Etching reveals grain boundaries and phase interfaces through selective dissolution and is the critical step before microstructural examination. The tables below summarize commonly used etchants for quick reference; compositions and etching times should be fine-tuned to the material condition.
Common Etchants for Ferrous Materials
| Material | Etchant | Typical Composition | Main Applications |
|---|---|---|---|
| Carbon and low-alloy steels | Nital | 2%–4% HNO₃ in ethanol | Reveals ferrite grain boundaries, pearlite, and quenched structures |
| Carbon and tool steels | Picral | 4% picric acid in ethanol | Reveals carbides and tempered structures with better contrast than Nital |
| Stainless steels | Aqua regia with glycerol | HNO₃ : HCl : glycerol ≈ 1:2:2 | Reveals structures of austenitic and martensitic stainless steels |
| Stainless steels (electrolytic) | Oxalic acid electrolyte | 10% oxalic acid in water | Reveals carbides and sigma phase; sensitization testing |
| Cast irons | Nital | 2%–4% HNO₃ in ethanol | Reveals the matrix; graphite must be examined in the unetched condition |
Common Etchants for Non-Ferrous Materials
| Material | Etchant | Typical Composition | Main Applications |
|---|---|---|---|
| Aluminum and its alloys | Keller's reagent | 2.5 mL HNO₃ + 1.5 mL HCl + 1 mL HF + 95 mL H₂O | Reveals grain boundaries and second phases |
| Copper and its alloys | Ferric chloride solution | 5 g FeCl₃ + 50 mL HCl + 100 mL H₂O | Reveals grain boundaries and annealing twins |
| Titanium and its alloys | Kroll's reagent | 2 mL HF + 6 mL HNO₃ + 92 mL H₂O | Reveals α/β structures |
Safety and Operation
Most etchants are strongly corrosive or toxic. Work in a fume hood with appropriate protective equipment, prepare reagents fresh before use, and dispose of waste liquid according to regulations. Follow the principle of light etching in repeated steps: etch briefly, then rinse, dry, and inspect immediately, re-etching only if contrast is insufficient. Nital-type reagents should be freshly prepared and must not be stored in sealed containers. An over-etched specimen must be re-prepared from the appropriate polishing step; extended observation cannot compensate for over-etching.
Practical Tips
- Select the initial etchant by material category; etch lightly and repeatedly with frequent inspection.
- Examine graphite in cast irons and inclusions in the unetched condition first.
- Handle strong acids in a fume hood and dispose of waste per regulations.
- The compositions and times listed are typical values; specimen evaluation and the actual operating procedure shall prevail.
MICROCRE learning notes · confirm the method for the specimen.