Common Etchants for Copper, Nickel and Cobalt: Copper & Alloys

Composition	Comments
2 5 mL NH4OH 25 mL water (optional) 25-50 mL H2O2 (3%)	General purpose grain contrasts etch for Cu and alloys (produces a flat etch for some alloys). Use fresh, add peroxide last. Use under a hood. Swab specimen 5-45 seconds.
100 mL water 10 g ammonium persulfate orientation.	General purposes etch for Cu and alloys. Immerse or swab for 3-60 seconds. Reveals grain boundaries but is sensitive to crystallographic
100 mL water 3g ammonium persulfate 1mL NH4OH	General purpose etch for Cu and alloys, particularly Cu-Be alloys.
70 mL water 5 g Fe(NO3)3 25 mL HCI	Excellent general purpose etch, reveals grain boundaries well. Immerse specimen 10-30 seconds

Common Etchants for Iron and Steel

Composition	Comments
90-99 mL methanol or ethanol 1-10 mL HNO3	Nital. Most common etchant for Fe, carbon and alloy steels, cast iron. Reveals alpha grain boundaries and constituents. Excellent for martensitic structures. The 2% solution is most common, 5-10% used for high alloy steels (do not store). Use by immersion or swabbing of sample for up to bout 60 seconds.
1 00 mL ethanol 4 g picric acid	Picral. Recommended for structures consisting of ferrite and carbide. Does not reveal ferrite grain boundaries. Addition of about 0.5-1% zephiran chloride improves etch rate and uniformity.
100 mL ethanol 5 mL HCI 1 g picric acid	Vilella’s reagent. Good for ferrite-carbide structures. Produces grain contrast for estimating prior austenite grain size. Results best on martensite tempered at 572-932 °F (300-500 °C). Occasionally reveals prior-austenite grain boundaries in high alloy steels. Outlines constituents in stainless steels. Good for tool steels and martensitic stainless steels.
Saturated aqueous picric acid solution grain plus small amount of a wetting agent	Bechet and Beaujard’s etch, Most successful etchant for prior-austenite boundaries. Good for martensitic and bainitic steels. Many wetting agents have been used, sodium tridecylbenzene sulfonate is one of most successful (the dodecyl version is easier to obtain and works as well). Use at 20-100 °C. Swab or immerse sample for 2-60 minutes. Etch in ultrasonic cleaner Additions of 0.5g CuCl2 per 100mL solution or about 1% HCI have been used for higher alloy steels to produce etching. Room temperature etching most common. Lightly back polish to remove surface smut.
150 mL water 50 mL HCI 25 mL HNO3 1 g CuCl2	Modified Fry’s reagent. Used for 18% Ni maraging steels, martensitic and PH stainless steels.
1 00 mL water 25 g NaOH 2 g picric acid	Alkaline sodium picrate. Best etch for McQuaid-Ehn carburized samples. Darkens cementite. Use boiling for 1-15 minutes or electrolytic at 6 V dc, 0.5 A/in2, 30-120 seconds. May reveal prior-austenite grain boundaries in high carbon steels when no apparent grain boundary film is present.
1 00 mL ethanol 100 mL HCI 5 g CuCl2	Kalling’s no. 2 (“waterless” Kalling’s) Etch for austenitic and duplex stainless steels. Ferrite attacked readily, carbides unattacked, austenite slightly attacked. Use at 20 °C by immersion or swabbing. Can be stored.
1 5 mL HCI 10 mL acetic acid 5 mL HNO3 2 drops glycerol	Acetic glyceregia. Mix fresh; do not store. Use for high alloy stainless steels.
100 mL water 10 g K2Fe(CN)6 10 g KOH or NaOH	Murakami’s reagent. Usually works better on ferritic stainless grades than on austenitic grades. Use at 20 °C for 7-60 seconds: reveals carbides sigma faintly attacked with etching up to 3 minutes. Use at 80°C (176°F) to boiling for 2-60 minutes: carbides dark, sigma blue (not always attacked), ferrite yellow to yellow-brown, austenite unattacked. Do not always get uniform etching.
100 mL water 1 0 g oxalic acid	Use for stainless steels at 6 V dc. Carbides revealed by etching for 15-30 seconds, grain boundaries after 45-60 seconds, sigma outlined after 6 seconds. 1-3 V also used. Dissolves carbides, sigma strongly attacked, austenite moderately attacked, ferrite unattacked.
100 mL water 20 g NaOH	Used to color ferrite in martensitic, PH or dual-phase stainless steels. Use at 3-5 V dc, 20°C, 5 seconds, stainless steel cathode. Ferrite outlined and colored tan.
40 mL water 60 mL HNO3	Electrolytic etch to reveal austenite boundaries but not twin boundaries in austenitic stainless steels (304, 316, etc.). Voltage is critical. Pt cathode preferred to stainless steel. Use at 1.4 V dc, 2 minutes.

Commonly Used Etchants for Magnesium and Alloys

Composition	Comments
25 mL water 75 mL 3-5 ethylene glycol 1 mL HNO3	Glycol etch, general purpose etch for pure Mg and alloys. Swab specimen seconds for F and T6 temper alloys, 1-2 minutes for T4 and 0 temper alloys.
19 mL water 60 mL ethylene glycol 20 mL acetic acid 1 mL HNO3	Acetic glycol etchant for pure Mg and alloys. Swab specimen 1-3 seconds for F and T6 temper alloys, 10 seconds for T4 and 0 temper alloys. Reveals grain boundaries in solution-treated castings and most wrought alloys.
100 mL ethanol 10 mL water 5 g picric acid	For Mg and alloys. Use fresh. Immerse specimen for 15-30 seconds. Produces grain contrast.

Commonly Used Etchants for Aluminum and Alloys

Composition	Comments
95 mL water 2.5 mL HNO3 1.5 mL HCI 1.0 mL HF	Keller’s reagent, very popular general purpose reagent for Al and Al alloys, except high-Si alloys. Immerse sample 10-20 seconds, wash in warm water. Can follow with a dip in conc. HNO3. Outlines all common constituents, reveals grain structure in certain alloys when used by immersion.
90-100 mL water 0.1-10 mL HF	General-purpose reagent. Attacks FeAl3, other constituents outlined. The 0.5% concentration of HF is very popular.
84 mL water 15.5 mL HNO3 0.5 mL HF 3g CrO3	Graff and Sargent’s etchant, for grain size of 2XXX, 3XXX, 6XXX, and 7XXX wrought alloys. Immerse specimen 20-60 seconds with mild agitation.
1.8% fluoboric acid in water	Barker’s anodizing method for grain structure. Use 0.5-1.5 A/in2, 30-45 V dc. For most alloys and tempers, 20 seconds at 1 A/in2 and 30 V dc at 20 °C is sufficient. Stirring not needed. Rinse in warm water, dry. Use polarized light; sensitive tint helpful.

Hints For Etching

Many etchants can be used by swabbing or by immersion. Swabbing is preferred for those specimens that form a tight protective oxide on the surface in air, such as Al, Ni, Cr, stainless steels, Nb (Cb), Ti and Zr. However, if the etchant forms a film, as in tint etchants, then immersion must be used as swabbing will keep the film from forming. Keller’s reagent reveals the grain size of certain aluminum alloys by forming a film. This will not occur if the etch is used by swabbing. Many etchants, and their ingredients, do present potential health hazards to the user. ASTM E 2014, Standard Guide on Metallography Laboratory Safety, describes many of the common problems and how to avoid them.

Etching Procedures

Microscopic examination is usually limited to a maximum magnification of 1000X — the approximate useful limit of the light microscope, unless oil immersion objectives are used. Many image analysis systems use relay lenses that yield higher screen magnifications that may make detection of fine structures easier. However, resolution is not improved beyond the limit of 0.2-0.3-um for the light microscope. Microscopic examination of a properly prepared specimen will clearly reveal structural characteristics such as grain size, segregation, and the shape, size, and distribution of the phases and inclusions that are present. Examination of the microstructure will reveal prior mechanical and thermal treatments give the metal. Many of these microstructural features are measured either according to established image analysis procedures, e.g., ASTM standards, or internally developed methods.

Etching is done by immersion or by swabbing (or electrolytically) with a suitable chemical solution that essentially produces selective corrosion. Swabbing is preferred for those metals and alloys that form a tenacious oxide surface layer with atmospheric exposure such as stainless steels, aluminum, nickel, niobium, and titanium and their alloys. It is best to use surgical grade cotton that will not scratch the polished surface. Etch time varies with etch strength and can only be determined by experience. In general, for high magnification examination the etch depth should be shallow; while for low magnification examination a deeper etch yields better image contrast. Some etchants produce selective results in that only one phase will be attacked or colored. Etchants that reveal grain boundaries are very important for successful determination of the grain size. A vast number of common etchants have been developed and it is displayed in Table 1 after this section.

ETCHING

Metallographic etching encompasses all processes used to reveal particular structural characteristics of a metal that are not evident in the as-polished condition. Examination of a properly polished specimen before etching may reveal structural aspects such as porosity, cracks, and nonmetallic inclusions. Indeed, certain constituents are best measured by image analysis without etching, because etching will reveal additional, unwanted detail and make detection difficult or impossible. The classic examples are the measurement of inclusions in steels and graphite in cast iron. Of course, inclusions are present in all metals, not just steels. Many intermetallic precipitates and nitrides can be measured effectively in the as-polished condition.

In certain nonferrous alloys that have non-cubic crystallographic structures (such as beryllium, hafnium, magnesium, titanium, uranium and zirconium), grain size can be revealed adequately in the as polished condition using polarized light. Figure (1) shows the microstructure of cold-drawn zirconium viewed in cross-polarized light. This produces grain coloration, rather than a “flat etched” appearance where only the grain boundaries are dark.

Figure 1: Mechanical twins at the surface of hot worked and cold drawn high-purity zirconium viewed with polarized light (200X).