In the fifties, Wittmoser [1] emphasized the importance of looking at graphite nodules alongside the primary structure rather than in isolation. Most etchants, however, reveal minimal information on the appearance of the structure immediately after solidification.
Etchants that are commonly used to revealing the microstructure of iron-carbon alloys .
The most popular etchants are diluted solutions of picric or nitric acid in water or alcohol e.g. Nital and Picral. They offer several advantages:
-Easy application at room temperature,
-Adaptable etching intensity,
-Safety in use,
-Clear depiction of substances in the structure.
Because of the above, such etchants were used extensively and were largely responsible for building up a general picture of the structure of cast iron (and steel). They worked by chemically attacking the surface of the treated specimen creating an impression in relief from which crystal orientation, cell boundaries and some the structural components could be seen. Figure 1 outlines the reflection of light on a nital-etched specimen.
Oxidizing etching treatments.
Another, much less well known group of etchants, form the so-called oxidizing etching treatments. These substances are particularly suited to revealing the primary structure of cast iron. Instead of attacking by means of chemicals, oxidizing etchants work by releasing a tiny layer of oxides to the surface of the specimen. The thickness of this layer depends largely upon the chemical composition of the underlying surface and the etching conditions.
A picture is created by shining light on the layer of oxide, through which it passes partially before being reflected by the metal substrate. Interference between both reflected light beams causes certain wavelengths to either intensify or diminish thus resulting in a colored pattern which is dependent upon the thickness of the oxide layer. The color pattern phenomenon is similar to that of oil on water. The thickness of the oxide layer must be in the range of the wavelengths of the applied light source.
Wittmoser used Jürich's etchant ( sulfuric acid, density 1.84 + 0.2% Boric acid) to reveal the existence of primary dendrites in the matrix [2]. Information on a special oxidizing (color) etching technique was published as far back as 1956 by Aubrion [3]. The first ever color pictures of nodular cast iron appeared in his article and is shown in the picture below. Figure 2, The first color etch of nodular cast iron appeared in 1956.
Color-etching.
Color etching has been increasingly used. Fargues [4] used such a method, as did Engler, Schmitz [5 ,6], and Motz, Wolters [7]. Almost all color etchants are based on a strong alkaline solution containing picric acid which is applied at temperatures between 65 and 100 degrees C. Etching times are most usually between 10 and 60 minutes.
Records of the following compositions can be found:
Reference
ml.H2O
gr. NaOH
gr. Picric-acid
[8]
100
25
2
[9]
100
25>
1,8
[6]
200
28
4+1 gr. Pot.disulphate
The etchant according to [9] is also known as Melnikov's, and was used to distinguish between cementite and iron-phosphide in steel.
The best results were, however, obtained with the following composition, developed during the seventies:
Composition of primary color etchant according to Becker [10]:
Ml. H2O
gr. NaOH
gr. KOH
gr. Picric-acid
50
10
40
10
Safety warnings
[8] gives a clear warning with the use of this type of etchant: 'Picric-acid-anhydride is highly instable and explosive! So beware of overheating and evaporation.'
The preparation of the concentrated 'Becker' etchant must be carefully controlled, as a lot of heat develops during the dissolution of the picric acid. A safe way to preparing this etchant is the following: dissolve NaOH and KOH in the water. Then add picric acid in 2 grams quantities, taking care to dissolve al solids. If nescessary, cool down between the acid additions.
Results This photo shows the structure as revealed by the traditional 'nital' etchant. The same specimen, treated with Becker etchant. The dendritic construction of the matrix becomes clearly visible.(both photographs magnification 50x)
Gray-scale
The intensity of the colors developed by this color or primary etchant, strongly depends on the etching conditions such as time, temperature and the number of times the same etchant solution has been used. As the etchant has a deep brown color of its own, the etching process cannot be followed and the best way to achieve consistent results is just by trial and error.
Next series of photographs shows the influence of longer etching times on color development.
Photo top left 10 min etching time, bottom right 90 min.
[1]A.Wittmoser, Graphitbildung in Eisen-Kohlenstoff-Gusslegierungen.
Giesserei April 1959, page 177/187.
[2] A.Wittmoser, A propos de la formation des nodules de graphite. Intern. Foundry Congress, Bruxelles 1951, page 433/449.
[3] G.Aubrion, Feststellung von Gefügeunterschieden bei Gusseisen mit Hilfe der Farbphotographie. Giesserei, 1956, page 203/210.
[4] J.Fargues, M.Hecht, J.C.Margerie, T.Chong, La formation et la prévention du graphite morcelé (Chunky) dans les fontes à graphite sphéroïdal, non alliées, lentement solidifiées. Fonderie-Fondeur d'aujourd'hui June/July 1985, page 14/23.
[5] J.Zhou, W.Schmitz, S.Engler, Untersuchung der Gefügebildung von Gusseisen mit Kugelgraphit bei langsamer Erstarrung. Giesserei-Forschung, 1987, Heft 2, page 55/70
[6] Z.Jiyang, W.Schmidz, S.Engler, Formation of Austenite Shell Around Nodular Graphite and its Effect on Detoriation of Graphite. AFS Transactions, 1990, page 783/786.
[7] J.M.Motz, D.B.Wolters, Über Erstarrungsgefüge und Eigenschaften in dickwandigen Gußstücken aus ferritischem Gußeisen mit Kugelgraphit. Teil 1: Mikroseigerungen. Giessereiforschung 40 (1988) nr.2, page 69/78.
[8] ASTM E407-70. Standard Methods for Micro etching of Metals and Alloys.
[9] A.Schrader, Ätzheft. Published by Gebrüder Borntrager-Berlin-Nikolassee, 1957.
[10] U.Becker, Haunsheim. Personal communication.
Figure 1 outlines the reflection of light on a nital-etched specimen. 
Figure 2, The first color etch of nodular cast iron appeared in 1956.
This photo shows the structure as revealed by the traditional 'nital' etchant.
The same specimen, treated with Becker etchant. The dendritic construction of the matrix becomes clearly visible.(both photographs magnification 50x)