Etchants
A quick reference for the most widely used metallographic etchants, and where to go when you need more than the basics.
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Common Etchants Quick Reference
| Etchant | Composition | Materials | Application |
|---|---|---|---|
| 2–5% Nital | 2–5 mL HNO₃ in 95–98 mL ethanol | Carbon and low-alloy steels, cast iron matrix | Swab 5–30 s |
| 4% Picral | 4 g picric acid + 100 mL ethanol | Carbon, alloy, and tool steels (cementite, pearlite) | Swab 10–60 s; store picric acid wetted |
| Vilella's | 1 g picric acid + 5 mL HCl + 95 mL ethanol | Tool steels, martensitic and ferritic stainless | Swab 5–60 s |
| Glyceregia | 10 mL HNO₃ + 20 mL HCl + 30 mL glycerol | Stainless steels (general) | Immersion 10–60 s; mix fresh — activity decays |
| 10% Oxalic (electrolytic) | 10 g oxalic acid + 100 mL H₂O | Austenitic stainless; sensitization per ASTM A262-A | 6 V, 30–90 s |
| Marble's | 4 g CuSO₄ + 20 mL HCl + 20 mL H₂O | Nickel-base superalloys | Swab 10–60 s |
| Modified Kalling's | 5 g CuCl₂ + 100 mL HCl + 100 mL ethanol | Nickel alloys, duplex stainless | Immersion 10–30 s |
| Keller's | 2 mL HF + 3 mL HCl + 5 mL HNO₃ + 190 mL H₂O | Aluminum and aluminum alloys | Swab 5–30 s; HF safety precautions |
| Weck's (Al) | 4 g KMnO₄ + 1 g NaOH + 100 mL H₂O | Aluminum alloys (color/tint etch) | Immersion 30–60 s |
| Barker's (anodizing) | 5 mL HBF₄ + 200 mL H₂O | Aluminum grain orientation (polarized light) | 20 V, 60–120 s — electrolytic anodizing, not a chemical etch |
| Kroll's | 2 mL HF + 6 mL HNO₃ + 92 mL H₂O | Titanium and titanium alloys | Swab 5–15 s; HF safety precautions |
| Ammonium hydroxide + peroxide | Equal volumes 28% NH₄OH + 3% H₂O₂ | Copper, brass, bronze | Swab 10–60 s; mix immediately before use |
| Klemm's I | 50 mL saturated Na₂S₂O₃ + 1 g K₂S₂O₅ | Copper alloys, duplex stainless (color/tint etch) | Immersion 30–180 s; needs deformation-free surface |
| Beraha's I | 10 mL HCl + 90 mL H₂O + 1 g K₂S₂O₅ | Ferrous alloys, duplex stainless (color/tint etch) | Immersion 30–180 s |
| Murakami's | 10 g K₃Fe(CN)₆ + 10 g NaOH + 100 mL H₂O | Carbides, tool steels, cermets, refractory metals | Swab 5–30 s (heated for refractories) |
| Acetic-glycol | 20 mL acetic acid + 1 mL HNO₃ + 60 mL ethylene glycol + 19 mL H₂O | Magnesium and magnesium alloys | Swab 5–30 s |
Compositions and times are typical starting points; adjust for the specific alloy and condition. Color/tint etchants require a deformation-free, colloidal-silica-finished surface.