Color etching of aluminum to show orientation of grains

A. Dear Murali Kumar Yeddalapalli

Start looking at threads 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:

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,

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

A. Try 5% fluoroboric acid etch.

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,