Why is an etching agent used during metallography?



The Metallography or. -graphy is the study of the structure of metals and thus represents a discipline of metal science. The activity is carried out by a metallograph. The task of metallography is the qualitative and quantitative description of the structure of metallic materials with the help of microscopic methods. In order to recognize the structure of a metallic material in the microscope, it has to be prepared. Knowledge of the properties and processing of a metal is important in order to avoid errors in preparation and structure assessment.

Due to the increasing number of composite materials and the emergence of new, further developed materials (ceramic / metal systems, metal / plastic systems, etc.), the term "materialography" is now preferred because it covers the entire spectrum of materials.

The Metallography/ Materialography often in quality assurance and damage analysis, as well as in research and development.

preparation

Just like non-metallic crystals (such as rock crystal), metals also have a crystalline structure that can be recorded qualitatively and quantitatively. In the Metallography this is usually done by preparing a bevel, which is polished to a high gloss and examined with an incident light microscope. A scratch-free section suitable for macro and / or microscopic observation must have a representative, sharp-edged, flat surface of the material to be examined, which clearly shows its structure and which does not show any changes caused during production such as deformation, chipping, scratches and smears . The production of perfect grindings is basically possible for all solid materials, but it is often very expensive.

Immediately after polishing, first statements about the purity of the material can be made due to the different reflective properties of metals and non-metals. Impurities usually stand out darkly from the metallic base material.

Abrasive pads are papers and fabrics on which the grains are applied. Polishing pads are mainly cloths (felt, velvet, synthetic fibers, linen). For better removal and to check the grinding and polishing marks, the specimen must be rotated 90 ° after each grinding or polishing step.

Grinding and polishing

  • Wet sanding on SiC paper with grain size P320 / P500 / P800 / P1000 / P1200
  • Pre-polishing with diamond suspension with grain sizes 6 µm, 3 µm, 1 µm on rayon or cotton cloth with alcoholic lubricant
  • Fine polishing with clay suspension on a wool or velvet cloth.

A very good and quick alternative:

  • Wet grinding on a 75 µm diamond disc
  • Polishing with a 9 µm diamond suspension on a honeycomb disc
  • Polishing with a 3 µm diamond suspension on a honeycomb disc
  • Polishing with a 3 µm diamond suspension on a hard, smooth disk
  • Polish with oxidic polishing suspension on a soft cloth

Careful cleaning between the individual grinding and polishing stages is necessary in order to avoid dragging along coarser cutting grains and abrasion. For drying, the liquids that evaporate hard (e.g. water, oil) are first rinsed off the sample with alcohol. In the hot air stream (hair dryer) this liquid dries quickly and without spots.

etching

The microscopic examination is initially carried out at a small magnification, which allows a good overview. In general, a polished ground surface shows no structure, since the incident light is reflected almost evenly. Lenses with higher magnification are used as required.

The section should be viewed in the unetched as well as in the etched state. The following can be observed on the unetched section:

If statements are to be made about the structure, it is necessary to etch the polished section - a wide variety of solutions (acids, alkalis, neutral solutions, molten salts, etc.) are used. The actual structure is examined on the etched section. Etching should always be carried out immediately after polishing, i.e. not the next day. If the structure is correctly contrasted, statements can be made about the heat treatment condition and quality, and in many cases conclusions can be drawn about the manufacturing process and the causes of defects in the event of damage.

The choice of etchant depends on the type of material to be etched and the size of the structures to be examined.

material Macro etchants[1]Micro-etchants[1]
cast iron  
  • Alcoholic nitric acid:
      98 ml of ethanol +
      2 ml of concentrated nitric acid
  • Unalloyed and low-alloy steels
  • Alcoholic nitric acid:
      90 ml of ethanol +
      10 ml of concentrated nitric acid
  • Etching agents according to Adler:
      Solution A:
      3 g ammonium chlorocuprate +
      25 ml of distilled water
      Solution B:
      15 g ferric chloride +
      50 ml concentrated hydrochloric acid
      After everything has completely dissolved, add solution B to A.
  • High-alloy steels, nickel
    Copper and copper alloys
    • 120 ml of distilled water +
      10 g ammonium chlorocuprate
      Add ammonia dropwise to the solution until the precipitate that forms disappears
    Aluminum and aluminum alloys
    • 10 ml of distilled water +
      10 ml concentrated hydrochloric acid +
      10 ml concentrated nitric acid +
      2.5 ml hydrofluoric acid (38-40 percent)
  • Hydrofluoric acid:
      99.5 ml of distilled water +
      0.5 ml hydrofluoric acid
  • Caustic soda:
  • High-alloy chrome-nickel steels  
  • V2A stain according to Goerens:
      100 ml of distilled water +
      100 ml concentrated hydrochloric acid +
      10 ml of concentrated nitric acid +
      0.3 ml economy stain
  • In addition to reflected light microscopy, electron microscopy is also used, in which not only polished sections but also fracture surfaces can be examined and the chemical composition of the structural components can be analyzed using spectroscopy.

    See also

    Technical assistant for metallography and physical material analysis

    swell

    1. ab Egon Kauczor: Metal under the microscope, Springer-Verlag, Berlin, Heidelberg, New York, Tokyo 1985, ISBN 3-540-15611-9 (Berlin …), ISBN 0-387-15611-9 (New York …)

    literature

    • Petzow: Metallographic, ceramographic, plastographic etching
    • Bargel / Schulze: Materials Science; VDI publishing house
    • Schumann: Metallography
    • Weißbach: Materials science and material testing

    Category: Materials Science