There are large volume of handbooks that prescribe suitable etchant for a particular metal or ceramic, But their theoretical justification is hardly ever mentioned. Then, is it possible to estimate (at least a first order approximation) composition of etchant for a metal/alloy (if its chemical constituents and possible present phases are known from standard thermodynamics and metallurgy) , solely from knowledge of organic and inorganic chemistry? Or just is it purely empirical trial-and error?
For example, nital or picral are good etchant for some ferrous alloys, but why is the acid nitric acid (cannot be sulfuric/hydrochloric/phosphoric?) instead of other strong acids; or picric acid instead of any other aromatic ortho-para trisubstituted (electron-withdrawing group) phenol? Is there any aromatic coordination chemistry or organometallic chemistry involved? Why is the other reagent alcohol instead of , say, water/ketone/aldehyde/ether; and why is the alcohol ethanol instead of, say, methanol, isopropanol or neopentanol?
Similar question can be posed for any other metal/alloy system and corresponding etchant. Exact metal or etchant are not asked here. And some related questions that may rise are
Is reactivity of particular metal contained in a microstructural phase with inorganic and organic reagents cannot be even first-order estimated with standard inorganic and organic chemistry? Why?
If coloring compound formation is the key, then how the color between transitional/ non-transitional metal and a ligand (can be organic chromophore) can be estimated with relatively simple first principle study?
Dear Dr. Sumit Bhowmick ,
the etching reagents for macroscopic examination or chemical polishing have been under study for many years, as shown by one of the texts most dear to me:
-Smithells Metals Reference Book, Edited by E. A. Brandes and G. B. Brook of which I am attaching the link to the 7th Edition (1992):
Available at: http://pmt.usp.br/academic/martoran/notassolidificacao/Smithells%20Metals%20Reference%20Book%207e.pdf
Smithells is the only single volume work which provides data on all key aspects of metallic materials.
Smithells has been in continuous publication for over 50 years. The 8th Edition represents the major revision:
by William F. Gale, Terry C. Totemeier, Elsevier, (2003) - 2080 pages.
In the metallography part, you can find quite precise indications on the reagents to be used for a specific material. Starting from this base and using the advice in the notes, to obtain an acceptable result it is necessary to make several attempts using different attack times, possible dilutions, etc.
If you have to get a result in a reasonable time, it is not useful to do other tests ....
My best regards, Pierluigi Traverso.
Of course, the task of selecting the optimal etchant for an alloy is a purely empirical task. General knowledge of chemistry is not enough to select the optimal composition of the etchant, since it is impossible to take into account all factors in a purely theoretical consideration of the issue. I will say more, chemistry is an experimental science and theoretical substantiations of certain processes in chemistry always appear after their experimental discovery.
Dear Dr. Sumit Bhowmick ,
the etching reagents for macroscopic examination or chemical polishing have been under study for many years, as shown by one of the texts most dear to me:
-Smithells Metals Reference Book, Edited by E. A. Brandes and G. B. Brook of which I am attaching the link to the 7th Edition (1992):
Available at: http://pmt.usp.br/academic/martoran/notassolidificacao/Smithells%20Metals%20Reference%20Book%207e.pdf
Smithells is the only single volume work which provides data on all key aspects of metallic materials.
Smithells has been in continuous publication for over 50 years. The 8th Edition represents the major revision:
by William F. Gale, Terry C. Totemeier, Elsevier, (2003) - 2080 pages.
In the metallography part, you can find quite precise indications on the reagents to be used for a specific material. Starting from this base and using the advice in the notes, to obtain an acceptable result it is necessary to make several attempts using different attack times, possible dilutions, etc.
If you have to get a result in a reasonable time, it is not useful to do other tests ....
My best regards, Pierluigi Traverso.
Dear Sumit Bhowmick
Developing a new set of etchants for the microstructure evolution and related processes are historically based in principles on chemical attack (corrosion process) followed by a doping process (physical process) which the mechanism of such process is described by transition state theory and the activated complex. This means that using reactive reagents with multi-stage reactions can create an activation for the corrode surface leading the metal ions to migrate into the grain boundaries or other phases of higher energy and then can improve the contrasting patterns and able to revealed a multiple features of the microstructure.
Therefore, I totally agree with you in such that developing etchant for a particular alloy system is estimated from physics principles, corrosion principles, chemistry (solid- liquid reactions) and metallurgy (phase formation and microstructure and energy distribution). In this respects you find the recently published work following a systematic methodology steps for developing an metallographic etchants for the microstructure evolution of aluminum alloy welds.I hope above contribution is helpful.
Best regards https://www.researchgate.net/publication/320339100_Development_of_Metallographic_Etchants_for_the_Microstructure_Evolution_of_A6082-T6_BFSW_Welds
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