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topic 55644

Color etching of aluminum to show orientation of grains




(2006)

Q. Hello
Is there anyone using NaOH to color etch aluminum? How can I tell if the specimen is over etched or not? Is NaOH a good etchant for color etching to see misorientation under polarized light? Any better method? Thanks.

Fangsong
Sheffield University - Sheffield, UK


ASM Handbook: Volume 9: Metallography And Microstructures

(2007)

A. Hi Fangsong,
The Sheffield University library has 30-some metallography books; e.g.,
  - ASM Handbook Volume 9: Metallography and Microstructures, ISBN 0871707063 =>
or previous edition listed as Metals Handbook, ISBN 0871700158.
  - Also, an earlier (8th Edn.) Metals Handbook, Vol. 8 :Metallography, Structures and Phase Diagrams.
  - Metallographic Etching by Gunter Petzow, ISBN 0871706334 (2nd Edn.) or ISBN 0871700026 (1st Edn.).

Plain NaOH solution is only used for macroetching. Etchants for orientation of aluminum grains are Barker's reagent (electrolytic), Poulton's reagent, Keller's etch, Graff/Sargent reagent and 2 g NaOH + 5 g NaF + 93 ml H2O.

Some info is on-line, e.g., Keller's etch is given at http://www.metallographic.com/Etchants/Aluminum%20etchants.htm

Ken Vlach
- Goleta, California
contributor of the year

Finishing.com honored Ken for his countless carefully
researched responses. He passed away May 14, 2015.
Rest in peace, Ken. Thank you for your hard work
which the finishing world continues to benefit from.




Etching Al3004 Aluminum

September 1, 2010

Q. Myself Murali studying mtech in NIT Tirchy. Regarding my project I need etchant for al 3004 series. Initially I have tried Kellers reagent but I was not able to get good microstructure. I want to see grains and grains boundaries clearly in my sample. Can you suggest other possibilities?

Murali Kumar Yeddalapalli
student - Tirchy, Tamilnadu, India

September 2, 2010

A. Dear Murali Kumar Yeddalapalli

Start looking at letters 26269 and 7041.

Finding the right etching is a matter of experimentation and patience, if you didn't disturb the surface already by applying too much pressure during the polishing steps.

Aluminum is a relatively difficult to prepare material for metallographic investigations; for detailed information, follow this link:

http://www.slideshare.net/corematerials/talat-lecture-1202-metallography-of-aluminium-alloys-2318129
^-- Ed. note: An outstanding link, Harry! Thanks. --^

Read on other sources, people advise to go in many steps to grit 4000 with grinding and slightly polish with SiO2 suspension.

Then the possible etchants (I partly used data from the famous German "Metallographie" book of Dr.sc.techn. Hermann Schumann, I hope I have permission for this):

Possible etchants for the macrostructure:
1. Tucker-etchant: 25 ml of water, 15 ml HF, 25 ml HCl, 15 ml HNO3
2. Flick etchant: 80 ml of water, 10 ml of HF and 15 ml of HCl. 10 - 20 sec. etching; after this dip the specimen in hot water, followed by dipping in concentrated HNO3 and rinsing, rinsing with ethanol and drying.
3. Keller etchant: 50 ml of water, 10 ml HF, 15 ml HCl and 25 ml of HNO3.
4. 0,5ml HF in 100 ml water
5. 1g NaOH in 100 ml water (to be made freshly before use)
6. Villela-etchant: 20 ml HF, 10 ml HNO3, 30 ml glycerine.

Others propose HF 10 ml, HCl 10 ml, HNO3 15 ml, water 65 ml
or: HF 5 ml, HNO3 10 ml, water 85 ml.
or: HF 5, HCl 20, HNO3 20, water 55.

It makes no sense to list more, these solutions should do the trick and you have to find the right conditions, which could cost some time. Good preparation is a skill and is often underestimated.

In all cases stop etching if the surface changes from brightly polished to dull appearance. If you have an underetched surface, you can continue, but an overetched means again grinding and polishing.

For the grain boundaries:
In most of the cases, the grain boundaries are the first ones to be attacked by the etchant, you can try diluted solutions like --
0,5 ml of HF in 100 ml water, typical etching times 15 sec. or try a mixture of 0,5 ml HF, 1,5 ml HCl and 2,5 ml HNO3 in 100 ml water.
Temperatures between 20 and 40° are used.
Etching times for the diluted NaOH can range to minutes.

Flick's reagent is especially mentioned as suited for grain boundary etching for most types of aluminium and alloys
90-100 ml water
0.1-10 ml HF (From this concentration range which is serious, you can already learn that playing with time, agitation and temperature is advisable as well.)

Furthermore I advise you to look on the internet to see what happens if you google "aluminium metallografie" or "aluminium metallography"

Success and best regards,

Harry van der Zanden
Harry van der Zanden
- Budapest, Hungary

----
Ed. note: Great posting, Harry. We appreciate it!




Etching 1100 Aluminum

(2006) -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. Hi
I'm trying to etch aluminum 1100 to reveal grain boundaries.I've tried many etchants but none seems to work for al1100. I tried keller's, NaOH,HF as well as barker's method(anodizing in 1.8% fluoboric acid).The most I could get was revealing cold work,flow lines etc but never the grain boundaries.Does anybody have any other idea?thanks.

Emre Gunduz
University - Boston, Massachusetts


(2006)

A. Try 5% fluoroboric acid etch.

Sumit Agarwal
- Providence, Rhode Island, USA


August 2013

A. Hi Emre. We appended your inquiry onto a thread where Harry Van Der Zanden suggested that it's not solely a matter of picking the right etchant, but of experienced technique, including avoiding too much pressure during polishing and avoiding over-etching. I suspect that careful attention to Harry's posting will solve your problem. Good luck.

Regards,

Ted Mooney, finishing.com
Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey



Aluminum Alloy (Al-Li-Mg) Over-etches in Seconds

May 16, 2016

Q. I'm very experienced at etching many of the Aluminum Alloys. I'm a Research Metallographer. I've recently been stumped by 1420 Al-Li-Mg that we've been casting. I'm using HBF4 5% as my electrolyte. It over-etches in seconds! Any advice would be greatly appreciated. The usual etches don't seem to be working.

Thanks!

Denise K. Balston
Metallographer - Spokane Valley, Washington USA



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