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topic 4884

What to use as anodize remover/stripper


Q. What is the best way to remove an anodized coating and prep the part for a new one. I would VERY MUCH like to use anything other than nitric acid, due to its availability and toxicity.

Chris Francis
- Hampton Virginia


A. Whoever anodizes your parts will strip them before they anodize. A warm bicarbonate of soda at about 1 lb per gal will strip it slowly. If you want it to go faster, you can add one teaspoon of lye per gal until you find one that works best for you. Guts ball, use Easy Off oven cleaner. Very aggressive.

James Watts
- Navarre, Florida


A. Caustic Soda, 50% mixture is a common anodizing stripper and etching solution that would not only remove previous anodize, it would etch and clean the surface and the surface would be ready for a new fresh coat of anodize.

If you're thinking industrial sized solution and not something for yourself personally then I would suggest either using straight caustic soda with an addition agent that keeps the caustic bead or flake from gathering on your tank and heating coils and what not, or you could buy a premixed solution from a vendor with the addition agents already available in the solution mixed with the caustic (I use the latter for its plain ease of use).

Matthew Stiltner
- Toledo, Ohio


Q. Won't the caustic soda mixture damage the aluminum, that is destroy the machined finish and possibly pit it?

chris francis
- HAMPTON Virginia


A. Chris, You just listed good reasons why you should have a professional do it. It is a series of tradeoffs. Time, temperature, chemicals, concentrations and thickness of the anodizing to be removed. Excess of any one of them will remove an excess of aluminum and possibly pit your part.

James Watts
- Navarre, Florida


A. Hi Chris, James is right, but from my experience, to pit a part with a caustic solution, takes an extended period of time. Granted, it depends a lot of the condition of the material in question. But I can never remember pitting a part with a caustic soda, maybe I'm just fortunate but its never happened to me. I'd say the maximum time to leave a part in any caustic soda solution should never exceed 1 minute to be safe. 15-30 seconds is a reasonable amount of time to prep a surface that is bare. To strip an anodize I generally etch around 45 sec - 1 min and it works fairly well 99% of the time.

The surface comes out with a semi-matte finish, but nothing so extreme as to dull the surface of the aluminum. There is a mixture of chromic acid + phosphoric acid that strips anodize only and limits the attack on the base material, but it's got to be used in excess of 150 °F, so it's probably too much out of range for average user needs such as yours. Not to mention the definite danger of these two chemicals mixed together and boiled can be very high.

Matthew Stiltner
- Toledo, Ohio


A. I have ran anodize line on type 2 for 30 years, and have removed the anodize coating on thousands of parts. Use a 10% solution of caustic soda at 110 degrees for 40 seconds to a minute, rinse parts.

Raymond Hendrix Troy, Tennessee


A. I believe we should take a step back here and look a little closer at this question. I don't know Chris' background from his brief letter, but with all due respect to him, if he is concerned about handling nitric acid (which won't strip anodize anyway) he obviously is not a person used to handling chemicals, as nitric acid is only one of many nasty chemicals in any typical metal finishers' shop. Hot caustic, even at 10%, is not something an inexperienced person would want to handle any more than he would want to handle nitric.

To me, telling Chris to use caustic for stripping is like telling an Indy race car driver who was just in a bad accident to switch to NASCAR- the odds are good that he can still get hurt.

The first sentence of Jim Watts' second letter expresses my advice to Chris- leave the stripping and refinishing to an experienced anodizer. If you are in a position where you must do this yourself, learn the proper handling procedures for hazardous chemicals. One reason that anodizing is done in the first place is because it is a relatively inert, chemically-resistant oxide. This means that removing it when necessary is not an easy task, and is almost impossible without handling something hazardous.

phil johnson
Phil Johnson
- Madison Heights, Michigan

thumbs up signHi Phil. Thanks for the important warning! But I really don't think we're "telling Chris to use caustic". Rather, a question has been asked about what is used to strip anodizing, and we've provided the technical answer that "caustic is what is most commonly used", along with issuing some warnings. In fact, we can't suggest to anyone at all that it is safe for them to do anything at all because we don't know what gaps there may be in their facility, knowledge, personal protective equipment, or training. The internet is a giant one-room schoolhouse with 99 readers for every poster ... with grammar school children & post doctorates often viewing the same pages, and where some readers are in advanced labs while others are in remote third world villages with access to almost nothing.

I think all we can do is try to provide the technical information that is requested, while constantly reminding all readers that proper facilities, technical knowledge, personal protective equipment, and hands-on training are essential. People should know, and should be repeatedly told (as you are doing), that almost all chemical operations involve dangers, and that anodizing almost always involves hazardous materials (as you mentioned). Thanks again!


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




Brian Allison
- Shelby, North Carolina


A. Anodize coatings can be stripped using a dilute aqueous solution of phosphoric and chromic acid. There will be no etching of the parent metal. See Mil-A-8625 [link is to free spec at Defense Logistics Agency,], especially the section on measuring the coating weight. The solution used for stripping the test panels can be used for production parts. Be sure to consider the dimensional change (hard anodize can be in excess of 0.002"). Don't mess with either chemical without proper safety precautions.

Sodium Hydroxide will strip the anodize coating (aluminum oxide) but will strip the aluminum even faster. I wouldn't do this in a production setting. I strongly recommend you don't do this, but Drano [linked by editor to product info at Amazon] (the plumbing cleaner) is a relatively strong NaOH solution. I know for a fact it etches aluminum in a hurry. As always, the resulting gas with Al and NaOH is hydrogen, which is highly explosive.

William Beeson
- Phoenix, Arizona, USA

October 1, 2008

Q. I tried stripping a clear anodize with KOH (I didn't heat the water though?). It seemed to strip it okay but left a dark film on the part which would not rinse off with water but would wipe off with a rag. The problem is there's a lot of engraving I can't clean the residue out of. How can I clean these parts so they can be re-anodized? Is the problem the KOH?

Thanks for your time.

Jeff A. Chapman
- Rochester, New York

October 27, 2008

A. Hi, Jeff. The principle here is that KOH will dissolve aluminum but it does not dissolve the copper, silicon, and other alloying materials that are usually present to varying degrees in aluminum alloy parts. They remain behind, and become concentrated, and turn the part gray or black. An anodizer removes them with desmutting compounds which can dissolve copper and silicon, often including quite dangerous nitric acid and horribly dangerous hydrofluoric acid.

It might help if you explained your situation in more detail; we don't know the alloy, and whether it's a casting or extrusion. It would be good if we knew whether this one particular part was a fluke that you encountered after previously stripping hundreds of parts, or whether this is a one-off and you had little idea what to expect.

If you are not an anodizing shop it might be best to just leave the parts as is and let the anodizing shop do the desmutting. It may not be a big deal with the proper acids. Best of luck!


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

February 21, 2015

A. Caustic Soda will attack any aluminium based product and destroy it [dependent on dilution]. If it touches Al it will turn it black. This is my experience of using a caustic soda & water based paint stripper bath.

Peter Robb
- Dunedin, Otago, New Zealand

March 18, 2016

Q. Stripping Nickel Acetate only.

How can I strip off the nickel acetate sealer from some Type III anodized parts.

We did some secondary machining on the parts after anodize, and then used Branson MC-3 in an ultrasonic tank to clean the parts, and it damaged the Nickel Acetate sealer. Preferably we cannot attack the Anodizing.

clay allen
Select... - wilton California

Hi Clay. Sorry, but I don't think that can be done. When we hear the term "sealer" we tend to envision some sort of "clear coat / top coat", but sealing in anodizing doesn't involve any sort of additional coating, rather it hydrates the aluminum oxide to cause it to convert to a more voluminous form that seals the pores.
Type III coatings are usually not sealed; are you sure they were?


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

November 11, 2016

Q. I am planning to start a business of de anodizing aluminium (hard coated) as far as i know there is always risk of damaging aluminium in using caustic as de-anodizing agent are there any other option for this process that can keep the unanodized part safe?

Pradee Papnai
- New Delhi, India

Ed. note: This question is answered in the posting from William Beeson above, Pradee.

How to remove 12µm Aluminum oxide on Aluminium alloy

December 16, 2016

Q. Hi. I would like to repair a damaged aluminum alloy anodized surface. I need to remove the 12µm Aluminum oxide on the surface first. I saw on the web that many suggest different methods. Actually is 10% HNO3 good enough to remove Al2O3 without attacking the Al surface? 10% NaOH at room temperature also gets the job done? I would like to try to keep the chemical concentration not too high as the waste treatment may have some problems.

Andrew Wong
- Hong Kong

December 2016

A. Hi Andrew. Raymond Hendrix told us that over the course of 30 years he's successfully stripped thousands of parts with 10% NaOH in 40 seconds to 1 minute, but at 110 °F. You can try it at room temperature, and see whether it works, and whether it harms your parts, but when advice is simple and clear and straight-forward, I am reminded of the aphorism "Don't tinker with a good line" :-)


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

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