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Removing anodizing from aluminum

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[editor appended this entry to this thread in lieu of spawning a duplicative thread]

Q. Hello all,
We are looking for new stripping solutions for anodized reworks (Type II and Type III). Currently, we use a 10% NaOH solution at 110 °F (suggested by Raymond Hendrix on thread 4884). While this solution is effective at stripping, the reworked parts are much darker than desired.

Other threads [Ed.note: including this one we appended Zander's posting to] have recommended various combinations of phosphoric acid and chromic acid. Can anyone provide any insight on the pros & cons of these various solutions, their preferred stripping solutions, visual characteristics of reworked parts with these solutions, or any other useful information? Ideally, we want there to be little to no visual difference in these reworked parts when compared with non-reworked parts. Thank you in advance for any help or advice.

Best,

Zander Naes
- Denver Colorado
October 13, 2023


A. Contact whom I assume is your dye vendor.

There's a stabilized and buffered sulfuric/phosphoric (? Gotta check sds) on the market for rack stripping that can be used on parts but you will want to have your line crew run some test specimens first as a "bucket test" in a drum or mini tank.

You of course can strip ano in hot caustic. See also, every internet review of commercial hardcoat high end cookware that some schmuck ran through a dishwasher. Exactly as the shipping papers said not to do.

But it's kinda twitchy to run, and may pit.

We run both an absolute garbage level strip tank so loaded up with junk metals that the manufacturer admitted the titration is 'an estimate', and a caustic etch with posted daily etch rates... guess which one our 20 year veteran crusty line guru uses.

BOTH.

Stripping Ano is as much an art as applying it :)

rachel_mackintosh Rachel Mackintosh
lab rat - Greenfield, Vermont
June 15, 2024




⇩ Related postings, oldest first ⇩



Q. From time to time I have wanted to remove anodizing from aluminum parts. While I'm not certain what the precise nature of the surface treatments are, they're usually not hard anodized. I'm curious if there are any relatively simple ways, using readily available chemicals that can remove the treatments. While sanding and polishing do the job, it can be a very long job.

rob howard
1998


"The Surface
Treatment &
Finishing of
Aluminium and
Its Alloys"

by Wernick, Pinner
& Sheasby

pinner
on AbeBooks

or eBay or

Amazon
(note: this book is two volumes)

(affil links)

A. There are 2 ways to accomplish the removal of the anodic coating from aluminum depending on the surface you want to end up with --

A NaOH (caustic) etch solution will remove the coating but will etch the surface leaving a diffuse (matte) appearance.

The chromic acid/phosphoric acid solution used in the Acid Dissolution Test ASTM B137 will remove the anodic coating without affecting the underlying surface. The composition of that bath is 80 grams of chromic acid in 3.5 liters of water, add 140 ml of 85% phosphoric acid, add water to make 4 liters. The bath is operated at 190 - 200 °F. Immerse the part for 15 minutes and then rinse well in water.

Lee C. Branch
Richmond, Virginia


? Would not a deoxidizer like 50% nitric do the job?

Bob Morrison
Ormond Beach, Florida


TUTORIAL FOR NEWBIES:

When aluminum alloys are etched (in caustic soda), the copper and other alloying materials are not dissolved/removed -- which will leave the surface grey or black. Nitric acid is often used as a de-smutter (sometimes called "de-oxidizer") to remove the remaining traces of copper after the aluminum has been etched. Different desmutting chemistry may be required, however, depending on the other materials in the aluminum alloy in question.

A. Hi Bob. To my knowledge, no, nitric acid does not remove anodizing.

As a semantic issue, although this step is often called "de-oxidize" it seems that it would be better for us to call it "de-smut" because 50% nitric acid is a very powerful oxidizing agent -- it's certainly not a de-oxidizer.

Regards,

Ted Mooney, finishing.com
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey


! Doesn't anybody know that good old oven cleaner [this product on eBay or Amazon affil links] has just enough acid to remove most anodizing without pitting or discoloring the aluminum surface? It works, try it.

Ken Perrella
- Utah
2002

Ed. note: Ken means 'caustic' not 'acid'

thumbs down sign Actually, that is not so. Oven cleaner is very caustic, and will remove anodize; but I have found it difficult to control, and it makes for a really ugly resulting finish. Completely undecorative.

tom pullizzi animated    tomPullizziSignature
Tom Pullizzi
Falls Township, Pennsylvania




Easy-Off
(6-pack)

on
Amazon

(affil links)

! I used oven cleaner on my oxidized frame on my GSXR and it didn't remove it. I sprayed it on and let it sit for 15 minutes and it did absolutely nothing. Sanding has been the only thing that will work.

DAVE L [last name deleted for privacy by Editor]
- BRISTOL, Rhode Island, U.S.
2002


A. Oven cleaner DOES work quite well. But Anodizing is the second hardest substance known to man, its going to take longer than 15 minutes! I recommend at least 45 minutes, after that you will actually see the anodizing fall off. And depending on the aluminum underneath, it sometimes does leave a horrid finish. But for a Motorcycle, since I am assuming you were polishing the frame on the GSXR... the resulting finish would NOT matter because the plan is to sand, cut and polish. I am sorry you spent the whole weekend sanding off the Anodizing, if you had given the oven cleaner another half an hour, you could have saved some elbow grease.

Steve Callen
- Walton, Kentucky, USA
2003




Multiple threads merged: please forgive chronology errors :-)



Which anodize stripper will not etch aluminum?

Q. I have some highly polished anodized fittings that a client wants stripped, and a new decorative anodized colour re applied. What is the best acid/alkali solution to use so as not to etch the alloy in any way? In what ratio should the stripping solution be mixed?

Kind regards,

Steve Power
- Nelson, New Zealand
2001


A. Although caustic soda is a popular stripping agent, it does not meet your requirement of not etching the base material. Rather, what you will want is the chromic-phosphoric mixture described by Lee Branch above. Good luck.

Ted Mooney, finishing.com
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey


A. We have had some success using a mixture of phosphoric acid (35 mL/L), chromium trioxide (20 g/L) and water to strip Type I (chromic acid), Type II (sulfuric acid) and Type III (sulfuric acid/oxalic acid) anodize coatings from aluminum. The solution is heated to 100 °C and usually takes less than 15 minutes to strip the anodize. Unfortunately, it smuts the base aluminum, to varying degrees, when it has been anodized in Chromic Acid.

Teddy McCracken
- Blacksburg, Virginia, USA




Q. We use the chromic- phosphoric acid solution for anodizing & Alodine coating stripping. But I search throughout and can not find any method to analyze the components of tank solution. Could anybody give any information or help?

hong huming
- xiamen china




Q. I am seeking a way to strip the anodic "Anodized" layer from previously anodized aluminum. I will re-anodize these parts after they are stripped. Any help would be greatly appreciated.

Craig Calhoun
- Houston, Texas
2001


A. Dear Sir,

To etch the anodised layer from the aluminium, you can use a mixture of 50-100 grams phosphoric acid (85%) and 30 grams chromic acid in one liter water. Five to 30 minutes, then thoroughly rinse afterwards. First try a dummy before you put your sample in. Another one: a mixture of 50 - 100 grams phosphoric acid (85%) and 50 grams sodium bichromate in one liter water; Same time. These mixtures don't attack the underlying Al, but the surface may become dull. Watch out for the phosphoric acid.

Martin Saille
- Ghent, Belgium


A. Martin neglected to mention that the chrome/phosphoric mixture should be heated to 175 °F. I have not used the other mixture, so I'm not sure as to the parameters.

Marc Green
Marc Green
anodizer - Boise, Idaho



Pure Lye

on eBay or

Amazon

(affil links)

A. Craig,

I don't know how helpful this will be but from personal experience, if you're looking for a simple stripper, a caustic soda [affil links] such as Red Devil lye has worked for me. WARNING! Lye will dissolve aluminum so if you're going to give it a try, keep a close watch on the parts as they're stripping. Be sure to COMPLETELY rinse all parts in Distilled or DI water after stripping. Also keep in mind that this will etch the surface of the aluminum and you may need to clean the part (with a weak solution of Nitric acid) prior to anodizing. This is just from personal experience. I can't say weather it will work for you but it has worked for me in the past.

Hope I was of some assistance.

P.S. You may want to test this procedure on scrap before you try it on anything important.

Erik Knifer
- Great Bend, Pennsylvania


A. Use oven cleaner [this product on eBay or Amazon affil links] . It works wonders ... fast!

Jared Beck
- Pearland, Texas
August 21, 2009




How to strip anodizing from automobile trim?

Q. I AM LOOKING FOR PROCESS INFORMATION ON STRIPPING ANODIZING OFF OF AUTOMOBILE TRIM. I OWN A METAL FINISHING SHOP AND HAVE SEVERAL CUSTOMERS THAT WANT ME TO RESTORE THEIR TRIM.POLISHING AND DENT REMOVAL IS NOT THE PROBLEM. JUST REMOVING THE ANODIZING IS. YOUR HELP IS GREATLY APPRECIATED.

THANK YOU,

BRUCE DELL
- Hubbard, Ohio
2002


Q. I am restoring a 1963 Chevrolet 2 door hardtop Impala. I have been able to clean and shine all of the chrome and steel, but am unable to clean the anodizing from the aluminum trim. I have buffed some of the trim, also have tried to clean and shine, but it will not come back to a good shine. Does anyone know how to remove the anodizing from the aluminum trim? I would be grateful for any suggestions.

R. Gerald Scales
hobbyist - Bowdon, Georgia
2007


A. Whereas it is somewhat practical to strip copper-nickel-chromium from automobile parts and then polish and replate, it is NOT, repeat not practical to strip the anodizing. The anodic oxide is 1/2 into the metal and 1/2 on top of the metal. When you strip it you lose the polished finish. Now you can re-polish to a shine, and bring down a 10 to 11 step anodize process cycle and re-anodize. I have never heard of a job shop that will take in this kind of work.

To strip the anodic film, dip in 6 oz/gal Sodium Hydroxide for about 3 to 5 minutes,rinse in cold water, then remove the smut in 30% Nitric Acid, rinse in cold water. Good Luck ! Take two aspirin and call me in the morning.

robert probert
Robert H Probert
Robert H Probert Technical Services
supporting advertiser
Garner, North Carolina
probertbanner


A. Hello,

The cheapest and in your case probably the best solution would be to use sodium hydroxide (aka caustic soda) solution. Its fairly inexpensive right now for the raw product (prices tend to dip and spike every few years, right now they are coming down from a 150-200% price spike).

Use this in a concentration of 5-7% of solution and see how it works. Be watchful of the process though, leaving parts in unattended will destroy them. Most parts such as car trim should probably strip completely down within 2-3 minutes. Heat this solution to 150 °F+ and you'll be fine. Use a polypropylene tank to keep it in as that has a heating max of 200 °F, where the cheaper plastic will start to flex and warp around 140 °F. Use a clean, room temp rinse tank to clean this off the parts, and get ready to buff and finish them off.

If you think things are taking a bit too long, bump the concentration by 1% at a time, anything over 10% is probably going to be nothing more than overkill and you'll just end up a lot of undissolved product at the bottom of your tank.

Good Luck,

Matthew Stiltner
plating company - Toledo, Ohio




Q. I am an amateur motorcycle enthusiast in need of some advice/insight. I recently purchased an 89 gsxr-750. The swing arm has already been stretched 6 inches and chromed. I would like to polish the frame of the bike, but I do not know what to do to get that annoying anodized coating off. Also would you recommend wet sanding the frame after stripping the anodized coating? I started working on a 6 inch section of the frame where the fairing will be covering just in case I botched it. At least this way I can cover it up. Any advice would be greatly appreciated.

Jamie Dalmida
Hobbyist - Boynton Beach, Florida, United States
2006


A. I've used oven cleaner to remove anodizing on different bicycle components. A pair of Sugino cranks and a pair of DA hubs. The oven cleaner (Zipp professional) took it off of the cranks in a few minutes. it took a lot longer and more steel wool [affil link on Amazon] to get it off of the hubs. I don't know if the coating on the hubs was thicker or if they had used some kind of sealant, but there was no mistaking the difference in time it took.

John McComb
- Oakland, California
2007


A. Use a phosphoric acid bath at about 20 percent phosphoric acid to 80 precent water (1 gal. of acid to 4 gal. of water ratio) heat liquid to 190 °F. Then submerge part depending on size for 1 to 3 minutes.

I'm an anodizing line operator.

Justin Ridenbaugh
- Newark, Ohio
September 16, 2008


A. I have read many articles on this subject and one stated that they took all of the anodized aluminum on their antique car and dipped it in drain cleaner for about 15 minutes to remove the anodizing. After cleaning and polishing it was coated with a clear plastic spray paint. The pictures of the finished product looked great. I am about to do the same thing to my car trim and hope it works. I would try the acid bath someone talked about but not sure what and where to buy this.

David Bennett
- Nashville, Tennessee USA
December 29, 2010


A. I read a bunch of these forums and found that the only thing that worked for me was Greased Lightning and elbow grease with a sander. Only after that can you really polish anything. I've included some photos on my Honda CX 500 build blog of my process here:
wrenchhacker.com/?p=187

Matt Heckroth
- Denver, Colorado, USA
January 6, 2012

Ed. note: That site no longer exists.



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