric acid solution
Solution: 100 ml ethanol, 2 to 11 g citric acid
Etching: 5 to 30 s, wipe etching, rinse specimen with hot water and dry in air current
Etching of: Mg-Cu, mg investment casting alloys
Please note: also for heat-treated casting and treated specimens

Tartaric acid solution
Solution: 90 ml dist. water, 2 - 10 g tartaric acid (Mg alloys with more than 6 % Al: 20 g tartaric acid)
Etching: 10 s to 20 min, polishing only dry, strong etch washing, wash with ethanol then and dry in a cold air current
Etching of: Mg-Al, Mg-Mn, Mg-Mn-Al-Zn
Effect: grain size in castings, flow marks, also for rolled, forged
and pressed specimens

Ammonium persulfate solution
Solution: 100 ml dist. water, 10 g ammonium persulfate (40 %)
Etching: Wipe etching until the surface has a brownish color
Etching of: Mg alloys in forged state
Effect: Grain boundary etching

Hydrofluoric acid solution
Solution: 90 ml dist. Water, 10 g hydrofluoric acid (40 %)
Etching: move the specimens for 3 to 30 s, if necessary, reduce the hydrofluoric acid content, use plastic trays
Etching of: Mg (techn. pure), Mg-Al-Zn, Mg-Zn-Th-Zr, Mg-SE
Effect: Mg17 Al12 is visible as dark phase, to distinguish Mg-Al from Mg-Al-Zn phases

Oxalic acid solution
Solution: 100 ml dist. water, 2 g oxalic acid
Etching: Caution! 6 to 10 s
Etching of: Mg, Mg-Al, Mg-Al-Zn, Mg-Mn, Mg-Th-Zr, Mg-Zn-Zr
Effect: Grain boundaries, also for continuously cast specimens

Additional references:
Deep etching

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