KNOWLEDGE
ETCHING
Surface treatment using chemical or electrochemical methods to reveal microstructure.
ETCHING
The surface obtained after the polishing process is suitable for examining certain material parameters. Some types of analyses that can be performed on polished surfaces include:
• Residue analysis,
• Determination of cast iron types and classification of graphite density,
• Crack examination,
• Porosity analysis,
• Examination of the distribution of certain secondary phases,
• Coating analysis, etc.
However, since polished surfaces reflect light uniformly, the details of the structure cannot be observed; contrast must be created to reveal them. This is achieved through etching.
Most metallurgical examinations are conducted after polished surfaces are treated with a suitable chemical solution (etching). Etching is necessary for analyzing various parameters such as grain size, deformation structure, segregation, microstructure, case hardening depth (carburization-nitriding), and decarburization.
Etching is performed using a wide range of chemical solutions that vary depending on the material and the parameters to be examined. In fact, even for a single type of material, multiple etching solutions may be available. A technical expert can formulate new solutions by applying their knowledge of materials and chemistry. Some solutions are quite simple, while others are complex and contain strong chemicals (acids or alkalis). Therefore, laboratories must be well-ventilated, and precautions should be taken to avoid inhaling the solution vapors.
| Etchant | Composition | Concentration | Conditions | Comments |
| Kalling’s No:1 | Distilled water CuCl2 | 33 ml | Immersion etching at 20° C | For etching martensitic stainless steels. Martensite will be dark and the ferrite will be coloured |
| HCl | 1,5 grams | |||
| Ethanol | 33 ml | |||
| Kalling’s No:2 | CuCl2 | 5 grams | Immersion etching at 20° C | For etching duplex stainless steels and Ni-Cu alloys and superalloys |
| HCl | 100 ml | |||
| Ethanol | 100 ml | |||
| Kellers etch | Distilled water | 190 ml | 10 –30 seconds immersion. Use only fresh etchant | Excellent for aluminium and alloys immersion for 10 – 20 seconds; titanium alloys immersion for 10-20 seconds |
| Nitric acid | 5 ml | |||
| HCl | 3 ml | |||
| HF | 2 ml | |||
| Kroll’s Reagent | Distilled water Nitric acid | 92 ml | 15 seconds | Excellent for titanium and alloys. Swab specimen upto 20 seconds. |
| HF | 6 ml | |||
| 2 ml | ||||
| Nital | Ethanol | 100 ml | Seconds to minutes | Most common etchant for Fe, carbon and alloys steels and cast iron- immerse sample up from seconds to minutes; Mn-Fe, MnNi, Mn-Cu, Mn-Co alloys, immersion upto a few minutes. |
| Nitric acid | 1-10 ml | |||
| Marbel’s Reagent | CuSO4 | 10 grams | Immerse or swab for 5-60 seconds | For etching Ni, Ni-Cu and Ni-Fe alloys and superalloys. Add a few drops to H2SO4 to increase activity. |
| HCl | 50 ml | |||
| Water | 50 ml | |||
| Murakami’s | K3Fe(CN)6 | 10 grams | Pre-mix KOH and water before adding K3Fe(CN)6 | Cr and alloys (use fresh and immerse) iron and steels reveals carbides. Mo and alloys uses fresh and immerse; Ni-Cu alloys for alpha phases use at 75°C; W and alloys use fresh and immerse; WC-Co and complex sintered carbides. |
| KOH | 10 grams | |||
| Water | 100 ml | |||
| Picral | Ethanol | 100 ml | Seconds to minutes Do not let etchant crystallize or gry-expolisive | Recommended for microstructures containing ferrite and carbides. |
| Picric Acid | 2 – 4 grams | |||
| Vilela’s Reagent | Glycerol | 45 ml | Seconds to minutes | Good for ferrite- carbide structures (tempered martensite) in iron and steel. |
| Nitric acid | 15 ml | |||
| HCl | 30 ml |
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