Metallographic etching is a chemical technique used to create accelerated differential corrosion between different phases in a metallic sample. The morphological characteristics, quantities and distributions of these phases can help investigators better understand structure property relationships in metals.
Read also: Robo-Met Recognized for Providing Ground Truth Data in NASA SBIR Award
The Robo-Met.3D system can be relied on to repeatably create fine polished surfaces on metallic samples, suitable for even SEM/EBSD investigations. To an optical microscope, such surfaces show limited phase contrast (Fig. 1). Etching can create the requisite contrast to optically investigate sample surfaces (Fig. 2)3.
Figure 1: 2D Microstructure (unetched for Ti6242)
Figure 2: Alpha-Beta Microstructure (etched with Kroll’s Reagent) with Defect Distribution for Ti6242
The dip wells in the Robo-Met.3D system are configured for immersion etching. Etching in the Robo-Met system must always be performed with the exhaust on, to ensure operator safety. The etching mechanism used also provides some guidelines to select etchants for use in the Robo-Met.3D. In addition to being metallographically appropriate to immersion etching the sample, the etchants need to:
Common etchants used in the Robo-Met system, along with the metal compositions they are used to investigate, are listed in Table 1 as a guideline.
Table 1: Etchants Appropriate for Immersion Technique Usage in the Robo-Met.3D System
Reagent Name |
Reagent Composition |
Metal Families Studied |
Nital (2-4%) |
Ethanol 100ml Nitric acid 1-10 ml |
Common etchant for Fe, carbon and alloys steels and cast iron |
Waterless Kalling’s Reagent |
CuCl 2 5 grams Hydrochloric acid -100 ml Ethanol 100 ml |
Ni-Cu, nickel-based superalloys, ferritic and martensitic 400 series stainless steels. This etchant will darken martensite, attack ferrite readily and etch austenite slightly. It will not attack carbides. |
Kroll’s Reagent |
Distilled water 92ml Nitric acid 6 ml Hydrofluoric acid 2 ml |
The primary alpha structure of titanium will appear white after etching. Alpha prime and acicular alpha structures will be outlined. Intergranular beta structure and beta grains will be darkened. |
Frye’s Reagent |
40 ml HCl 5 g CuCl2 30 ml Water 25 ml Ethanol |
Fry's reagent for martensitic and precipitation hardenable grades of stainless steel. |
Marble’s Reagent |
CuSO4 10g Hydrochloric acid 50ml Water 50 ml |
For etching Ni, Ni-Cu and Ni-Fe alloys and superalloys. |
A large amount of water is used to flush and clean the metallographic samples and platens. We studied
etching during serial sectioning with Robo-Met.3D, with a tap established in the rear effluent port of the
system to monitor pH and estimate flows. We found that for the etchants studied (Marbles, Nital, Kroll’s
and Kalling’s reagents):
This enabled us to conclude that subject to variations in applicable environmental safety regulations,
discharging the metallographic waste water directly into the lab waste stream should be safe.
Do not hesitate to contact us if you have questions about etching in the Robo-Met.3D system.
Read also: 2020 Research Using Robo-Met's Materials Analysis
Questions? Contact us here. Connect with us on Facebook, Twitter, LinkedIn, and Instagram.