Villela's Reagent
Definition
Etchant for stainless and high-chromium steels: 1 g picric acid + 5 mL concentrated HCl + 100 mL ethanol. Targets prior austenite grain boundaries and reveals the strained zone where chromium oxide passivation defeats Fry's.
- Composition
- 1 g picric acid, 5 mL concentrated HCl, 100 mL ethanol
- Target metals
- Stainless and high-chromium steels
- Use
- Restoring erased or obliterated serial numbers
- Contrast
- Fry's reagent, which fails on chromium oxide passivation
Common questions
Why does Fry's reagent fail on stainless steel where Villela's succeeds?+
Stainless steel forms a passive chromium oxide layer that resists Fry's etchant, while Villela's picric acid and hydrochloric acid mixture is formulated to penetrate that layer and reveal the strained prior austenite grain boundaries left by stamping.
Is picric acid hazardous to keep in a metallurgy lab?+
Yes, dry picric acid is a shock-sensitive explosive hazard, so it is stored wetted with water or alcohol and mixed fresh under standard laboratory safety protocols rather than kept as a dry stock reagent.
Related terms
- Davis Reagent
- Variant etchant for steel using cupric ammonium chloride (Cu(NH4)2Cl4) in dilute HCl. Slower acting than Fry's, sometimes preferred when contrast on Fry's...
- Fry's Reagent
- Classical etchant for plain carbon and low-alloy steel: 40 g cupric chloride (CuCl2) + 90 mL concentrated HCl + 100 mL water....
- Heat-Tinting
- Restoration by controlled heating of the polished surface. Strained metal oxidises at a different rate from unstrained metal, producing colour contrast (straw...
- Magnetic-Particle Method
- Non-destructive restoration for ferromagnetic metals. The polished surface is magnetised and dusted with fine iron particles; the particles cluster at the strained...
- Peak Contrast
- The narrow window during a chemical etch when the original digits are most legible. Continuing past peak contrast overetches the unstrained metal...
- Stamped Marking
- A serial, chassis, engine or part number impressed into metal by a hardened steel die under high force. The visible impression is...
- Strained Zone
- The region of cold-worked, plastically deformed metal lying beneath an original stamp. The crystal lattice is distorted, dislocation density is elevated, and...