De-smutting/De-Oxidizing of aluminum

[editor appended this entry to this thread which already addresses it in lieu of spawning a duplicative thread]
Q. Hello fellows,
Q. In Germany nitric acid is not allowed for private purposes respectively only concentrations with max. 3%.

But there are also desmutters on the market which containing hydrochloric acid and ammonium bifluoride. Does anyone have experience with that?

Thanks

Chris Gough
- Germany Berlin
September 30, 2023

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⇩ Closely related postings, oldest first ⇩

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Q. Can anyone tell me why a nitric acid bath is necessary in the aluminum anodizing process?

Chris [last name deleted for privacy by Editor]
- Hampton Virginia
1999

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Q. What mixture ratio do you mix (mixture of sulfuric/nitric/ammonium bifluoride) to make the desmutter and deoxidizer and where is the best place to buy these chemicals?

Roy Clarkson
- Rock Hill, South Carolina
2003

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Multiple threads were merged: please forgive repetition, chronology errors, or disrespect towards other postings [they weren't on the same page] :-)

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De-ox for cast aluminium parts

Q. I am trying to select the correct de-ox for cast aluminium (Japanese motorcycle parts), unfortunately I can't find out the exact content of the parts. A hobby plating supply company offers a de-ox they say it is good for this type of alloy, but I'm in Ireland, so it would cost too much to have some shipped.

So far I have read that nitric acid is commonly used - but is seriously dangerous, also I have read that sulfuric acid can be used. Any suggestions (including rough measurements if possible) would be welcomed, I would prefer to use sulfuric, but I also want a good job done on the parts, so I am willing to experiment!

Thanks,
Robert

Robert Harrison
- Dublin, Ireland
2002

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Q. I work in a job processing dept. Alodining and anodizing parts but they changed our etch from sulfuric acid to nitric acid because it wasn't passing the fog test. But now the parts are not turning out the way they used to. Some spots look like the Alodine is not taking in all areas. We degrease the part then put them in Turco for 12 min then the rinse tank for 3 min and then the nitric acid for 5 min and rinse tank again then the Alodine for 2 min. ... some turn out good but its seems like it's a hit and miss. With the sulfuric acid they all turned out fine but they weren't passing the fog test ... can you tell what you think the problem is?

Suzie Scott
- Amherst, Nova Scotia, Canada
2003

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Multiple threads were merged: please forgive repetition, chronology errors, or disrespect towards other postings [they weren't on the same page] :-)

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Does deoxidizing in iron base deoxidizer or dilute nitric acid affect the bond quality of P-2 etch?

Q. We etch aluminum fittings that are part of satellite assemblies. We use the P-2 etch process, but from time to time notice a residue left on the surface of the 2024 aluminum lap shear panels after etching. During a test, deoxidizing seems to remove this film. Will deoxidizing the aluminum with either of the above solutions affect our bond quality when of our epoxy adhesive [on eBay or Amazon] primer?

Frank Lary
space systems - San Diego, California, USA
2006

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Anodising directly over natural oxide layer

Q. Can anodizing be successfully performed on a freshly CNC machined aluminum piece that has been degreased and cleaned but not deoxidized? I.e., is it possible to anodize over the naturally occurring oxide layer?

Chris Amrit
- Leicester, UK
August 14, 2017

Q. Thanks for the reply Ted.

I have no expertise in this area really. I am product designer/engineer and we want a two color (black/other) aluminum part (without adding masking/mask removal to the process).

The reason for asking this particular question is that it sounds like the double anodizing problem comes down to this.

So if we still did a desmut process (which I assume does not remove the natural oxide layer or any previous anodising?) it should work.

We plan on trying this:
Anodizing the whole part black and seal
Cut more from the part making sure there is somewhere for contact in hidden area
Anodise the whole part again in a lighter color the freshly cut areas.
Seal again.

If I am understanding your answer correctly then it should work in theory.

Chris Amrit
- Leicester, UK

Q. Yes I saw that thread before but it doesn't really answer my question.

So I guess we just have to try it ...

Chris Amrit [returning]
- Leicester, UK

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Q. Hi everyone,

First, this is an amazing site.

My question is in regard to de-smutting of aluminum parts. I'm anodizing small aluminum parts and would like to experiment by inserting a de-smutting step in my process. I noticed an entry in the Q&As: "The aluminum surface is desmutted by immersing it in an aqueous solution of 30% nitric acid." My question, how long should I insert my small parts (less than a sq/ft) in the 30% nitric acid solution?

Thanks!

russell grigsby
- Freeland, Washington, USA
November 6, 2019

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De-smutting of bright 7075 aluminum before anodizing

Q. I am fairly new to anodizing and have a question related to how long I should desmut highly polished 7075 aluminum so as to make sure I won't have any problems with the copper/zinc in the 7075 when I anodize and dye a bright red, while at the same time making sure I don't trade off too much of the polished finish by keeping the part in the desmut tank too long.

MICHAEL MCLELLAN
- EMERSON, New Jersey, USA
January 14, 2020

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Q. Thanks Robert, I am not etching.. I have highly polished 7075 and after the degreaser tank, I put the parts in a desmut/ deoxidizer solution before the anodize tank. The first time I did this I was told that if you leave polished parts in the desmut/ deox for too long, it will dull the shine. So I took the parts out in less than a minute and ended up with a lot of smut forming on the part in the anodize tank. I am rather new to anodizing.

MICHAEL MCLELLAN [returning]
- EMERSON, New Jersey, USA
January 23, 2020

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Q. Hi Janis,

I am using the 740 degreaser. The parts are hand polished using aluminum oxide sandpaper from 320 to 1500 grit, then finally with an aluminum paste. The parts are then washed by hand in hot soapy water several times, then they sit for 30 minutes in a container full of acetone [on eBay or Amazon], are dried and stored in sealable plastic bags until ready to anodize. When I am ready to anodize, that is when they go into the 740 degreaser. My anodize tank is a 5 gallon plastic container with 2 lead cathodes at each end as well as an aquarium tank bubbler at each end. I hang my parts using titanium wire. My sulfuric acid bath is at 17.5% concentration. I anodize at 6 amps per square foot for 90 minutes at 68-70 °F. The parts are perfectly clean (no black streaks) when they go into the anodize bath. It is the anodize bath that brings out the black streaks in the aluminum. I notice it when I finish anodizing.

When I say I am new at this, I had been anodizing 6061 with no problem and even started skipping the desmut step with no problems. I didn't realize that the 7075 had so much zinc and copper in them relative to the 6061 and requires desmutting after resurfacing with the sandpaper. A rookie mistake and first time I have ever anodized 7075. I believe I just need to burn off the surface zinc and copper with a good desmut before the anodize tank to fix the problem. I have read recommended desmutting times from 1 minutes to 10 minutes. The anodizing manual I bought suggests not to leave polished parts in the desmut solution for more than 2 minutes. My desmut solution is 18% sulfuric acid and 6% Sodium Nitrate [affil links].

Thanks!
mike

MICHAEL MCLELLAN
- EMERSON, New Jersey, USA
January 24, 2020

January 24, 2020 A. 17.5 % by weight or by volume? Current needs to be up to 12 amps per square foot. "Bubbler" may not be enough vigorous. Temperature? Proprietary deoxdizers are available that will not etch, your home-made formula is etching. If you cannot afford a proprietary deox in 5 gallon pails, then use straight Ferric sulphate at 8 oz/gal, it will not etch. Robert H Probert Robert H Probert Technical Services Garner, North Carolina A. Can you wipe those "black streaks" away after pulling the parts from anodizing bath? if you can't wipe them off, then that is not what is generally called "smut" (be it after etching or after sealing), those streaks are most likely to do with age and batch of the material, I've had perfectly fine looking 7075 and 7020 parts come out of the same batch being anodized next to parts that have those darkish streaks, there are quite a few posts here without a definitive fix to this problem, search for "7075 streaks" or similar keywords, I posted a picture of a largish gold colored 7xxx part with "zebra stripes" in one of those topics That being said, I would lower the acid content in the anodizing bath to around 13-14%, and increase the current density so the parts anodize at around 17-19V, calculate the time using the 720 rule to get the thickness you want - the aim is to grow the layer to desired thickness faster than what you're doing now (6 amps/sqft is quite low), it may help, may not, that high acid concentration and low current density conditions lead to long anodizing times till you reach the desired thickness, remember - the formed oxide layer is continuously being dissolved by the acid, thus you will consume more aluminum to grow the layer than necessary, you can deox all you want, but the more Al you consume, the more alloying elements eventually are being trapped in the oxide layer and/or dissolve in the acid, do not let the acid get warm, or again it will lead excess oxide being dissolved and producing a soft layer in general, no amount of deoxiding (desmutting) prior to anodizing will remove alloying elements deep into the surface so only pure aluminum would be exposed for anodizing, deox/desmut is generally for dealing with what is right on the surface of the part Regarding the prep work, I think you're overdoing it a bit, you can sand and polish the part and leave it oily, that can be better than having the clean aluminum being exposed to atmosphere, some corrosion might even form because you cleaned your parts so thoroughly and then let them sit for extended period of time exposed to air (even if kept in a sealed bag), do not use water based cutting fluids for this, they usually have quite high ph, and 7xxx are known to stain (corrode) if left sitting wet for a while I'm not doing this sort of process on regular basis, but when I have to, I use WD-40 [on eBay or Amazon] while sanding, polish also using some WD-40, then spray some on the part so it stays wet, leave it till its time to anodize, normally it wouldn't be more than couple hours, then degrease, no etch, no deox, straight into the anodizing bath, 50 min at 10 amps/sq.ft. seems to be minimum in my experience for parts to dye charcoal black (or any other color) Janis Ziemelis - Riga, Latvia January 24, 2020 Michael You are operating at the low end of the current density range. It is possible the electrolyte is attacking the anodize due to the extended duration in the solution. Try upping your current density and reducing time proportionately. Willie Alexander - Green Mountain Falls, Colorado January 24, 2020

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January 25, 2020

Q. Thanks Guys, this is very helpful!

My anodize bath is 17.5% by volume. I have been getting my anodizing supplies from US Specialty so I was using their proprietary Desmut/Deox solution. My power supply is small, just 5A/30V. For these particular parts, I can up the current density to 7 or 8amps per square foot.

I didn't try to wipe the black streaks off once they came out of the ano tank, and after dying and sealing, the black/dark streaks were even more apparent.

I will definitely up the current density and reduce time in the tank as much as I can. I also just reduced the sulfuric acid concentration.

To tell you a little more about these 7075 parts: they are 21 year old aluminum parts from Japan ... actually from Honda Racing in Japan and used on the Honda team race bikes ... they are very rare as these parts are supposed to be destroyed at the end of the race season. I am restoring an old Honda racing motorcycle (that avoided the crusher) for display in a museum, so I have been very nervous about ruining these rare and probably irreplaceable parts.

I understand that the 7075 alloy was invented in Japan and first used in the Japanese Zero fighter planes. I heard this alloy was a key reason with the planes were so agile in hard maneuvers. Just a little history I learned about 7075.

Thanks again for the help!!

mike

MICHAEL MCLELLAN [returning]
- EMERSON, New Jersey, USA

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Black residue depositing on clad aluminum during desmutting?

Q. Hello,

We've been noticing an issue with our deoxidizer tanks. When we deox/desmut clad alloys (mostly 2024-T3 and a bit of 7075-T6 clad Al) there is a black residue that develops on the parts, only on the edges. Reprocessing these parts after thoroughly scrubbing the edges with a scotch-brite pad and subsequent cleaning with acetone does not prevent the formation of this film. The deox is a proprietary chromate-based mixture with nitric acid.

A brand new bath does not have this problem, but after fairly little volume processed and only a day or two, the issue resurfaces. We do not have this problem on the bare versions of these alloys, the bath's etch rate is within tolerance, and the problem is there at both ends of the acceptable etch rate range. Longer processing times do not solve the problem either, and in fact sometimes make the issue worse!

Some general qualitative testing indicates that the residue is dissolved slowly by nitric acid, but if that's the case, the bath should be taking care of it. Salt fog testing does not indicate a major corrosion resistance issue for our chemical conversion tanks, so far as we can tell.

I'm wondering if anyone has encountered a similar problem or may be able to help point me in the right direction to find a solution. Thanks!

Dalton Mackelprang
- Cedar City, Utah, USA
March 20, 2020

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Q. 1. The deox is chromate (chromic acid) based; it does not contain ferric sulphate. It also has a small amount of hydrofluoric acid in it. We have another deox chemical that we use that does contain ferric sulphate, but we have not had the same problem with it as we have with the chromate deox. The bath also contains nitric acid. We have some tanks that are sparged with air agitation and one that is simply recirculated (it's a long tank, so air agitation doesn't work very well for that one). The one giving the issue is air-agitated, but we've had the same issue to a lesser degree on the recirculated tank.

2. The etch rate is performed only on a clad aluminum test panel and is used for solution maintenance in accordance with one of our customers' requirements (and the manufacturer instructions). The deposit does not appear on the bare versions (2024-T3 Clad smuts, 2024-T3 Bare does not) which is why it is so puzzling to me that only the edges of clad parts get this deposit (since that's where the bare substrate material is exposed).

Thanks!

Dalton Mackelprang [returning]
- Cedar City, Utah, USA
March 24, 2020

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Q. I've cleaned some motorcycle carbs with hydrochloric acid but they have a dull grey substance on them, so they need desmutting.

In the UK ther are quite a lot of regulations on acids.

The strongest I can find is 15% sulfuric acid. Is this strong enough?

Nick Mumby
- nottingham UK
July 20, 2021

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Q. Hey all,

I am a materials engineer doing some small-scale R&D trying to effectively anodize some Al 6061 gaskets and we are hoping for a Type III or "hard coat". My question revolves around the De-Ox/De-smut step before actually anodizing in the H2SO4 bath. What is a good acid and concentration other than Nitric (which hasn't really done the job thus far and isn't recommended these days anyhow)?

Many thanks for any and all help/advice!

-Perry

Perry Spott
- Milwaukee Wisconsin
May 26, 2021

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