Search

Metallographic Etching

Metallographic etching is one of the process steps in mechanical sample preparation for subsequent microscopic examination. In metallography, "etching" has been established as a synonym for techniques that generate optic contrast on prepared materials where the microstructure of the treated sample is not visible after metallographic polishing.

QATM is a leading manufacturer and supplier of high-quality metallographic consumables as well as a range of innovative electrolytic polishers & etchers. The QATM application experts combine decades of experience with thousands of processed materialographic samples as well as diverse materials from a wide range of industry sectors and will be happy to assist with your application.

Metallographic etchers


QATM offers metallographic etching devices for any requirement

Metallographic Etching and optic contrasting methods

Whether a microstructure is visible under polarized light (with a sensitive tint plate in the optical path) or not, strongly depends on the phases of the analyzed material. In this context the material's Bravais lattice type is of great importance. While BCC and FCC metals like steel, aluminum, copper, or chromium need to be etched to see a microstructure, HCP metals and alloys like α-titanium, cobalt, hafnium, zinc alloys or the orthorhombic bismuth show their microstructure in the polished state.

There are various techniques of optic contrasting. Birefringent crystals, for example, (like spherulites in partially crystalline polymers like polyamides or high-density polyethylene), can be visualized via phase contrast methods. Fluorescence microscopy and dark field microscopy are also important contrasting methods for failure analysis. However, they are usually applied to visualize microcracks, pores, or similar material inhomogeneities.

In many cases however, the incident light from the microscope is reflected quite uniformly. Therefore, contrast must be generated in another way to enable any kind of optical evaluation, e. g. by metallographic etching.  

Metallographic Micro & Macro Etching

To etch polycrystalline solids is an important part of metallography. It can be carried out based on different physical and chemical processes, which help to reveal the micro structure or macro structure of the workpiece. This contrast generation is essential for light microscopic analysis.

The choice of the according preparation method is strongly influenced by the analytical objective of the process. In metallography, a distinction is made between micro- and macro-etch applications; the former is used to analyze the microstructure of materials at specific points, the latter aims at showing differences in the microstructure across the geometry of parts. Apart from different reagents and process times, the material itself plays an important role for the obtained results.

The metallographic preparation prior to the etch process is also essential. While micro-etching always requires polished surfaces (usually produced by a metallographic polishing machine), a fine-ground sample may be sufficient for macro-etching. In general, etch processes in metallography can be based on physical mechanisms (thermal), electrochemical reactions (electrolytic), or spontaneous redox reactions (chemical).

It needs to be clarified whether mounted samples previously processed by a hot mounting press can be used or whether samples without mounting material yield better results. This sometimes is the case if a material is electrolytically polished and etched. Electric conductivity, thermal shock and high temperature behavior of the material must be taken into consideration for thermal or electrolytic processes as well. 

Comparison of metallographic etch processes

 ThermalChemicalElectrolytical
DevicesTube/muffle furnaces with temperature control, crucible tongs, inert gases (N2, Ar) Trays, crucible tongs, heating plates, other standard accessories of a wet-chemical laboratory Electrolytic etcher
ConsumablesCleaning media (alcohols/water, etc.)
Etchant, cotton wool, cleaning media (alcohols/water, etc.) Electrolytes, cotton wool, cleaning media (alcohols/water, etc.)
MaterialsOxide ceramics, carbide ceramics, cobalt-based alloys, nitride ceramics, titanium, steel
Ferrous materials, non-ferrous metals, oxide/carbide/nitride ceramics, rock, nickel, aluminium and other main group metals and alloys, titanium and other secondary group metals, semiconductor materials  Metals that appear in the electrolytic series of voltage. It is often used in the field of aluminium alloys, iron and copper metals
TemperatureMost important method parameter lies below the sintering temperature
RT and temperatures up to the boiling point of the medium (generally < 300 °C) RT to slightly elevated temperatures (< 100 °C)
Time expenditure/process10 - 60 minFew seconds to 30 minutes 1 - 30 min
HandlingChallenging (temperature control) Simple to challenging (complex geometries, metals susceptible to corrosion) Simple to very complex (method development)
PPE/workplace equipmentActive extraction unit, thermal protection, gloves, apron, visor Fume cupboard, protective clothing, protective gloves, protective goggles
 Detailed requirements depend on the furnace volume and target temperature Detailed requirements depend on the properties of the media used
ReproducibilityGood to a limited extentGood to a limited extentGood
CostsHigh investment costs / low follow-up costs Low investment costs / medium follow-up costs High investment costs / medium follow-up costs

Chemical etching in metallography

Chemical etch processes are the most common in metallography. They are popular due to the excellent cost-efficiency and simple application. In most cases, they is done by immersion: The sample surface to be etched is completely immersed in the according medium and moved around. Another technique, suitable for some applications, is with the use of a swab: Here, cotton pads or very soft tissues are wettened with etchant and the sample surface is wiped. This method is typically applied when immersion is technically not possible due to the susceptibility of the material to the etchant. Care must be taken to not scratch the prepared surface. In most cases, chemical etching is a selective corrosion or oxidation. This is called structure etching.

In the case of oxidative etching, a redox reaction takes place between a component of the medium, often H+/H2 and the solid, which often is metallic. This reaction occurs with a higher reaction velocity depending on crystallographic orientation (with grain surface etching) and crystal distortion (with grain boundary etching). The composition of the phases also leads to different electrochemical potentials and thus different oxidation rates. This causes a relief formation, which becomes microscopically visible as shading contrasts.

With some systems, e. g. the "Kalling 2" reagent, reduced metals or salts are deposited on the etched sample. These can be removed using cotton wool. Only then the etched microstructure becomes visible. The informative value of a pure structure etch process is limited because the mentioned mechanisms overlap, and not much information regarding the grain orientation is accessible. The primary goal is rather related to the determination of grain size distribution and phase composition of certain materials.

This makes the development of microscopic methods like the automated determination of grain orientations almost impossible. The same applies, to a certain degree, to inhomogeneities. It should be noted that non-metallic inclusions can be reliably represented with pure structural processes.

This can be well illustrated using the example of a low-alloy steel. The ferrite has a lower potential than precipitated cementite or graphite, which leads to a faster oxidation of the phase. Distorted grain boundaries are removed more slowly, and they form protruding areas in this case. Due to the lamellar structure of the pearlite, this phase results in a homogeneous relief etching in the grain, which can be recognized by dark grey stripes. Depending on the orientation of the grain to the grinding plane, these stripes are more or less clearly visible. In this case, Nital or V2A etchant would be a typical reagent

With different metallographic etchants, so-called color or precipitation etching is possible. This technique provides more microstructural information and is much more difficult to perform in a repeatable way.

In addition to the phase- and orientation-selective attack of the medium, a layer belonging to the redox system is deposited. This layer is of different thickness, depending on the local reaction velocity. This leads to interference phenomena of the incident light, which manifests itself in a strongly orientation-dependent discoloration of the grain surfaces, which becomes visible under polarized light. If the sample is over-etched, the interference disappears due to an excessive layer thickness.

A well-known metallographic color etch technique applied on low-alloy steels is the one according to Klemm. Different color reagents based on the anodic formation of sulfide films are applied for steel. The reagents according to Behara and LePera, differ in additives and the sulfite carriers used. The different reagents are chosen depending on the specific alloy system to be analyzed.

Color etch processes are also very common in inorganic-nonmetallic applications, e. g. for cement clinkers. The mechanisms of many of these processes have not yet been fully understood. However, they often work reliably when quantifying phases.

AlFe10, Fcc Aluminium matrix with needles of FeAl 3 , etched electrolytically with Barkers' reagent

AlFe10, Fcc Aluminium matrix with needles of FeAl3, etched electrolytically with Barkers' reagent

AlMg 4.5 etched with 7% NaOH solution

AlMg 4.5 etched with 7% NaOH solution

Austenitic V2A steel, etched with Beraha 2 etchant

Austenitic V2A steel, etched with Beraha 2 etchant

Alpha and beta brass, etched with 10% aqueous ferrinitrate

Alpha and beta brass, etched with 10% aqueous ferrinitrate

For chemical etch applications in metallography, the following important parameters must be considered after selecting the basic method:

  • Concentration of the used solution: Most etchants used in metallography are aqueous or alcoholic mixtures. They can usually be diluted as desired. This is especially recommended for very base metals to extend the exposition time and to not over-etch the samples. In case of very corrosion-resistant metals, a restriction of the medium can lead to better results.
  • Time: This parameter must be determined individually for a sample type. It has the strongest influence on the final result. Excessively long exposition may over-etch the specimen (usually recognizable by indefinable coatings and strong discoloration of the surface) while excessively short exposition results in an incomplete representation of the microstructure (poorly visible grain boundaries, very low discoloration of grain surfaces, etc.)
  • Age of solution: This is a common reason for faulty results, especially in case of color etching. For this reason, many reagents should be freshly prepared each working day or week to obtain optimum results. Structure etch solutions are usually stable over a longer period of time: The solution can be used multiple times. In most cases, color changes or precipitation failure are sure signs that the solution is too old.
  • Temperature of the medium: Usually high temperatures accelerate the process, whereas low temperatures slow it down. To ensure good reproducibility, materials should be etched at constant temperatures. Variations in the range of a few °C are usually negligible. Very corrosion-resistant materials often need to be etched at elevated temperatures.
  • Cleaning: The specimen must be cleaned immediately after exposing it to the etchant in order to not over-etch the material. In most cases, rinsing is done first with water, then with ethanol. Mounted samples are often "tapped" to remove the reagent that has penetrated the gap to the maximum possible extent. Especially with very base metals, e. g. Mg- or Zn-alloys, only alcohols should be used, since water is an oxidizing agent for these materials. The specimen is dried directly after cleaning to prevent the formation of dry spots. The use of aggressive detergents and surfactant mixtures is not recommended in any case.

Electrolytical etching in metallography

Like the chemical process, electrolytic etching is based on the formation of numerous galvanic elements on the polished sample's surface. In this case, it is necessary to apply an external voltage to the sample to force the desired redox reaction.

In addition to the factors listed above, locally varying electrical conductivity and the set voltage or current of the electrolysis cell influence the removal rate. When an automatic metallographic etch machine is used, flow rates and cell geometry also have an impact on the displayed microstructure. Electrolytic methods usually show higher removal rates than chemical methods, which is why they can be used as metallographic polishing processes, too. This is the greatest advantage of the process, since the replacement of metallographic polishing steps makes it possible to produce completely deformation-free surfaces and reveal the true microstructure, which is not possible otherwise.
The transition between electrolytic polishing and etch processes is mainly determined by the applied current density. Electrolytic etch processes are almost exclusive to the field of metallography. Since they are automatically controlled, they provide a higher reproducibility than purely chemical ones. These are still performed manually and require a higher level of user experience. Electrolysis in metallography can also be described as anodizing a metal. Generally, more noble, or at least equivalent, metals are connected as cathodes, while the sample serves as the anode.

QATM Producten & Contact

QATM offers a wide range of innovative machines for metallographic sample preparation. Accompanying QATM consumables are thoroughly tested and selected for perfect interaction with our machines. Contact us for a consultation, quote or to talk to one of our dedicated application specialists!