Removing anodizing from aluminum

A. Hi Ira. The caustic soda [affil links] or oven cleaner [on eBay or Amazon] should work, but just removing the anodizing doesn't necessarily mean it will be attractive -- and it will probably quickly get corrosion volcano/pits. So it's a big job with hazardous chemicals, with little reward. But good luck.

Regards,

A. You can use 70 gpl caustic solution at 60 °C to strip the anodic film. Be careful, as the reaction is exothermic, creating fumes. Then you have to rinse in water and then neutralize the part in 200 gpl sulfuric acid or nitric acid solution, and then rinse in water well.

A. Hi Tom. Oven Cleaner is usually considered an amateur rather than a professional approach because it was neither made for that purpose nor do you know quite what is in it -- which leaves you with a lot of 'splanin' to do to your customers about how your quality control system works, and to your employees and the regulators about how your safety program works.

Depending on the particular parts, an alternate and perhaps preferable approach would be to mask the components either for one-side anodizing or for one-side stripping. Good luck.

Regards,

Thanks Brian. Thanks Marc, I sent you Arnold's e-mail address; please contact him privately with your sourcing info.

But, Marc! I most certainly WAS NOT sipping girlie drinks on some beach! I was washing down fried bologna sandwiches with draft beer at Robert's Western World in Nashville -- arguably the very best bar in the whole world, especially when Eileen Rose is So Lonesome She Could Cry and Rich Gilbert is shredding Ghostriders ...

You DO see why I couldn't hurry home, right?

Regards,

A. Hi Alaattin,
A Sodium hydroxide solution will indeed strip off the anodising from the aluminium parts.
But, with Aluminium being amphoteric, i.e., it reacts with acid and alkali, the sodium hydroxide will carry on dissolving the aluminium under the anodic layer once that has gone. This will be a fast and violent reaction at the temperature and concentration you have stated.
Therefore you will have no control over the amount of metal that is removed. It would not take long to completely dissolve the components.
So the chromic acid and phosphoric acid solution is the best option. This solution will only remove the anodic layer. It takes about 30 to 45 minutes to remove the anodic layer and there is no violent reaction during the process. The aluminium underneath will be unaffected by the acidic solution. The concentration should be phosphoric acid at 35 ml/l, and chromic acid at 20 g/l, heated to boiling, It will not work even if only a few degrees cooler than that. But it does work well when performed as described.
Good Luck
Mark

A. My ears must have been ringing. It's Sunday, so I'm not at work. I'll try to remember to get the name of the stuff tomorrow. We have used it with success, but not extensively enough to be able to completely recommend it. But it does work. Due to the nature of my customers, changing an existing process can be quite a challenge.

That being said, the manufacturer is not a supporting advertiser on this site, so I'm not sure if I'll be able to post the product name.

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Ed. note: Please post it Marc, it's no problem! We don't want to interfere with learning, just to discourage spam and avoid opinions which lead to heat rather than light: "Iridite is better than Alodine" (or vice versa), "Atotech is better than Enthone" (or v/v), and to discourage those people who aren't really looking for camaraderie or offering help, but just constantly prowling the site for topics where they feel justified trying to advertise their products for free :-)

As my always insightful partner Tom observed at the dawn of the internet: "The Price/Demand curve for Internet Advertising: Infinite demand at zero cost / zero demand at infinitesimal cost" -- Thomas J. Pullizzi, 1995 :-)

A. Hi James, even such authoritative tomes as the previously-mentioned 1200+ page Wernick, Penner, & Sheasby tell us to either use titanium or titanium-clad racks, or suffer the wear & maintenance of caustic stripping of aluminum :-(

But we appended your inquiry to a thread which addresses a chromic-phosphoric anodize stripper which is non-etching. Whether it does a good job on rack stripping hasn't been discussed on this site yet, but maybe now it will be :-)

I am not an anodizer, but my suspicion is that consumption of raw aluminum rack components by the caustic is a smaller issue than their consumption by the anodizing process, such that improved rack strippers may address the round off error rather than the heart of the problem; even if the stripping removed no aluminum, the next anodizing cycle consumes raw aluminum to build the anodic film which you then remove, and "rinse and repeat". Good luck.

Regards,

A. Chrome/phosphoric strip will minimize the loss of aluminum but may be an environmental issue. This can also be accomplished by removing the racks from the caustic strip as soon as they are bare. Of course that takes some babysitting. Leaving them too long or forgetting them in the caustic will result significant loss.

Well, it only took me 4 1/2 months (sorry guys, I spaced this off), but the product I was referring to earlier is Stripper 5275, from SIC Technologies (http://www.sictechnologies.com/Misc.html).

It's a non-chrome stripper. I played around a bit with it on a small scale, and it does work as advertised. That being said, we decided not to facilitate it (limited space for an additional tank and ventilation), so my experience with it is limited. I do not know how long the bath will last. It seemed to take a long time to remove the anodizing, however with increased heat and concentration, the time shortened up. We never spent enough time to optimize the process.

It was pretty viscous, and had a sweet smell, I would guess it was a phosphoric/sulfuric acid based chemistry.

A. We used caustic-based formula for stripping anodizing racks for 3-4 years; later we gave it up. But we did not try either phosphoric/sulfuric acid or chrome/phosphoric based formula because of environmental issues.

At first we viewed stripping with a hand grinding machine (pneumatic) suspiciously. But we saw it as easier, faster, and faultless. So, we have used this method for 2 years now ... if I were you I would at least try this method, I advise everyone.

Sincerely...

A. Hi Parthi. We appended your inquiry to a thread which offers at least 4 methods: oven cleaner, caustic, chromic-phosphoric acid, or sanding. Please introduce yourself and your situation so people can advise you which is most appropriate, least dangerous for you, and most environmentally friendly. Telling us that the sheet is this thin is of some help, thanks, but it doesn't tell us whether you are a hobbyist with one old piece of dirty aluminum, or a process planner with acres of aluminum sheet to strip the anodize from every day, and the best stripping method also depends on what you intend to do with the aluminum next -- re-anodize it, paint it, clearcoat it, etc. :-)

Good luck, and Regards,

A. To remove anodized coating from automobile parts, build some troughs using 10 inch pvc pipe about 8 feet long. Cut an open slot about 90 inches long, and about 8 inches wide out of one side put a drain in the end, also close off both ends with a cap.
Using a 5 gallon bucket of water, add about 8 ounces of caustic soda [affil links] slowly while stirring.
Pour this solution into your trough, then place your parts inside. After a few minutes it will look like it is boiling as it removes the coating. Most parts need about 20-30 minutes, but some take longer. KEEP A CLOSE WATCH ON THEM, IF LEFT TOO LONG IT WILL EAT INTO THE PARTS.
Do this in an open area, USE RUBBER GLOVES, AND A VERY GOOD MASK, DO NOT INHALE, OR GET ON YOUR SKIN.
When finished, remove parts, rinse quickly with water, then brush off immediately, this will remove the black coating. Then buff to a new looking finish. Drain liquid into bucket, and re-use at any time. More acid^^caustic will be needed if used several times. I DO THIS ALL THE TIME -- IT WORKS PERFECT.

A. Hi Shawn. Yes it will, and that's what most shops use to strip anodizing. Without too much simplification:
'lye' = 'caustic soda [affil links] ' = 'sodium hydroxide'.
However, it also dissolves the underlying raw aluminum, so babysitting and some experience regarding the timing is helpful.

Regards,