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topic 22496

De-smutting of aluminum

A discussion started in 1999 but continuing through 2019


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


A. It is not "necessary", it is a readily available cheap desmut chemical. Proprietary desmut/deoxidize chemicals are available, but at a higher cost. Most of them work better because they do two functions.

Probably the most effective deoxidize desmut is a mixture of sulfuric/nitric/ammonium bifluoride. Effective, but slightly nasty.

James Watts
- Navarre, Florida


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

"Surface Treatment & Finishing of Aluminium and Its Alloys"
by Wernick, Pinner & Sheasby
from Abe Books
info on Amazon


A. Hi Roy. You can buy proprietary desmutters from an aluminum pretreatment specialist like Chemetall (Oakite), Macdermid or U.S. Specialty Chemical. One advantage is technical help -- they'll tell you how and where to control the composition and help you make sure you get it right.

If you want to buy raw chemicals yourself, any commodity chemical supply house will have them. Wernick, Pinner & Sheasby =>
suggests 100 g/l chromic acid, 60 g/l sulphuric acid, & 3-4 g/l ammonium bifluoride as one possible composition.
Good luck.

Ted Mooney, Teds signature
Ted Mooney, P.E. - Pine Beach, New Jersey
Striving to live Aloha

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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 sulphuric acid can be used. Any suggestions (including rough measurements if possible) would be welcomed, I would prefer to use sulphuric, but I also want a good job done on the parts, so I am willing to experiment!


Robert Harrison
- Dublin, Ireland


A. Although you, I, and many others use the term 'de-ox', I think you are really looking for a 'de-smutter' not a deoxidizer -- which I mention to help us agree about what the chemical is supposed to do. The desmutter is supposed to remove the copper, silicon, and other undesirable materials from the surface because only the aluminum is anodizable. Sulfuric acid may be pretty powerless at removing some of them.

The overwhelming majority of professional anodizers (who must work very hard to minimize costs) do not attempt to 'home-brew' their solutions. Amateurs (who usually know much less) should not strive to formulate their own processes. The best mixes are the result of years of research and are proprietary; and sometimes they don't involve just mixing, but also require synthesis of organic chemicals from precursors.

It's better to look up 'plating supplies' in your city phonebook and buy a proper desmutter; but you can search the published metal finishing literature and expired patents for "aluminum desmut" if you insist. Good luck.

Ted Mooney, Teds signature
Ted Mooney, P.E. - Pine Beach, New Jersey
Striving to live Aloha


A. A world wide company called Atotech offers De ox for casting aluminium alloys. The trade name is ALUMETCH G
This is used as additive along with Phosphoric acid and nitric acid. They claim that this does the job for cast alloys except 380 cast alloys
Hope this information will help

Dr. H.B.Rudresh
- Bangalore, India

Ed. note: Thank you Dr. Rudresh. A number of different companies offer desmut processes, so readers are suggested to please scan our Directory of Chemicals for additional sources. Good luck.

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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


A. This is only a guess, Suzie. But straight sulphuric acid is an unlikely etchant or desmutter, so I'm wondering if the original process had hydrofluoric acid or bifluorides in it that the new nitric acid formulation lacks? That might leave it incapable of acid etching your parts consistently, depending on what they are.

It is always possible for a pretreatment like an etch to adversely affect salt spray performance, but it may not necessarily be the culprit. Do you know what the salt fog requirement is?

Ted Mooney, Teds signature
Ted Mooney, P.E. - Pine Beach, New Jersey
Striving to live Aloha


A. There is another issue that may be causing problems for you. Nitric carry over into the sulfuric bath can greatly reduce anodizing. The following information is the answer to a question regarding nitric desmut from a Lecture series entitled "Theories of Anodized Aluminum, 100 Questions and answers". This response, along with other issues on our anodizing line, encouraged us to change from nitric to a sulfuric based desmut. When aluminum is etched with alkali, the surface of aluminum turns to a color from gray to black. This black substance sticking on the surface is called 'smut.' Smut is formed when impurities or alloy contents of Si, Mg, Fe, or Cu included in the aluminum, deposit on the surface. The aluminum surface is desmutted by immersing it in an aqueous solution of 30% nitric acid. Why is nitric acid used? Is it impossible to use acids other than nitric acid for desmutting? Acids other than nitric acid, such as sulfuric acid can be used for desmutting.

However, nitric acid can remove smuts completely and in a much shorter time than sulfuric acid. Nitric acid is an oxidizing acid, whereas sulfuric acid is a non-oxidizing acid; this is the major difference between these two acids. In general, dissolution of metals is faster in oxidizing acidic solutions rather than in non-oxidizing acidic solutions. For instance, corrosion of metals is faster near the surface of the sea than at a large depth in the sea. The reason for this is that the quantity of dissolved oxygen at large depths in the sea is small, but a large quantity of dissolved oxygen can be found near the surface of the sea. Considering the principle "effective desmutting is possible in oxidizing acidic solutions," nitric acid is not absolutely necessary for desmutting. An aqueous solution of sulfuric acid to which hydrogen peroxide is added, may be used for desmutting. Nitric acid was being used in the past for the reasons mentioned above, but is not preferred nowadays. When nitric acid is neutralized, the nitrate ion (NO3-) transported to the sulfuric acid bath for anodizing aluminum, hinders the formation of anodic oxide film. Similar to chlorine ion, the NO3- ion causes pitting corrosion on the oxide film during electrolysis.

Ron Oscarson
- Spokane, Washington USA

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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 primer?

Frank Lary
space systems - San Diego, California, USA

"Handbook of Aluminum Bonding Technology and Data"
by J. D. Minford
from Abe Books
info on Amazon


A. No experience with the P-2 Etch. But from the chemistry-- sulfuric acid 350 g/L, ferric sulfate 150 g/L, 145°F, used for 11 minutes -- I expect traces of iron, sulfate & copper on the 2024 surface after rinsing. Would worsen with increased use of the etch. A brief 20-50 vol% nitric acid deox/desmut should clean up the surface. Rinses well. Follow with a warm DI rinse and hot air drying prior to bonding.

The nitric deox/desmut is good for 2024 test panels prior to chem film & salt spray testing, whereas a nitric-sulfuric-ferrous deox/desmut leaves detrimental residue.
Do some surface analysis & bond testing.

Ken Vlach [dec.]
- Goleta, California
contributor of the year 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.

Anodising directly over natural oxide layer

August 14, 2017

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 2017

A. Hi Chris. My answer is no ... not because I am sure it will never work, but because I feel that it will not reliably work. Finishing sequences are designed to be robust, not to operate on the razor's edge between success and failure.

I think something to consider is that the "de-ox" step is probably a misnomer; it's actually a "de-smut" step designed to deal with non-aluminum contamination, not to remove oxidized aluminum.


pic of Ted Mooney
Ted Mooney, P.E. RET - Pine Beach, New Jersey
Striving to live Aloha

August 16, 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

August 2017

thumbs up sign Hi Chris. If I understand what you are trying to do, please see letter 48706 which is on that topic and includes very good graphics of two-color anodized items :-)

Luck & Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET - Pine Beach, New Jersey
Striving to live Aloha

August 17, 2017

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

November 6, 2019

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?


russell grigsby
- Freeland, Washington, USA

November 6, 2019

A. Hey Russell,

The acid dip desmut dwell time cam be anywhere from 10 seconds to 5 minutes depending on the type of smut left on the material and the design of the part. It is also dependent on what steps took place beforehand; a more aggressive etch will leave more smut, which can lead to higher times needed. Without any knowledge of the part or the process I would guess around 30 seconds.

A few things to note, if you run a large amount of 2000 series aluminum through this solution, the copper content should be monitored, as it can have detrimental affects on the baths ability to desmut. if you are running high silicon aluminum alloys, silicon in the smut will not be removed.

Hope this helps!

William Hogan
Amphenol Aerospace - Sidney, New York USA

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