Metallographic Etchants / Stainless Steel

Etchants for Stainless Steel

Etching stainless steels can be somewhat difficult due to the anti-corrosive nature of stainless steel. Austenitic or 300 series stainless steels typically have higher chrome as well as a significant amount of nickel (e.g. 304 stainless steel - 18% chrome, 8% nickel) which makes them harder to etch. Martensitic or 400 series stainless steels are easier to machine, however, they are not as corrosion resistant so they are easier to etch.

Stainless Steel Microstructure
Stainless Steel Classification System
  • 300 Series (Austenitic): Chromium (18%) and nickel (8%) - Excellent corrosion resistance, non-magnetic
  • 400 Series (Martensitic): Higher carbon content - Heat treatable, magnetic, moderate corrosion resistance
  • 200 Series (Austenitic): Manganese and nitrogen - Cost-effective alternative to 300 series
  • Duplex Grades: Mixed austenitic and ferritic structure - Superior strength and corrosion resistance
  • Precipitation Hardening: Special heat treatment - High strength with good corrosion resistance
Etching Considerations for Stainless Steel
  • Surface Preparation: Ensure proper surface preparation before etching to achieve consistent results
  • Etchant Selection: Choose appropriate etchants based on grade (austenitic grades require more aggressive etchants)
  • Etching Time: Monitor carefully to prevent over-etching, especially for sensitive grades like 316L
  • Heat-Affected Zones: Use specialized etchants for welded samples to reveal both base metal and HAZ microstructures
  • Sigma Phase Detection: Use Murakami's reagent for clear identification of sigma phase in duplex grades
  • Electrolytic Etching: Consider for difficult grades and large samples to ensure uniform results
  • Storage: Store etched samples in a dry environment and prepare fresh etchants for each use

The selection of appropriate etchants is crucial for revealing the microstructure of stainless steels. While Adlers and Kalling's reagents are commonly used, specific grades may require specialized etchants for optimal results. The table below provides detailed information about recommended etchants for stainless steels and their applications. For a comprehensive list of etchants, visit our Etchant Database.

Need help with the initial sample preparation steps for Stainless Steel? Check out our guide

Recommended Etchants

Etchant Composition Conditions Applications
Adlers Etchant
  • Ferric chloride: 45 gm
  • Copper ammonium chloride: 9 gm
  • Hydrochloric acid: 150 ml
  • Distilled water: 75 ml
  • Immerse for several seconds
  • A very effective etchant for 300 series, austenitic, duplex stainless steels
Carpenter Etchant
  • Ferric chloride: 8.5 gm
  • Cupric chloride: 2.4 gm
  • Alcohol: 122 ml
  • Hydrochloric acid: 122 ml
  • Nitric acid: 6 ml
  • Immerse for several seconds
  • A nice etchant for 300 series, austenitic, duplex stainless steels
Kalling's No. 2
  • Cupric chloride: 12 gm
  • Hydrochloric acid: 20 ml
  • Alcohol: 225 ml
  • Immerse or swab
  • Seconds to minutes
  • For 400 series or martensitic stainless steel
Modified Murakami's
  • K3Fe(CN)6: 30 gm
  • Potassium hydroxide: 30 gm
  • Distilled water: 150 ml
  • 1 second to several minutes
  • Immersion or swabbing
  • Mix potassium hydroxide into water before adding K3Fe(CN)6
Color Etchant
  • Hydrochloric acid: 20-30 ml
  • Selenic acid: 1-3 ml
  • Ethyl alcohol: 100 ml
  • Immerse at room temperature
  • 1-4 minutes
  • Color etching
  • Colors carbides and gamma prime phase in heat resisting steels
Delta Ferrite Etchant
  • Ammonium bifluoride: 20 gm
  • Potassium metabisulfite: 0.5 gm
  • Distilled water: 100 ml
  • Immersion at room temperature
  • 1-2 minutes
  • Color etching
  • Etching of austenitic stainless steel and welds
  • Reveals delta ferrite in welds
V2A Etchant
  • Hydrochloric acid: 119 ml
  • Nitric acid: 12 ml
  • Distilled water: 119 ml
  • Immersion or swabbing
  • 20 degrees Celsius
  • For etching austenitic stainless steels
Weck's No. 2 Reagent
  • Potassium Permanganate: 25 gram
  • Sodium Hydroxide: 25 grams
  • Distilled water: 250 ml
  • Immersion or swabbing
  • 20 degrees Celsius
  • Often used for stainless steels, nickel-based alloys
  • Reveals grain boundaries, phases, and other microstructural features
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Expert Guidance

Etching Best Practices

  • Ensure sample is properly polished before etching
  • Clean sample thoroughly before etching
  • Use fresh etchant solutions
  • Monitor etching progress under microscope
  • Rinse and dry immediately after etching
  • Store etched samples in a desiccator

Troubleshooting Guide

Common Etching Issues and Solutions
  • Over-etching: If the surface appears too dark or grain boundaries are too prominent, reduce etching time by 30-50% and monitor progress under the microscope. For sensitive grades like 316L, use shorter etching times.
  • Under-etching: If grain boundaries are not clearly visible, increase etching time by 10-20 seconds or ensure etchant is fresh. For duplex stainless steels, consider using a stronger etchant like Vilella's reagent.
  • Uneven etching: Ensure sample is completely dry before etching and maintain consistent immersion/swabbing motion. For large samples, consider using electrolytic etching for more uniform results.
  • Staining: Rinse immediately with water after etching and dry with compressed air to prevent water spots. For martensitic grades, use ethanol rinse to prevent rust formation.
  • Poor contrast: For austenitic grades, try Beraha's reagent for better phase differentiation. For ferritic grades, consider using Murakami's reagent to enhance carbide visibility.
  • Etchant shelf life: Prepare fresh aqua regia-based etchants for each use and discard after 24 hours. Store mixed etchants in dark glass bottles away from light.
  • Sigma phase detection: If sigma phase is suspected, use Murakami's reagent at 60°C for 10-15 seconds to clearly distinguish it from other phases.
  • Heat-affected zones: For welded samples, use a combination of Vilella's and Glyceregia etchants to reveal both the base metal and HAZ microstructures.
Class 5 specimen preparation chart

Class 5 Procedure

See the standard metallographic approach for Medium Hard, Ductile Metals like Stainless Steel.