Common Etchants Reference Guide

Comprehensive quick reference guide to common etching reagents and their applications for metallographic sample preparation. Includes both commercially available pre-mixed solutions and other commonly used etchants that can be prepared in-house.

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Get a comprehensive printable reference guide with common etchants organized by material type, including compositions, applications, and etching times. Includes both commercially available pre-mixed solutions and other widely used etchants.

Quick find

Jump to the etchant family for your material. The PDF download includes additional etchant families (titanium, copper, cast iron, tool steel) and a safety appendix.

Carbon & low-alloy steel Stainless steel Aluminum & Al alloys Nickel alloys

High-consequence hazards

Three reagent families on this page need more than standard PPE. Read these before mixing anything else.

DANGERPicral / Picric acid solutions

Picric acid is shock-sensitive and explosive when dry. Store wetted with at least 30% water by weight; 10% is the regulatory shipping minimum, not a safe lab-bench storage threshold.

Specific hazards

Dry picric acid (or its metal picrate salts) can detonate from friction, shock, or heat.
Picric acid forms shock-sensitive picrates with metals — never store in metal-capped bottles or near copper, lead, or zinc.
Toxic by ingestion and skin absorption; stains skin yellow as a warning sign of contact.

PPE & handling

Always confirm the bottle is wetted (visible water layer) BEFORE opening.
Use plastic or amber-glass containers with plastic-lined caps; never metal caps or metal-lined closures.
Mix in a fume hood; standard PPE — gloves, goggles, lab coat.
Never evaporate to dryness when concentrating; never grind dry crystals.

Storage & disposal

Store wetted with ≥30% water by weight in a labeled plastic or amber-glass bottle, away from acids and metals (Cu, Pb, Zn, Fe).
Inspect periodically for water-layer loss; refill before crystals begin to dry out.
Bottles with crusted necks or dried crystals must be handled by hazardous-materials specialists — do not open.
Spent picral solutions remain toxic; collect for organic-acid waste stream.

This callout is a summary, not a substitute for the SDS. Review the manufacturer's safety data sheet and your institution's chemical-handling SOP before mixing or applying any etchant.

EXTREME HAZARDKroll's Reagent

Contains hydrofluoric acid (HF). HF is bone-seeking and can cause delayed, lethal injury from small skin exposures.

Specific hazards

HF penetrates skin and reaches deep tissue and bone, causing systemic toxicity hours after exposure.
Symptoms can be delayed; a small splash that does not feel painful is still a medical emergency.
NOₓ fumes from HNO₃ are toxic to the respiratory tract.

PPE & handling

Mix and apply only in a fume hood.
HF-rated nitrile or neoprene gloves over standard nitrile, full face shield, lab coat, splash apron.
Calcium gluconate gel must be on hand BEFORE you mix the etchant.
Never let the surface dry between application and rinse — HF residue continues to attack.

Storage & disposal

Store in HF-compatible (PE/PP) bottle, never glass.
Neutralize spent etchant with calcium hydroxide or carbonate before disposal.

This callout is a summary, not a substitute for the SDS. Review the manufacturer's safety data sheet and your institution's chemical-handling SOP before mixing or applying any etchant.

DANGERAqua Regia

3 HCl : 1 HNO₃. Generates NOₓ, Cl₂, and NOCl; pressurizes if sealed — never store mixed.

Specific hazards

NOₓ + Cl₂ + NOCl evolution; severe respiratory and eye irritant.
Reacts violently with organics, ammonia, sulfides, and many metals — generates heat and toxic gases.
Pressure buildup ruptures sealed containers.

PPE & handling

Mix only in a fume hood, in small volumes, immediately before use.
PPE: nitrile gloves, full face shield, lab coat, splash apron.
Add HNO₃ to HCl, slowly, with cooling.

Storage & disposal

Never store mixed. Discard unused aqua regia by slow dilution into a large volume of cold water in the fume hood; neutralize and dispose.

This callout is a summary, not a substitute for the SDS. Review the manufacturer's safety data sheet and your institution's chemical-handling SOP before mixing or applying any etchant.

Preview

Note: Etchants marked with (Available as pre-mixed solution) are available commercially as pre-mixed solutions. Other etchants are commonly used in practice and can be prepared in-house.

Carbon Steel / Low Alloy Steel

2% Nital

(Available as pre-mixed solution)

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Composition: Ethanol, HNO₃ (2%)

Application: General purpose, reveals ferrite grain boundaries and pearlite. Ideal for softer steels.

Time: 5-30 seconds

3% Nital

(Available as pre-mixed solution)

Composition: Ethanol, HNO₃ (3%)

Application: Standard concentration for most carbon steels. Most commonly used.

Time: 5-30 seconds

Picral

(Available as pre-mixed solution)

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Composition: Ethanol, Picric acid (2-4g/100ml)

Application: Reveals cementite and pearlite, does not attack ferrite boundaries. Superior for pearlite structures.

Time: 10-60 seconds

Sodium Metabisulfite (10%)

Composition: Na₂S₂O₅ (10g), Water (100ml)

Application: For revealing prior austenite grain boundaries in quenched and tempered steels

Time: 10-30 seconds

Stainless Steel

Vilella's Reagent

(Available as pre-mixed solution)

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Composition: Picric Acid (1g), HCl (5ml), Ethanol (100ml)

Application: General purpose, most versatile for stainless steel. Works well for most grades.

Time: 5-30 seconds

Adler's Etchant

(Available as pre-mixed solution)

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Composition: Cu(NH₄)Cl₂ (9g), HCl (150ml), FeCl₃ (45g), DI Water (75ml)

Application: 300 series austenitic stainless steels and superalloys

Time: 5-30 seconds

Aqua Regia

Composition: 3 parts HCl : 1 part HNO₃

Application: For austenitic stainless steels. Very aggressive - reveals grain boundaries and twin boundaries. Handle in fume hood.

Time: 5-15 seconds

Aluminum & Aluminum Alloys

Keller's Reagent

(Available as pre-mixed solution)

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Composition: DI Water (190ml), HNO₃ (5ml), HCl (3ml), HF (2ml)

Application: Standard etchant for aluminum and aluminum alloys. Reveals grain boundaries and precipitates. Most common.

Time: 10-30 seconds

Tucker's Reagent

Composition: HCl (45ml), HNO₃ (15ml), HF (15ml), Water (25ml)

Application: For revealing grain structure in aluminum alloys, especially heat-treated materials

Time: 15-45 seconds

Nickel Alloys

Inconel Etchant

(Available as pre-mixed solution)

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Composition: Multiple Solutions (HNO₃, HCl, H₂O₂)

Application: Macro etch for nickel alloys including Inconel. Use fresh solution only.

Time: 10-30 seconds

Glyceregia

Composition: HCl (15ml), Glycerol (10ml), HNO₃ (5ml)

Application: For nickel-based superalloys. Reveals gamma prime and other phases. Excellent for superalloys.

Time: 10-30 seconds

Safety Guidelines

Always work in a well-ventilated area or fume hood
Wear appropriate PPE (gloves, goggles, lab coat)
Handle acids and corrosive chemicals with extreme care

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