This page is about making a specimen you can actually examine. Look up sectioning, mounting, grinding, polishing and etching material by material, and when something goes wrong, pick the defect you see and your material to get likely causes and fixes in order. For identifying what you see, use the Microstructure Atlas; for choosing an instrument, the Characterization Method Finder.
Steps, times, forces and speeds are typical values from the sources (mostly equipment makers' procedures for automatic machines) and change with equipment, consumables and the size and number of specimens. On a new material, adjust while checking that each step has removed the previous scratches. Compositions were compared between sources; where they differ, a range or the difference is given. Standards are cited by name, number and scope only.
Failure table 13
Identify the defect from its appearance and check the causes in order of frequency. Material-specific tips also appear in each material's procedure.
Parallel scratches in one direction, scratches coarser than the current abrasive, or scratches that show up after etching.
Usual step Grinding
Causes and fixes (most common first)
The previous step's scratches were not fully removed (too short, a grit skipped, too little force)
Fix Continue each step until all of the previous scratches are gone; rotate 90° to check. Allow about three times the time needed to establish the new pattern, and do not skip grits.
Coarse particles carried into a later step (specimen, holder, hands, cloth): randomly oriented scratches
Fix Clean ultrasonically (30–60 s) between steps, change gloves, dedicate each cloth to one abrasive and keep cloths covered.
Deformation hidden by the final polish was revealed by etching (not caused by the etchant)
Fix Extend the final polish and lower the force; if needed, go back to an earlier grinding step. Alternating polish and light etch confirms it.
Sectioning damage was not removed by the first grinding step (one or two deep scratches)
Fix Extend the first grinding step or go back to a coarser grit.
Worn, glazed or loaded paper or cloth (no longer cutting)
Fix Replace or dress paper and cloth and recharge the abrasive; on very hard materials SiC wears out quickly, so use diamond.
SiC embedded in a soft metal scratches during later steps
Fix Finish grinding on finer SiC with light force, or switch to diamond on a rigid disc.
Depends on the material: Soft
Material-specific tips
Sources: PACE troubleshooting; Struers Metalog Guide; Metkon 12 polishing artifacts
The edge drops below the focal plane, so coating thickness and case depth cannot be measured correctly.
Usual step Mounting
Causes and fixes (most common first)
A gap between specimen and resin (resin shrinkage)
Fix Cool hot mounts under pressure; for cold mounting use a slow-curing, low-shrinkage epoxy with the specimen at room temperature; consider vacuum impregnation.
The mount is much softer than the specimen
Fix Use an edge-retentive mount (DAP, mineral-filled epoxy, conductive phenolic).
A soft long-nap cloth or a long final polish
Fix Use a firmer low-nap cloth and keep the final polish short.
Too much force or unnecessary polishing time
Fix Reduce force and keep polishing time to what is needed.
Thin layers or oxide scales are unsupported
Fix Protect the edge with electroless nickel plating before mounting.
Depends on the material: Edge or surface layer of interest
Sectioning or compression mounting delaminated or chipped the coating
Fix Section perpendicular to the coating with a diamond or CBN blade on a precision saw (abrasive cut-off wheels can delaminate it), vacuum-impregnate in castable epoxy and reduce grinding force. Inspect the cut at once and re-section if it has delaminated (later polishing cannot recover it).
Depends on the material: Edge or surface layer of interest, Brittle, Porous
Material-specific tips
- Carburized, nitrided and decarburized cases Use an edge-retentive resin (DAP, mineral-filled epoxy), cool under pressure, and plate thin layers with electroless nickel before mounting (PACE).
- Zinc coatings and zinc Mount in slow-curing cold epoxy (negligible shrinkage, good adhesion) to avoid gaps (Struers).
- Thermal spray and hard coatings Edge delamination and chipping come from force and heat in sectioning and mounting: use precision sectioning, castable vacuum impregnation and lower grinding force (PACE).
Sources: PACE troubleshooting; Struers Metalog Guide; Metkon 12 polishing artifacts; Struers zinc coatings; PACE thermal spray
Hard particles, phases or coatings stand proud of a softer matrix; one phase is out of focus.
Usual step Polishing
Causes and fixes (most common first)
A resilient soft cloth follows the topography and removes the soft phase
Fix Use a less resilient, firmer or napless cloth.
The final polish is too long
Fix Cap the final polish at 1–3 minutes (if needed, a brief colloidal silica step).
Phases or layers of very different hardness side by side
Fix Use diamond on rigid composite discs to keep the surface flat, with minimal force and controlled speed.
Alumina was used on very hard carbides
Fix On materials with hard particles, polish with diamond to the end.
Depends on the material: Hard, Phases or layers of different hardness
Material-specific tips
Sources: Struers Metalog Guide; PACE troubleshooting; Metkon 12 polishing artifacts; PACE cermets
Teardrop-shaped streaks trailing from inclusions, pores or hard particles, all in the same direction.
Usual step Polishing
Causes and fixes (most common first)
One-directional motion between specimen and cloth (holder and platen not matched in direction and speed)
Fix Rotate holder and platen in the same direction at the same speed, and use a specimen holder (automatic machine).
A long-nap cloth, too much slurry, or settled suspension
Fix Use a napless or short-nap cloth kept moist but not wet; shake suspensions before use.
Too much force, or long polishing on a resilient cloth
Fix Reduce force (PACE suggests about 13–18 N, 3–4 lbf, for a 32 mm specimen) and shorten soft-cloth polishing.
Freehand polishing
Fix Use a specimen holder.
Sources: Struers Metalog Guide; PACE troubleshooting; Metkon 12 polishing artifacts
Yellow, brown or rainbow patches, outlines of water drops, uneven darkening.
Usual step Cleaning and drying
Causes and fixes (most common first)
Water or etchant dried on the surface
Fix Rinse with water, displace with ethanol and dry with clean warm air at once; if stained, lightly re-polish and re-etch.
Liquid trapped in a gap between specimen and resin seeps out
Fix Eliminate the gap (cool under pressure, vacuum impregnation, low-shrinkage resin) and store specimens in a desiccator.
Colloidal silica dried on the surface, or oily compressed air
Fix Run plain water on the cloth for the last 10–30 s of a colloidal silica step, and dry with oil-free air.
Hot water, or a hot specimen or etchant
Fix Avoid hot water; let the specimen cool before etching.
A water-sensitive material was ground, polished or cleaned with water
Fix For Mg use only ethanol-based lubricants and cleaning; for Zn keep aqueous steps short and finish with water-free lubricants and ethanol.
Depends on the material: Reacts with or is damaged by water
Material-specific tips
- Cast irons (gray, ductile) Water was left on the specimen: rinse with ethanol right after water and dry with warm air (PACE).
- Magnesium alloys A white powdery surface is oxidation from water: re-polish with ethanol-based lubricants, dry all equipment, work at low humidity, and re-section if severe (PACE).
- Zinc coatings and zinc Zinc is water-sensitive: keep aqueous steps short, use water-free lubricants and finish with ethanol; mount in low-shrinkage epoxy so gaps do not trap liquid (Struers).
Sources: PACE troubleshooting; Metkon 12 polishing artifacts; Struers Metalog Guide; PACE magnesium; Struers zinc coatings
Dark background with dissolved features and boundaries widened into trenches; pits may be mistaken for porosity.
Usual step Etching
Causes and fixes (most common first)
Etching time too long
Fix Re-polish and start short (the lower end of the source's range; 5–10 s for nital).
Etchant too concentrated
Fix Dilute to the published composition (nital no stronger than 5%).
Warm specimen or etchant
Fix Let specimen and etchant come to room temperature first.
A swab etchant was used by immersion (HF or aqua-regia type)
Fix Swab Kroll's, glyceregia and similar briefly instead of immersing.
Depends on the material: Passive, hard to etch
Material-specific tips
Sources: PACE troubleshooting; PACE carbon steel; PACE etchant pages
Little or no structure even after a long etch; weak contrast.
Usual step Etching
Causes and fixes (most common first)
A disturbed layer from polishing remains
Fix Re-polish or add a chemo-mechanical (colloidal silica) step.
Exhausted or expired etchant
Fix Mix fresh (ammonium persulfate decomposes in 1–2 days, NH₄OH + H₂O₂ loses activity in minutes, and HF-based etchants degrade).
The wrong etchant for the alloy
Fix Match the etchant to the alloy (nital hardly works on stainless steel: use oxalic electrolytic, glyceregia, etc.).
Too short or too weak
Fix Lengthen time gradually or use a slightly stronger solution (2% → 3% nital).
Oil, fingerprints or polishing residue
Fix Clean with ethanol before etching.
A passive or oxide film formed after polishing
Fix Etch right after polishing; for stainless, activate with a brief glyceregia swab before electrolytic etching.
Depends on the material: Passive, hard to etch
Material-specific tips
- Austenitic stainless steels (304, 316, etc.) A passive film from polishing: activate with a brief glyceregia swab before the oxalic etch, or lengthen the electrolytic time (PACE).
- Duplex stainless steels (2205, 2507, etc.) Vilella's and oxalic do not separate ferrite and austenite; switch to Beraha's I or Klemm's I (PACE).
- Titanium and titanium alloys The HF-containing etchant has degraded: mix fresh, or switch from swabbing to immersion; Ti oxidizes quickly after polishing, so etch promptly (PACE).
- Nickel-base alloys and superalloys (Inconel, Hastelloy, etc.) These alloys resist etching: change etchant (Adler's, aqua regia) or etch electrolytically (PACE).
- Ceramics (alumina, zirconia, silicon nitride, silicon carbide) Look first with DIC or polarized light; if grain boundaries still do not show, thermal etch (PACE).
Sources: PACE troubleshooting; PACE carbon steel; PACE stainless steel; PACE copper alloys; PACE titanium
Pits appear during or after etching on a surface that was clean after polishing.
Usual step Etching
Causes and fixes (most common first)
Etchant too strong or time too long
Fix Dilute or shorten; or choose a milder etchant.
Stale etchant
Fix Use fresh etchant.
Etchant pooled and dried on the surface
Fix Rinse at once, displace with ethanol and dry with warm air.
Galvanic action with a metal clip
Fix Insulate clips or swab-etch instead.
Electrolytic voltage too high or time too long
Fix For oxalic electrolytic, drop from 6 V to 3–4 V or shorten.
Depends on the material: Passive, hard to etch
Material-specific tips
Sources: PACE troubleshooting; PACE aluminum; PACE stainless steel
Smooth-bottomed dark holes where graphite, inclusions or particles should be (unlike sharp-edged real pits). Re-polish briefly and look at the same spot: real pores stay, pull-outs shift.
Usual step Polishing
Causes and fixes (most common first)
Long-nap cloth
Fix Use a napless or short-nap cloth and check after every step.
Excessive force
Fix Reduce force (about 13–18 N, 3–4 lbf, for inclusion work on steel).
Polishing too long
Fix Shorten the final polish (about 2 minutes at most for inclusion rating in steel).
SiC paper used too late on cast iron
Fix Move to diamond early and finish with 1 µm diamond on a short-nap pad.
Depends on the material: Graphite to retain
Porous or brittle specimen polished without impregnation
Fix Vacuum-impregnate with low-viscosity epoxy first and start from a finer grit.
Depends on the material: Porous, Brittle
Hard particles and a softer matrix or binder remove at different rates
Fix Lower the force and use softer cloths for the fine steps; if relief also appears, do 9 and 3 µm on rigid composite discs.
Depends on the material: Hard, Phases or layers of different hardness
Material-specific tips
- Tool and high-speed steels Carbide pull-out comes from excessive force or the wrong pad: reduce force, use softer pads for the fine steps and check after each step (PACE).
- Cast irons (gray, ductile) Reduce force to 22 N (5 lb) per specimen, use 1 µm diamond on a short-nap pad, and do not use SiC paper too late. For malleable iron try a rigid composite disc for fine grinding (PACE).
- Welds (weld metal, HAZ, base metal) Pull-outs and smearing can mimic weld porosity and cracks; re-polish briefly and look at the same spot (real defects stay) (PACE).
- Aluminum alloys Pulled-out oxides (black voids) cannot be recovered: keep force at 22–45 N, use alumina paper for planar grinding, and re-section if needed (PACE).
- Copper, brass, bronze Lead inclusions dissolve on etching and look like voids; image lead as-polished (PACE).
- Nickel-base alloys and superalloys (Inconel, Hastelloy, etc.) For pull-out of γ′ and carbides: lower force, extend polishing, use softer final cloths and consider vibratory polishing (PACE).
- Thermal spray and hard coatings Unfilled porosity or insufficient impregnation: re-impregnate under vacuum with low-viscosity epoxy, grind with diamond instead of SiC, and use fluorescent dye to tell pores from pull-outs (PACE).
- Cemented carbides (WC-Co) and cermets The binder polishes faster than the carbide: use softer cloths and lighter force; for relief, do the 9 and 3 µm steps on rigid composite discs (PACE).
- Ceramics (alumina, zirconia, silicon nitride, silicon carbide) Try in order: lower force, firmer pad, extend the previous step, then vibratory or ion-beam polishing. Vacuum-impregnate porous material with low-viscosity epoxy (PACE).
Sources: PACE troubleshooting; Struers Metalog Guide; Metkon 12 polishing artifacts; PACE cast iron; PACE cermets; PACE thermal spray
The surface looks bright but bland: faint grain boundaries and blurred second phases.
Usual step Polishing
Causes and fixes (most common first)
Finished with mechanical polishing only (a disturbed layer remains)
Fix Finish chemo-mechanically with colloidal silica or fine alumina on a soft cloth at light force; use polish–etch–polish cycles.
Excessive force in the final step
Fix Use light force for the final step (PACE suggests 9–13 N, 2–3 lbf).
Damage from coarse grinding carried forward
Fix Add fine grinding steps (800 and 1200 grit, P1500 and P2500) for pure Al, Cu and low-carbon steel.
Dull abrasive or poor lubrication
Fix Use fresh abrasive and adequate lubrication.
Mechanical twins introduced by sectioning or grinding force (not from processing history)
Fix Reduce sectioning and grinding force, use a thin precision blade, and remove the deformed layer step by step.
Depends on the material: Prone to mechanical twinning
Material-specific tips
- Carbon and low-alloy steels (annealed, normalized) On soft low-carbon steel add 800 and 1200 grit and alternate polish → light nital → polish (PACE).
- Cast irons (gray, ductile) Graphite looking black means matrix smeared into the cavities: extend 1 µm by 1–2 min, check the pad is not loaded, and if needed re-grind lightly on fresh alumina paper (PACE).
- Copper, brass, bronze Reduce force to 22–31 N and match pad stiffness to the abrasive size. If deformation reappears after each etch, coarse-grinding damage remains: return to the SiC sequence with light force (PACE).
- Titanium and titanium alloys Mechanical twins come from sectioning and grinding force: reduce it and use a thin precision blade (Metkon); finish with colloidal silica + H₂O₂ (Witt & Nowell).
- Finishing for EBSD (any metal) For EBSD any residual mechanical damage kills the patterns: extend the colloidal silica finish (minutes to hours) and add H₂O₂ for Ti (Witt & Nowell).
Sources: PACE troubleshooting; Metkon 12 polishing artifacts; PACE carbon steel
Color in only part of the surface, wipe marks, blotchy color. Note: different colors in different grains are normal (they depend on orientation).
Usual step Etching
Causes and fixes (most common first)
Residual preparation damage (tint films are sensitive to it)
Fix Polish more carefully than for black-and-white etching, to a damage-free surface (electropolishing is not required).
Uneven application or a contaminated surface
Fix Immerse and agitate gently; clean with ethanol before etching; polish with uniform pressure.
Old or cloudy reagent
Fix Use freshly made reagent (discard when cloudy); add the K₂S₂O₅ to Beraha's reagents just before use.
Dried with compressed air
Fix Rinse Klemm's with water then ethanol and let it air-dry.
Material-specific tips
Sources: Vander Voort, color metallography; PACE troubleshooting; PACE etchant pages; PACE stainless steel
Bright specks that do not move and a finish that will not come scratch-free; Si or C may show in EDS.
Usual step Grinding
Causes and fixes (most common first)
SiC or fine diamond on a soft metal
Fix Reduce force and use diamond on a rigid disc; for soft materials use grain sizes of 3 µm and finer only on resilient cloths; re-run 1 µm on a fresh pad.
Loaded or worn paper
Fix Replace papers often and keep enough water flowing.
Too much lubricant
Fix Use less lubricant and keep mechanical polishing short; finish solders by vibratory polishing.
Depends on the material: Soft, Heat-sensitive
Material-specific tips
- Aluminum alloys Embedded SiC from fine grinding shows as bright specks: re-run 1 µm on a fresh pad and replace glazed paper (PACE).
- Solders and tin (Sn–Pb, lead-free) Use less lubricant, keep mechanical polishing short, and finish with vibratory polishing to remove embedded particles (Buehler).
Sources: PACE troubleshooting; Struers Metalog Guide; Metkon 12 polishing artifacts; Buehler lead-free solder; PACE stainless steel
Temper colors on the polished surface, a dark band near the cut, scattered hardness on hardened steel.
Usual step Sectioning
Causes and fixes (most common first)
Inadequate coolant or forcing the cut
Fix Keep coolant flowing and let the wheel cut without forcing. Heat damage cannot be polished out: grind below the affected zone or re-section.
Cut-off wheel bond not suited to the material
Fix Use a soft-bond wheel for hard, brittle materials and a hard-bond wheel for soft, ductile ones; diamond blades for cemented carbides and ceramics.
Dry grinding or glazed paper
Fix Grind wet only and replace glazed paper.
Heat of hot compression mounting (150–180 °C)
Fix Mount in room-temperature castable epoxy (precipitation-hardened Al, solders, etc.).
Depends on the material: Heat-sensitive
Material-specific tips
Sources: PACE troubleshooting; PACE tool steel; Struers Metalog Guide; PACE aluminum; PACE stainless steel; Buehler lead-free solder
Safety
Some etchants are seriously hazardous — hydrofluoric acid, picric acid, solutions that can release cyanide, and others. Before mixing or using one, always check the SDS of each chemical and your institution's safety rules, and work in a fume hood with protective equipment.
For every etchant
Before mixing or using any etchant, read the SDS (safety data sheet) of every chemical in it and follow your institution's safety rules and the instructions of the person in charge. Compositions and times on this page are guides for choosing a procedure; they do not replace your institution's safety procedures. The standard (ASTM E407) also states that setting safety practices is the user's responsibility.
Mix and use etchants in a fume hood. Always wear gloves when handling chemicals, and add a face shield for the more dangerous solutions.
Order of mixing: solvents first, then dissolve the salts, then add the dangerous acids last, slowly and with stirring. Sulfuric acid goes in last, slowly into water. Never just pour one liquid into another.
Collect used etchant and rinse liquids as your institution requires; do not pour them down the drain or put them in ordinary waste.
Sources: Vander Voort, lab safety; ASTM E407-23; Univ. of Saskatchewan picric SOP
Solutions containing hydrofluoric acid (HF) or ammonium bifluoride
HF on the skin may cause no immediate pain or visible damage. Severe pain may be the only symptom for hours, and visible damage may appear only 12–24 hours later. Burns can become deep before they are noticed, so treat dilute solutions (such as 0.5% HF) the same way.
HF can cause severe electrolyte problems in the body, and calcium gluconate (gel, solution or medication) is used to treat it. Before using any HF-containing etchant, keep calcium gluconate first-aid gel within reach and know how to use it (the ASM Handbook safety article also says laboratories using HF should stock an antidote kit).
If exposed: rinse at once with plenty of water (safety shower) for 15 minutes, remove contaminated clothing, apply calcium gluconate gel after rinsing, and get medical attention immediately. For the eyes, rinse at an eyewash for 15 minutes and get medical attention.
HF attacks glass: mix and store it in polyethylene (PE) or polypropylene (PP) containers labeled for HF.
Solutions with ammonium bifluoride (NH₄HF₂) — Beraha's tint etchants, LB1, Weck's reagent for Ti — also release HF; use the same precautions as for HF.
Fluoroboric acid (HBF₄, in Barker's reagent) also causes severe skin burns and eye damage, gives hydrogen fluoride on decomposition and attacks glass (SDS). Handle it with the same precautions as HF and store it in PE bottles.
Sources: CDC hydrogen fluoride; NIOSH Pocket Guide: HF; Columbia Univ. HF first aid; Vander Voort, lab safety; PACE etchant pages; PACE cast iron; Fluoroboric acid SDS
Solutions containing picric acid
Dry picric acid is an explosive (GHS: explosive, division 1.1). Grinding, shock or friction can set it off.
It reacts with metals (especially copper, lead, mercury and zinc) to form shock-sensitive picrates, which detonate even more readily when dry. Do not use metal caps, spatulas or bench tops with it.
Never let the solution dry on a surface or on the bottle threads. Store the solid wetted with water, check about monthly that it has not dried out, and top up with water. Do not keep mixed etchant for long (discard at the end of the day, or within 7 days, depending on the source and your rules), and rinse it off the specimen before it can dry.
If you find dry crystals in the bottle or on its threads, do not touch or move it; contact your safety or waste-management staff.
Sources: MHLW model SDS: picric acid; Vander Voort, lab safety; Univ. of Saskatchewan picric SOP; PACE etchant pages
Nitric acid in alcohol (nital and similar)
Nital with up to 3% nitric acid in ethanol can be mixed and stored safely in small quantities. Stronger solutions should be mixed daily, used in an open dish and never stored in a tightly stoppered bottle (pressure builds up).
Methanol is a cumulative poison: it is absorbed through the skin as well as by ingestion and inhalation, and damages the central nervous system. Use ethanol whenever possible; if methanol is used, keep nitric acid at 5% or less.
Tightly stoppered bottles have exploded from pressure build-up (NOx). Drill a small vent hole in the cap or vent the bottle from time to time.
Mixtures of methanol with more than 5% nitric acid can decompose violently if heated; PACE's tin etching table also says not to exceed 10% for nital.
Put the alcohol in first and add the nitric acid to it.
Sources: Vander Voort, lab safety; PACE safety fundamentals; PACE etchant pages; PACE tin alloy etching
Aqua regia and glyceregia (HCl + HNO₃)
Mixing releases heat, and the solution decomposes with time, giving off chlorine and nitrogen dioxide. Mix a small amount just before use, do not let it stand for more than about 15 minutes, and discard it right after use.
Never close the mixed solution in a sealed container (it pressurizes); discard a pressurized container at once. Work in a fume hood.
Glyceregia contains nitric acid and glycerol. Strong mixtures of nitric acid with glycerol (or glycols) can be explosive, and the solution decomposes readily: mix only a small amount and discard it immediately after use. Never use glycerol in anhydrous solutions containing nitric and sulfuric acids (nitroglycerin can form).
Sources: Vander Voort, lab safety; PACE etchant pages; PACE stainless steel
Potassium ferricyanide solutions (Murakami's)
Potassium ferricyanide is poisonous but stable and reasonably safe to use. In contact with acid, however, it releases hydrogen cyanide: keep the solution alkaline, never acidify it, and never add it to acid waste.
Hot solution can boil over; use it in a fume hood and let it cool before disposal.
Potassium and sodium cyanide are a different matter: extremely toxic, and particularly hazardous in contact with acids or acid fumes. Only trained people familiar with them should use them, so etchants containing them are not given on this page.
Sources: Vander Voort, lab safety; PACE etchant pages; PACE stainless steel; ASTM E2014-17
Sodium or potassium hydroxide solutions
Strongly caustic to skin and eyes. Heat warm-use solutions in PE or PP containers, not in glass.
Sources: PACE etchant pages
Solutions containing hydrochloric acid (including ferric and cupric chloride etchants)
Concentrated hydrochloric acid attacks the skin strongly, causing severe burns, is very dangerous to the eyes and irritates the nose and throat. It gives off hydrogen chloride gas, so handle it in a fume hood.
When diluting, add acid to water, never the reverse. Concentrated acids in contact with metals can liberate hydrogen, a fire and explosion hazard.
Sources: Vander Voort, lab safety; PACE safety fundamentals
Oxalic acid solutions
Harmful if swallowed; causes serious eye damage and irritates the skin and respiratory tract (GHS classification).
It reacts violently with strong oxidants, so never mix it with nitric acid. It can react with some silver compounds to form explosive silver oxalate.
Sources: MHLW model SDS: oxalic acid; Vander Voort, lab safety
Solutions with oxidizers (ammonium persulfate, potassium permanganate, hydrogen peroxide)
Ammonium persulfate is an oxidizing solid: mixed with combustibles it ignites easily and burns violently. It may cause allergy or asthma if inhaled and allergic skin reactions (GHS respiratory and skin sensitizer, category 1). Avoid breathing the dust and store it away from combustibles.
Potassium permanganate is a fire and explosion hazard, especially in contact with organic material. Never mix it with sulfuric acid: a violent explosion can result.
3% hydrogen peroxide is reasonably safe to use, but the 30% solution is a very powerful oxidant. Both are very damaging to the eyes.
Sources: MHLW model SDS: ammonium persulfate; Vander Voort, lab safety
Tint etchants with potassium metabisulfite (Klemm's, Beraha's, LB1)
Potassium metabisulfite causes serious eye damage (GHS category 1). Contact with acids liberates a toxic gas, so keep it out of acid waste. Add it to Beraha's I reagent (which contains HCl) just before use and work in a fume hood.
Sources: Potassium metabisulfite SDS; PACE stainless steel
Electrolytic etching (oxalic acid, NaOH, Barker's, etc.)
Electrolytic etching uses a DC power supply with conductive acidic or alkaline solutions; conductive solution and wet hands can deliver a painful shock, so make sure your hands are dry before adjusting the power supply.
Electrolytic etching can generate fumes and gas, including hydrogen. Work in a fume hood or a well-ventilated area.
Sources: PACE safety fundamentals
Perchloric acid electropolishing solutions (no recipes on this page)
Perchloric acid is a strong oxidizer. It may explode when heated; above 72% it is unstable and, when dry or drying, can explode from shock or vibration. It reacts violently with combustible and organic material and can ignite paper and similar materials at room temperature.
Contact with organic material forms highly unstable perchlorates. Perchloric acid–acetic anhydride electrolytes are not recommended. Keep such solutions cold (oxidizing power rises with temperature), do not store them, and never electropolish specimens mounted in phenolic or other plastics in them (explosions have resulted). The fumes need a special hood with a water wash-down.
This page gives no compositions or conditions for perchloric-acid electrolytes. If electropolishing is needed, do it where dedicated equipment and written procedures exist, together with the person in charge.
Sources: MHLW model SDS: perchloric acid; Vander Voort, lab safety
Solutions with hexavalent chromium (CrO₃) (not given on this page)
Chromium trioxide is a strong oxidizer classified under GHS as a category 1A carcinogen, fatal by inhalation or skin contact, and a respiratory sensitizer, among other hazards.
This page does not give procedures for etchants containing hexavalent chromium (Palmerton's reagent for zinc, Graff–Sargent's reagent for aluminum, etc.). Alternatives are listed under each material.
Sources: MHLW model SDS: chromium trioxide; PACE zinc alloy etching; PACE aluminum
Solutions not given on this page
The following are especially hazardous, so neither compositions nor procedures are given. They are used only where dedicated equipment, written procedures and training are in place.
- Etchants containing potassium or sodium cyanide (used for gold and other precious metals) — extremely toxic (see the ferricyanide entry above)
- Perchloric-acid electrolytes (e.g. for EBSD of Ti, Al and steel) — explosion hazard (see perchloric acid above)
- Hexavalent-chromium etchants (Palmerton's, Graff–Sargent's, etc.) — carcinogenic (see above)
- Tint etchants made with cadmium or lead salts (Beraha's CdS and PbS reagents) — they use heavy-metal salts
- Chemical etching of ceramics with molten salts, boiling concentrated acids or concentrated HF — treated as serious hazards in the PACE guide. For ceramics, try optical contrast and thermal etching first
Etchants 29
Compositions are given in mL (volume) and g (mass); acids are concentrated reagent grade unless stated otherwise. Read the safety notes before using any etchant: For every etchant
Hazards Nitric acid + alcohol
- Composition
- 1–10 mL HNO₃ + 90–99 mL ethanol; usually 2% (2 mL HNO₃ + 98 mL ethanol). Methanol is also used, but it is a cumulative poison absorbed through the skin, so prefer ethanol (with methanol, keep HNO₃ at 5% or less). Mix solutions above 3% daily and do not stopper them tightly
- Use and time
- Immerse or swab; 2% for 5–30 s (start at 5–10 s). Harder steels 3–5% (not above 5%)
- Zinc coatings: 0.5–2% for a few seconds (over-etches easily). Mg alloys: 1–2% for 5–15 s, rinse with ethanol
- Reveals
- Ferrite grain boundaries and pearlite (both), martensite, carburized and nitrided cases, the matrix of cast irons. The most common etchant for irons and steels.
Sources: PACE etchant pages; PACE carbon steel; Bursa Technical Univ. lab notes; Metkon etchant table; Struers zinc coatings; PACE magnesium
Hazards Picric acid
- Composition
- 2–4 g picric acid + 100 mL ethanol (4% is usual)
- Use and time
- Immerse 5–60 s (cast iron: swab 10–30 s). Rinse with ethanol, then water, and dry at once
- Reveals
- Attacks cementite, not ferrite: resolves pearlite lamellae, spheroidized carbides and tempered-carbide distribution. It does not reveal ferrite grain boundaries; combine with nital when both are needed.
Sources: PACE etchant pages; PACE carbon steel; Bursa Technical Univ. lab notes; Metkon etchant table
Hazards Picric acid HCl
- Composition
- 1 g picric acid + 5 mL HCl + 100 mL ethanol
- Use and time
- Immerse or swab 5–60 s; rinse with ethanol
- Reveals
- The standard for martensitic and PH stainless steels and hardened tool and high-speed steels: prior-austenite grain boundaries, tempered martensite, carbides.
Sources: PACE etchant pages; PACE stainless steel; PACE tool steel; Bursa Technical Univ. lab notes
Hazards Metabisulfite
- Composition
- 10 g Na₂S₂O₃ + 3 g K₂S₂O₅ + 100 mL water (Vander Voort's 10/3 reagent; a ready-made product is 5–15% thiosulfate and 2–5% metabisulfite by weight)
- Use and time
- Immerse 30–120 s. Polish to 0.05 µm (damage-free surface)
- Reveals
- Tints ferrite (cyan/orange), martensite (brown) and bainite (intermediate); retained austenite stays unstained (colors depend on orientation and composition).
Sources: Vander Voort, color metallography; PACE etchant pages
Hazards Metabisulfite
- Composition
- 50 mL saturated aqueous Na₂S₂O₃ + 1 g K₂S₂O₅
- Use and time
- Immerse 30 s–3 min (until colored). Rinse with water then ethanol and air-dry only (no compressed air)
- Reveals
- Tints twinned grains in brass, the phases of duplex stainless, and ferrite and pearlite in steels (colors depend on orientation and composition).
Sources: Vander Voort, color metallography; PACE stainless steel; PACE copper alloys; PACE etchant pages
Hazards Metabisulfite
- Composition
- 50 mL saturated aqueous Na₂S₂O₃ + 5 g K₂S₂O₅
- Use and time
- Immerse 60–120 s; warming to about 40 °C improves staining
- Reveals
- Colors ferrite blue and martensite brown to black; retained austenite stays white (e.g. to sort out the matrix of austempered ductile iron).
Sources: PACE cast iron; PACE copper alloys
Hazards HF HCl Metabisulfite
- Composition
- Stock: 1000 mL water + 200 mL HCl + 24 g NH₄HF₂
- Working: 100 mL stock + K₂S₂O₅ (0.1–0.2 g for martensitic, 0.3–0.6 g for austenitic and ferritic grades; PACE uses 1 g), added just before use
- Use and time
- Immerse 30 s–2 min (until colored)
- Reveals
- Tints the grains of stainless steels, including twinned grains. PACE's first choice for telling the phases of duplex stainless apart.
Sources: Vander Voort, color metallography; PACE stainless steel; PACE cast iron
Hazards HF Metabisulfite
- Composition
- 20 g NH₄HF₂ + 0.5 g K₂S₂O₅ + 100 mL water (dissolve in hot water)
- Reveals
- In duplex stainless steels it colors the austenite, not the ferrite.
Sources: Vander Voort, color metallography
Hazards Oxalic acid Electrolytic
- Composition
- 10 g oxalic acid dihydrate + 100 mL water
- Use and time
- Electrolytic: specimen anode, stainless cathode, 6 V DC (electrodes 25 mm apart). 15–30 s for general structure (Ni alloys 5–30 s at 3–6 V); for the sensitization test (ASTM A262 Practice A) follow the standard
- If pitting appears, lower to 3–4 V or shorten
- Reveals
- Grain boundaries in austenitic stainless (step/dual/ditch per A262 Practice A), sigma phase, carbides.
Sources: PACE etchant pages; PACE stainless steel; PACE nickel alloys; Bursa Technical Univ. lab notes; MHLW model SDS: oxalic acid
Hazards Strong alkali Electrolytic
- Use and time
- Electrolytic: specimen anode, 2–6 V DC, 5–30 s
- Reveals
- Reveals sigma phase in stainless steels as orange to brown.
Sources: PACE stainless steel
Hazards HCl + HNO₃
- Composition
- 15–30 mL HCl + 5–10 mL HNO₃ + 10 mL glycerol (ratios vary between sources: 15:10:10 on PACE's etchant page, 10 HNO₃ + 20–30 HCl + 10 glycerol in its stainless guide, 15:5:10 in its Ni-alloy guide)
- Use and time
- Swab 10–60 s (do not immerse: heat builds locally). Rinse with water then ethanol. Use freshly mixed; never store
- Reveals
- Grain boundaries, twins, carbides and sigma phase in austenitic stainless and Ni-base alloys; also used as a brief activating swab before oxalic electrolytic etching.
Notes
- Strong mixtures of nitric acid and glycerol can be explosive; mix only a small amount and discard right after use (see the safety section)
Sources: PACE etchant pages; PACE stainless steel; PACE nickel alloys; Vander Voort, lab safety
Hazards HCl + HNO₃
- Use and time
- 5–15 s; use freshly mixed and discard at once
- Reveals
- Inconel and other hard-to-etch Ni-base alloys.
Sources: PACE nickel alloys; Vander Voort, lab safety
Hazards HCl
- Composition
- 10 g CuSO₄ + 50 mL HCl + 50 mL water (a ready-made product is 10–20% HCl and 2–10% CuSO₄ by weight)
- Use and time
- Immerse or swab 5–60 s; a few drops of H₂SO₄ make it more active
- Reveals
- Grain boundaries and carbides in Ni-base superalloys (Inconel 600/625/718), austenitic stainless and Co-base alloys.
Sources: Metkon etchant table; PACE etchant pages
Hazards HCl
- Composition
- 5 g CuCl₂ + 100 mL HCl + 100 mL ethanol
- Use and time
- Swab or immerse 5–60 s (duplex stainless 30 s–2 min)
- Reveals
- Grain boundaries in stainless steels, ferrite in duplex grades, sigma phase, and the structure of Ni alloys such as Monel.
- Materials
- Duplex stainless steels (2205, 2507, etc.) · Austenitic stainless steels (304, 316, etc.) · Martensitic and PH stainless steels (410, 420, 440C, 17-4 PH, etc.) · Nickel-base alloys and superalloys (Inconel, Hastelloy, etc.)
Sources: Metkon etchant table; PACE etchant pages; PACE stainless steel; PACE nickel alloys
Hazards HCl
- Composition
- 45 g FeCl₃ + 9 g copper ammonium chloride + 150 mL HCl + 75 mL water
- Reveals
- Hastelloy and other highly corrosion-resistant Ni alloys; grain boundaries in ferritic, martensitic and PH stainless steels.
Sources: PACE nickel alloys; PACE etchant pages
Hazards HCl
- Composition
- 5 g CuCl₂ + 40 mL HCl + 30 mL water + 25 mL ethanol
- Reveals
- Strain patterns (slip lines, deformation bands) from cold work in low-carbon and maraging steels.
Sources: Bursa Technical Univ. lab notes; PACE etchant pages
Hazards Ferricyanide Strong alkali
- Composition
- 10 g K₃Fe(CN)₆ + 10 g KOH (or 7–10 g NaOH) + 100 mL water; dissolve the alkali in the water first
- Use and time
- Cemented carbide: 10–60 s at room temperature. Carbides in steels: cold, or at 50–80 °C for 3–10 min. Use fresh. Never acidify
- Reveals
- WC grains and the Co binder distribution in WC-Co (cold); carbides in steels (Cr carbides in D2/A2, carbides in high-speed steel); sigma phase in stainless.
Sources: PACE etchant pages; PACE cermets; PACE tool steel; Metkon etchant table; Bursa Technical Univ. lab notes
Hazards HF HCl
- Composition
- 2 mL HF + 3 mL HCl + 5 mL HNO₃ + 190 mL water (the 1.0 : 1.5 : 2.5 : 95 mL in PACE's Al guide is the same ratio); mix in PE/PP
- Use and time
- Immerse 8–20 s. Rinse thoroughly with cold (or warm) water; do not rinse aluminum with alcohol. Use fresh
- Ti alloys: immerse 3–10 s (PACE Ti guide) or 10–20 s (Metkon)
- Reveals
- Grain boundaries and second-phase particles (Al₂Cu, Mg₂Si, MgZn₂) in wrought Al alloys (1xxx–7xxx), cold-worked vs recrystallized grains. May leave a Cu stain on 2xxx alloys; switch to 0.5% HF there. Also used on Ti alloys.
Sources: PACE etchant pages; PACE aluminum; PACE titanium; Metkon etchant table
Hazards HF
- Composition
- 0.5 mL HF + 99.5 mL water (in PE/PP)
- Reveals
- Grain boundaries and second-phase particles in Al alloys; an alternative to Keller's on Cu-bearing 2xxx alloys.
Sources: PACE etchant pages
Hazards HF Electrolytic
- Composition
- 4–5 mL fluoroboric acid (HBF₄, 48%) + 200 mL water
- Use and time
- Specimen anode, Al or stainless cathode, 20–30 V DC for 30 s–3 min. Rinse with water and view in polarized light (with a sensitive tint plate)
- Reveals
- The anodic film thickness depends on grain orientation, so grains show different colors in polarized light (grain size and orientation contrast).
Notes
- Fluoroboric acid is corrosive; use in a fume hood with PPE (PACE)
- Fluoroboric acid gives HF on decomposition and attacks glass: use HF precautions and PE bottles (SDS)
Sources: PACE etchant pages; PACE aluminum; Vander Voort, color metallography; Fluoroboric acid SDS
Hazards Strong alkali
- Composition
- 1–10 g NaOH + 100 mL water (1% for Al microstructure; 10–20% for macrostructure)
- Use and time
- Al: 1%, swab or immerse 30–60 s (remove smut with HNO₃). Macro: 10–20% at 50–70 °C for 30–60 s. Zn: 10% for 1–5 s
- Reveals
- Al microstructure (an HF-free option); forging flow lines, segregation and weld macrostructure in Al; pure and low-alloy zinc.
Sources: PACE aluminum; PACE etchant pages; PACE zinc alloy etching; Bursa Technical Univ. lab notes
Hazards Strong alkali Oxidizer
- Composition
- 4 g KMnO₄ + 1 g NaOH + 100 mL water (PACE's etchant page uses 4 g NaOH, warm)
- Use and time
- Immerse 20–60 s (until colored)
- Reveals
- Colors the grains of Al alloys; cast alloys color more easily than wrought ones.
Notes
- KMnO₄ stains skin and benches; handle in a tray (PACE)
Sources: Vander Voort, color metallography; PACE aluminum; PACE etchant pages
Hazards HF
- Composition
- 1–3 mL HF + 2–6 mL HNO₃ + about 100 mL water (Metkon: 2 HF + 6 HNO₃ + 92 water; PACE: about 2 mL HF + 6 mL HNO₃ + 100 mL water); in PE/PP
- Use and time
- Swab 3–15 s (immersion over-attacks with HF). Rinse thoroughly with cold water. PACE's Ti guide also lists immersion for 10–30 s, but its etchant page specifies swabbing
- Reveals
- Alpha (light) and beta (dark) phases, transformed-beta colonies and grain boundaries in Ti alloys (CP-Ti, Ti-6Al-4V, etc.); also Zr alloys.
Sources: PACE etchant pages; PACE titanium; Metkon etchant table
Hazards HF
- Composition
- 100 mL water + 25 mL ethanol + 2 g NH₄HF₂ (the original 50 mL ethanol produces etch artifacts)
- Use and time
- Immerse a few seconds at 20 °C
- Reveals
- Colors Ti grains by orientation (mechanical twins are visible); with polarized light also used to measure alpha case.
Sources: Vander Voort, color metallography; PACE titanium
Hazards Oxidizer
- Composition
- 10 g (NH₄)₂S₂O₈ + 90 mL water
- Use and time
- Swab or immerse 5–60 s. Mix fresh (it decomposes within 1–2 days)
- Reveals
- Grain boundaries and annealing twins in OFHC copper, brass, nickel silver and bronze; alpha and beta phases in brass.
Sources: PACE copper alloys; PACE etchant pages; MHLW model SDS: ammonium persulfate
Hazards HCl
- Composition
- 5–10 g FeCl₃ + 5–50 mL HCl + 100–120 mL water, ethanol or methanol (adjust strength to the alloy)
- Reveals
- Grain boundaries and phases in copper alloys, Sn segregation (dendrites) in bronze, tin–copper alloys.
Notes
- If you make it with methanol, remember that methanol is toxic and is absorbed through the skin. Use ethanol where possible (see also the nitric acid and alcohol entry in the safety section)
Sources: PACE copper alloys; Bursa Technical Univ. lab notes; PACE tin alloy etching
Hazards Oxidizer
- Composition
- Concentrated NH₄OH : 3% H₂O₂ : water = 1 : 1 : 1 (water optional)
- Use and time
- Swab 5–30 s. It loses activity within minutes, so mix just before use
- Reveals
- Grain boundaries in copper and brass.
Notes
- Concentrated ammonia gives off strongly irritating vapor. Handle it in the fume hood (SDS)
Sources: Bursa Technical Univ. lab notes; PACE copper alloys
Hazards Picric acid
- Composition
- 4.2–6 g picric acid + 5–30 mL acetic acid + 0–15 mL water + 100 mL ethanol (ratios vary between sources)
- Use and time
- 10–30 s; rinse with ethanol, never water
- Reveals
- General microstructure of Mg alloys (Mg–Al, Mg–Al–Zn, etc.).
Notes
- Glacial acetic acid is corrosive and flammable. Keep it away from ignition sources and use it in the fume hood (SDS)
Sources: PACE magnesium; Univ. of Saskatchewan picric SOP; Steel Data (etchant No. 126)
Hazards HCl
- Composition
- 1–5 mL HCl + 100 mL ethanol, methanol or water
- Reveals
- Pure tin, tin–lead and tin–antimony–copper alloys; grain boundaries and second phases in zinc.
Notes
- If you make it with methanol, remember that methanol is toxic and is absorbed through the skin. Use ethanol where possible (see also the nitric acid and alcohol entry in the safety section)
Sources: PACE tin alloy etching; PACE zinc alloy etching
Why every scratch from the previous step must go (the deformed layer)
A grinding scratch is not just a groove: beneath it lies an invisible layer of plastic deformation. The next step has to remove that layer as well as the groove. Struers' guide makes it the rule that every step removes the damage of the previous one while introducing as little as possible of its own.
If the layer remains, the surface may look flat as-polished, but scratches and disturbance appear after etching. The etchant did not make them; it revealed deformation the final polish had hidden. Soft metals (Al, Cu, Sn, austenitic stainless, Ti) are most prone to it, and the sources recommend extending the final polish, a chemo-mechanical colloidal silica step, or polish–etch–polish cycles.
The depth of deformation cannot be judged from the visible grooves. In a study that ground iron, copper and titanium on SiC papers and measured the deformed depth by EBSD and FIB, the EBSD depths for the same paper were much larger than those seen by FIB — the damage extends well beyond what is visible, and even the measured value depends on the method.
The more a measurement depends on the surface, the more this matters. EBSD uses diffraction from the top 10–50 nm, so slight deformation weakens the patterns. Retained austenite in hardened steel can transform under grinding and polishing deformation, so its X-ray measurement is made on an electropolished surface.
How to check: rotate the specimen 90° at each step and look for any scratches in the previous direction. PACE suggests continuing for about three times as long as it took to establish the new scratch pattern.
Sources: Struers Metalog Guide; Metkon 12 polishing artifacts; PACE troubleshooting; Pinard et al. (deformation depth); Witt & Nowell (EBSD); Lambda Technologies, retained austenite
What etching does
Grain boundaries are disordered and higher in energy, so they dissolve faster than the grain interiors and become grooves. Light is not reflected straight back from a groove, so boundaries appear as dark lines.
Chemically different phases dissolve at different rates; where phases of different potential touch, a local cell forms and one phase is attacked preferentially. Even within one phase, attack depends on crystal orientation, so grains of the same orientation look equally bright.
Tint etchants (Klemm's, Beraha's, Weck's and others) deposit a thin film, usually a sulfide, instead of dissolving the surface. The film grows to different thicknesses depending on orientation and composition, and interference produces color in bright field. A narrow range of colors indicates preferred orientation. The films are sensitive to residual damage, so preparation must be better than for black-and-white etching.
In electrolytic etching the specimen is the anode under DC, and the attack is controlled by voltage and time (e.g. oxalic acid). Barker's anodizing of Al builds a film whose thickness depends on orientation, so grains show different colors in polarized light. Thermal etching of ceramics heats the specimen somewhat below its sintering temperature to groove the grain boundaries.
Sources: Bursa Technical Univ. lab notes; Vander Voort, color metallography; PACE etchant pages; PACE ceramics
FAQ
Nital or picral — which should I use?
Nital reveals both ferrite grain boundaries and pearlite, so use it for general steel structures and grain size. Picral attacks cementite but not ferrite boundaries, so it suits pearlite lamellae and spheroidized or tempered carbides. Combine them when you need both. Picral contains picric acid: never let it dry.
Scratches appeared after etching. Is the etchant at fault?
Usually not. Etching has revealed deformation or shallow grooves that the final polish had hidden. Go back an earlier step, re-polish, clean and re-etch; a light etch–repolish cycle confirms it.
Should I finish with alumina or colloidal silica?
It depends on the material and the purpose. PACE uses 0.05 µm alumina as its default final step for steels because colloidal silica can attack the matrix and lift fine inclusions. For EBSD, titanium (with hydrogen peroxide), ceramics and soft metals that smear, chemo-mechanical polishing with colloidal silica helps. Either way, do not let it dry on the surface, and finish with water.
Can I etch Al or Ti without hydrofluoric acid?
For Al, 1% sodium hydroxide, Weck's reagent (a permanganate–NaOH tint etch) and Barker's anodizing (fluoroboric acid, corrosive) all work. For Ti, most etchants contain HF; PACE also lists an HF-free option (Ogden–Holden: dilute HCl and H₂SO₄, used boiling), but it involves boiling acid. If you use HF, have calcium gluconate gel at hand and follow your institution's procedure.
I found dry crystals on the neck of a picric acid bottle. What should I do?
Do not touch it, move it or open the cap. Dry picric acid and picrates can explode from shock or friction. Contact your safety or waste-management staff immediately. In normal use keep the solid wetted with water and inspect it regularly.
Why must magnesium not be polished with water?
Mg reacts with water and oxidizes quickly (a white, powdery surface) and evolves hydrogen. From sectioning through polishing and post-etch rinsing, use ethanol-based lubricants and cleaning, dry right after polishing and store in a desiccator or ethanol. The swarf is pyrophoric, so never cut it dry.