Metallographic Etching

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.  

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 FeAl3, etched electrolytically with Barkers' reagent}

_{AlMg 4.5 etched with 7% NaOH solution}

_{Austenitic V2A steel, etched with Beraha 2 etchant}

_{Alpha and beta brass, etched with 10% aqueous ferrinitrate}

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.