Materialographic Etching

About etching

Etching is a chemical or electrolytic process used after metallographic grinding and polishing procedures.

Etching enhances the contrast on surfaces in order to visualize the micro- or macrostructure.

Etching in materialography exerts a controlled influence on the surface profile or optical properties at grain boundaries, phases or grain surfaces, thus enabling microscopic inspection and additional use of optical filters in the microscope. Preparation before the etching process must be adapted to the investigation target.

How to do etching

Method selection steps:

  • Initial polishing of the sample requires a smooth surface without deformations or scratches.
  • The correct etching fluid for the material is selected from the listed literature.
  • In certain cases it is possible to enhance the contrast with light microscope filters additionally to the chemical etching process. This is referred to as optical etching.
  • In the case of electrolytic etching a suitable electrolyte, voltage and sample exposure time should be selected.

 

 

Materials and etchants

Some examples of a wide variety of etchants are listed below:

Alcoholic nitric acid

Carbon steels:

Alcoholic nitric acid.

Beraha

High alloyed steels

Colour etchants (Beraha, Lichtenegger), with V2A-lye or Adler-etchant containing hydrochloric acid.

Electrolytic etching with oxalic acid

High alloyed steels

Electrolytic etching with oxalic acid.

Barkers electrolyte

Aluminium and -alloyes:

Barker

Klemms reagent

Copper and -alloys:

Modifications of Klemm's reagent.

Types of etching

There are several types of etching, which are described below.

Chemcal etching

Chemical etching

Chemical etching involves the complete immersion of a prepared sample, usually ground (for macro etching) or fine polished (for micro etching), into an etching fluid (etchant).
Macro etching enables investigation of the sample surface by eye or magnifying glass (magnification up to 25x). Micro etching enables microscopic inspection at magnifications up to 1,000x (light microscopy) or more (electron microscopy).

Dissolution etching enables specific attacks at grain boundaries, surfaces or phases.
During precipitation etching (also known as colour etching), a thin layer is formed on the surface with a certain thickness depending on the chemical composition or orientation of grains.

Electrolytic etching

Electrolytic etching

Electrolytic etching requires a homogeneous conductive workpiece. The initial required polishing of the surface can be in the form of short mechanical grinding and polishing which results in a more plane surface, but with deformation and/or scratches. Electrolytic polishing provides less deformation with edge rounding and possible washout of phases.

The principle of electrolytic etching is the same as that of chemical etching, except that the specimen is placed as an anode in a galvanic cell, resulting in the removal of material from the specimen surface. For special investigations on aluminium based materials an electrolytic etching forming an anodized layer on the well polished sample surface can be applied and analysed under polarized light (Barker-etching).

Troubleshooting - Etching

Problem:

Incorrect preparation of the specimen
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Solution:

Scratches and deformation are acceptable for both methods. Depending on the preparation target, however, scratches and deformations can overlay or influence the phase or structure that should be visible after etching.
Incorrect electrolyte
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Solution:

Using an incorrect electrolyte or etchant means it will not be possible to verify a surface structure according to the required parameters.

Voltage and exposure
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Solution: 

In the case of electrolytic etching the voltage and exposure time are critical to revealing the correct surface structure for a valid verification. Exposure time, and sometimes temperature, are important parameters when using chemical (wet) etching.

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