Oberhoffer's Etchant
Oberhoffer's Reagent
Material Specific
Composition
FeCl₃ + SnCl₂ + HCl + ethanol + water (two-solution mix)
Mix fresh; do not store mixed
Method
Immersion (mixed fresh)
Time
60–180 s
Alternative Names
Tags
Reveals
Phosphorus segregation patterns (ghost banding, dendritic segregation); primary solidification structure in as-cast and as-forged low-carbon steels and irons
Typical Results
Classical Oberhoffer's reagent, the specialty etchant for revealing phosphorus segregation in low-carbon steels, cast irons, and as-forged sections. Supplied as a two-solution kit (FeCl₃ + SnCl₂ in ethanol/water plus HCl/water) that must be combined immediately before use. The SnCl₂ is what selectively attacks high-P regions, leaving low-P "ghost" bands light.
Application Method
Method:Immersion (mixed fresh)
Typical Time:60–180 s
Preparation Notes
PACE Oberhoffer's is supplied as a two-solution kit. Combine equal volumes of Solution A and Solution B immediately before each use; do not store the mixed reagent.
| Solution | Recipe (per ~125 mL) |
|---|---|
| Solution A | 120 mL ethanol · 5 mL DI water · 7.2 g ferric chloride (FeCl₃) · 0.12 g stannous chloride (SnCl₂) |
| Solution B | 115 mL DI water · 10 mL hydrochloric acid (HCl) |
The SnCl₂ in Solution A is what produces the phosphorus selectivity. Use the combined solution within minutes of mixing.
Application Notes
Combine equal volumes of Solution A and Solution B immediately before use. Immerse the polished surface 60–180 s at room temperature. Swab very gently if needed. Rinse with water then ethanol.
Troubleshooting
SnCl₂ oxidizes over time; always use a freshly mixed solution. Ghost banding is only visible on as-cast or as-forged structures with segregation present; fully wrought, recrystallized steel shows no Oberhoffer contrast, which confirms uniform composition rather than etch failure.
Storage Notes
DO NOT STORE the combined reagent. Solutions A and B may be stored individually in amber glass at room temperature. Mix only the volume needed for the session.
Alternative Etchants
- Klemm's Reagent (general tint)
- 2% Nital (companion grain etch)
Similar Etchants
- Stead-type P-segregation etchants