Concentrations used in preparing chemical etchants 2

[kyleamf]

Hello all,
I work for a bowling manufacturing company and we are looking to install a materials failure analysis addition to our engineering test lab. I have been looking online everywhere for chemical suppliers (and have finally found one or two good ones) but I wasn't sure what concentrations we should purchase.

For example, in preparing nital, I understand that it is about 2 parts nitric acid and 98 parts alcohol - but what original concentration of nitric acid do you use to mix with the alcohol?

The idea is, we are trying to buy just a few key chemicals (glycerin, nitric, hydrochloric, hydrofluoric, & ethanol) so we can keep the cost down, but at the same time have the ability to create a variety of etchants. Let me know if you have any suggestions. Thanks,

Kyle

 

[arunmrao]

Please read this book,which will provide you a good insight.

Principles of Metallographic Laboratory Practice
by George L Kehl

 

[dbooker630]

In my lab I have been purchasing a 68-70% solution of nitric acid and water to mix into denatured ethyl alcohol. I also have sulfuric and hydrochloric acids. Usually if you buy Certified ACS or ACS Plus you will be OK without having to buy the most expensive grade.

Your safety people may not allow you to use picric acid because of concerns about explosion. I fought that battle and eventually lost.

There are several choices for suppliers, I use Fisher Scientific

http://www.fishersci.com.

 

[CoryPad]

Usually the acids are concentrated so that you can dilute them to lesser concentration if necessary. In our lab, we have:

HCl 36.5 %
HNO₃ 70 %
H₂SO₄ 96 %
HF 50 %

I have used picric acid at every lab I have been in over the last 17 years. Your mileage may vary.

 

[kyleamf]

Thanks for your help. So I guess I still have one final question, and I'll use glyceregia as another example.

Using 1 part nitric, 2 parts glycerol and 3 parts hydrochloric, does it matter what specific concentrations you use for the individual components? I haven't been able to find anything online that says so (although I understand that nitric is unstable in certain solutions - like alcohol, if you're preparing nital - past 5% conc.). I can buy concentrated solutions and dilute them, but if I need to, say, dilute the hydrochloric acid for this "recipe", is there a specification (or just a rule of thumb) that says how much to dilute it?

 

[kyleamf]

Actually I believe I have just had my question answered by a friend. According to him, the concentrations used depend on how fast you want the etching to go. Or in other cases, certain concentrations are specified and required, or certain concentrations can be violently reactive, etc.

I guess it's just a etchant-to-etchant basis.

Thanks again for all your help.

Kyle

 

[unclesyd]

As posted by CoryPad always use the laboratory reagent grade chemicals. If there is any other concentration requirements they will be called out in the recipe.
Normally there will be a don't store label if the enchant is at all unstable. Glyceriga falls into the don't store category. I've never seen a need for Nital to be over 5% in 46 years in and around metallurgical laboratories.
You need to get the book referenced by Arunmrao or any book that has all the enchants and directions.

Or you can subscribe to a database like the one at the Kaker site.

http://www.kaker.com/etch/etch.html

 

[kyleamf]

unclesyd thanks a lot. I think we will probably end up buying the book too, then.

 

[TVP]

Kyle,

The book that unclesyd mentioned was once the bible of metallography, but it is long since out-of-print. George Vander Voort's book Metallography: Principles and Practice published by ASM International is probably the best reference if you want a thorough source on techniques, lab practices, etc. ASM HANDBOOK Volume 9 Metallography and Microstructures is also an authoritative reference. Buehler and Struers also have a lot of excellent information on techniques, etchants, failure analysis, etc. on their sites. Here are some links for more information:

http://asmcommunity.asminternational.org/portal/site/

http://www/AsmStore/ProductDetails/?vgnextoid=ff2f2b41ce0f8110VgnVCM100000701e010aRCRD

http://www.buehler.com/application_support/recommended_books.htm

http://www.buehler.com/technical_information/Buehler Book/book.asp

http://www.struers.com/default.asp?top_id=5&doc_id=73