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Aluminum Etch turned Aluminum Black

Quickstart:
     Before anodizing aluminum or chromate conversion coating it, it usually must be 'de-smutted' (the non-aluminum portion of the alloy dissolved away, because those other materials will not form transparent aluminum oxide, but discolored oxides). If the aluminum is to be etched before anodizing, de-smutting becomes even more important because the etching solution dissolves only the aluminum, leaving even more of those other ingredients on the surface.

Q. ETCHING THE 7075 AND 2024 SERIES ALUMINIUM AT 115 °F, SOMETIMES AFTER THE DESMUT IN NITRIC ACID BATH IT STILL IS NOT ABLE TO REMOVE THE BLACK SMUT FULLY AND IS LEAVING YELLOW-ISH SPOT. WHAT COULD BE THE REASON?

CURRENTLY OUR ETCHING TANK LOOKS LIKE GREEN-ISH YELLOW. WHAT IS THE CAUSE FOR THAT?

JAYDEVSINH VAGHELA
- HAMILTON, ON, CANADA
August 5, 2024

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⇩ Related postings, oldest first ⇩

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Q. I am etching 7075 aluminum parts at ambient temperature, with a mixture of sodium dichromate, sulfuric acid & DI water in a polypro tank. The solution has been used for some time & has gradually darkened, picked up about 800 ppm iron but is otherwise unchanged(?)(i.e., by analysis of H2SO4 & Cr6+ approx. decrease of <10%).

The problem is that recent batches of parts are coated with a fine black film after final DI water rinse. A swab sample of this deposit, when examined by XRF (ED as attachment to SEM) shows only the presence of aluminum ( atomic # > oxygen). This film can be removed but I would like to know where it comes from. Anyone with an idea? PS The 7075 parts are racked on a 6061 frame.

G W Green
- Montreal, Canada
2000

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Q. I work for a small DoD contractor, and we do repair of various aircraft electronic assemblies and also paint shipboard antennas and the like. We are attempting to branch out into some simple Chemical Conversion Coating treatment of aluminum brackets and the like. We are using the old standby of Alodine 1201 ⇦this on eBay or Amazon [affil links] .

My plating line is as follows: Wash all parts with warm tap water and Dawn brand dish washing detergent, and thoroughly rinse off all traces of detergent, Alumiprep 33 ⇦this on eBay or Amazon [affil links] at a 3:1 ratio with DI water, a rinse cycle of tap water (pH of about 6.5), Alodine 1201 ⇦this on eBay or Amazon [affil links] at a 2:1 ratio with DI water, and two more rinse cycles with tap water, then blow dry with heated air (from a hair dryer ⇦this on eBay or Amazon [affil links] ).

All of the pieces have come out fine, with no problems except one type. There is a small standoff of 1/4" hex aluminum, about 3" long. It is drilled and tapped at both ends, however, does not have any sort of helicoil or steel insert. I am not sure of the exact aluminum alloy, I would suspect it is probably 6061 or something similar, as that is very common on Navy equipment.

When I process these particular parts through the Alumiprep 33, they come out with a dark, almost pewter finish on them. The finish does not rub off. If it is wetted, and a white paper towel is rubbed vigorously on them, they will darken the paper towel, slightly.

With the dark finish, then Alodining the part makes it look real bad. These parts are going to be painted, so the appearance is not a big issue.

I know the Alumiprep solution is still good, because right after these parts were run, I ran some other parts, and they came out fine.

I did contact the manufacturer's tech support. He suggested that possibly for some reason the corrosion/oxidation etc. which Alumiprep is supposed to remove from aluminum is being deposited on these particular pieces. Almost like for some reason the chemical reaction polarity is reversing. He suggested trying the parts in fresh, unused Alumiprep - that did not help.

I also prepared a very dilute solution of HF (5% in H2O, pH of about 3.0 on pHydrion paper) - this solution does not seem to be helping either. The aluminum in the HF appears to have some sort of whispy strands coming from it in the HF - something like whiskers - but more whispy.

As I said, I am not sure of the exact Aluminum alloy these are made from. One other point of interest (potential) is these parts were very heavily corroded and pitted - much more so than other parts from the very same antenna. In fact, I have never seen aluminum this badly corroded. When they parts were tumbled to remove the corrosion, the pits looked very much more like rust on iron - like an old iron part from some old tractor which had been cleaned up. I could send photo's of the parts, if that would help.

Sorry for such a long post, but that is all my information.

Scott A. Wasmer
- Odon, Indiana, USA
2006

Ed. note: Please don't apologize for a detailed posting:-)
The detail is what makes the inquiry interesting as well as answerable!

If you professional finishers are having a chuckle at my post - well, that is okay. I know I am not real smart in this area, as I said, I am just setting up my little line.

I did call one of my supplier's technical people - he gave me some ideas, and they did not work. Then he stated that he was absolutely certain that my material must not be aluminum. I guess he so much believes in his "do-all" product, that it HAD to work.

I am not the sharpest tool in the shed, but I do know what Aluminum is. So I went back to this web site, and contacted one of the sponsors here. What a great guy - told me to try some Nitric Acid to De-Smut. It worked great!

Just goes to show me that reading and talking to suppliers, who are in the business of selling and promoting product is not always the best resource. Thanks again for the help here... I have found very much great information, and surely will be back again.

Scott Wasmer [returning]
- Crane, Indiana, USA

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Q. Why do aluminium parts turn black when cleaned in Sodium hydroxide solution? How should it be removed?

Aijazullah Tajir
- Hyderabad, India
February 29, 2012

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Q. What is the procedure to prepare etching for Al 6061

Sean John
- Melaka, Malaysia
November 14, 2012

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Q. Hi,

we are facing a problem after etching our aluminum parts* by getting black smut/coating which doesn't disappear in our nitric acid bath -- therefore parts look like dull yellowish.

I have found this article** about this issue; however I want to learn that what experts say.

* Unfortunately I don't know what kind of parts we treat but I am showing the parts.
** https://patentimages.storage.googleapis.com/84/dd/a7/dd3aadf4f12524/US3692583.pdf

ender toprak
- Ankara, Turkey
March 18, 2021

Q. haha, hi Drew.

I have found some articles citing fluoride as a desmutting agent. and I have prepared a mini desmutting bath by using Ammonium bifluoride as a source of fluoride. Something happened as a exothermic reaction after dipping the AL part ... however, the black smut didn't dissolve :(

ender toprak [returning]
- Ankara, Turkey