Removing anodization without spoiling the aluminum

A. Back in the good ole days, we used to use oven cleaner ⇦ this on eBay or Amazon [affil links] to remove the anodizing from aluminum. Then, polish it FOREVER with Mothers Mag & Aluminum Polish ⇦ this on eBay or Amazon [affil links] . It will look as bright as chrome, but won't last unless you powdercoat it with clear resin.

A. I'm a retired ME with a metal finishing background, and as you'll find below, also have way too much time on my hands.

I presume the trim is off the car. To clean as well as strip the anodic coating at the same time, you might try a product by Alcoa that's simply called "Alcoa Cleaner". The stuff was intended for bare aluminum only because every spec of the anodic coating will go south during the cleaning process. Of course, this is what you want if the intent is to remove the anodic coating so you can follow up by polishing/buffing to a mirror luster.

I can't get into the endless details of polishing/buffing aluminum alloys to a mirror luster without writing a book.

Boeshield T-9

on eBay or

Amazon

(affil links)

In addition to polishing/buffing, you have to seal the part if you don't want to constantly mess with hand waxing to keep the surface sealed or polish frequently to remove oxidation. Regardless of any product claim, Carnauba-based waxes of any brand last about as long as a $20-dollar bill on a busy sidewalk.

There are only two sealants out there by reputable companies that I know for sure actually do what the manufacturer states. One is the "Alcoa Sealer," the other being "Boeshield T-9" ⇨

However, for your specific application, I recommend the Boeshield T-9. It was developed by Boeing (the aircraft folks) to a military specification defining a material for inhibiting both oxidation from aluminum surfaces as well as preventing ferrous alloys from rusting. It is now licensed to a 3rd party, but has the same original formulation.

The reason I'm recommending the Boeshield T-9 is because the Alcoa Sealant will slightly degrade a mirror luster because it imparts a "micro-oxidation" due to the isopropenol/inorganic acid content.

However, the Boeshield T-9 seals in an entirely different way and reminds you of the new "wet-look" you get when spraying WD-40 on something. It's a spray on, wipe off product, but differs in that it mysteriously seals and retains the "wet look" for 6-9 months, depending on inland vs coastal use. To reapply takes seconds to spray and wipe off.

Hope this helps.

Greybeard

A. If the parts are anodized (clear or colored) you can try Easy-Off oven cleaner ⇨
I used to use this method when I used to race remote control cars. I wanted to polish the shock housings and they came with gold anodizing. Do this in a well ventilated area, and you may want to try it out on an extra or scrap part first. I can't remember that exact time required, and it may take multiple applications. Make sure you wear rubber gloves ⇦ this on eBay or Amazon [affil links] and don't get it near your eyes. Good luck.

Ed. note: ... and goggles ⇦ this on eBay or Amazon [affil links]

A. If it's a clear coat rather than anodizing, you should be able to remove it with Aircraft Stripper (methylene chloride). It is nasty stuff -- use only outside, standing upwind, and wearing gloves and goggles.
Alternately you can get the coating professionally removed by a plating shop if you prefer. Good luck.

A. Depending on your definition of "spoiling", stripping back to original is impossible. The aluminum oxide thickness is half on top of the original surface, and half underneath the original surface. When you strip you will be 1/2 the thickness of the oxide underneath the original surface. Robert H Probert Robert H Probert Technical Services Garner, North Carolina Dear Chris, A. Antimony three oxide(Sb2O3) plus sulfuric acid 40% at 80 °C will dissolve aluminum oxide depositing grey antimony which is the end of Al2O3 dissolving, to remove antimony in turn immerse workpiece in nitric acid 30% at RT. Until the white substrate of aluminum is appeared. Hope this is okay with your case. Hadi Khosravi - Tehran, Iran A. I work as a Design/R&D engineer and I have explored anodizing before to solve problems related to corrosion. Let me tell you what I have come to believe & preach... Anodized aluminum is the most resilient, corrosion resistant, cost effective (non-polymer) finish known to man! A good quality anodized finish will literally last forever! The only thing I know for sure to chemically strip off the Al2O3 layer is Nitric Acid! It is Waaaay more dangerous than the H2SO4 (Sulfuric Acid) used in the electrolysis. And besides, it's hard to get hold of because it is a constituent ingredient in the manufacture of TNT & therefore it's sale and distribution is regulated in Canada & U.S., but I'm not sure about the U.K. I don't recommend even trying to use the stuff, because it reacts let's say "violently" with "certain" materials (including water). However ... You can strip the color from the anodized layer by reapplying an electrolytic bath at a higher voltage than the original finish. This will actually bleach the color out while creating an even deeper barrier layer & more pores. I have done this when my colors didn't come out right or the connection to the part was lost before it was done. It works best if the electrolyte bath is slightly heated. As for another way to strip off the coating... Technically one should be able to separate the oxygen from the aluminum again by some combination of reverse electrolysis and ion exchange from a metal salt electrolyte. Matrix Azaleph - Victoria, B.C. Canada

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 / MIL-PRF-8625 [on DLA], 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.

Ed. note: Readers can search the site for additional threads about "phosphoric chromic anodize strip". William is pointing out that this method dissolves the anodizing without dissolving any raw aluminum whereas it's a matter of timing for oven cleaners and other caustic materials because they dissolve raw aluminum as well as anodizing.

A. Stripping anodized aluminum is fast and easy. You need sodium hydroxide to do it. It is available commercially at chemical supply places. You can use Crystal Drano ⇦ this on eBay or Amazon [affil links] (29% sodium hydroxide). Use your hot water and about 1/2 cup to a gallon. Let part sit for a few minutes, part will start to bubble. That is when the anodized coating is off and now you're going into the aluminum. Use rubber gloves ⇦ this on eBay or Amazon [affil links] and well ventilated area! Wash part immediately in cold water.

Now to the restoration part. You can sand and polish at will, that takes a good buffer, compound and know how. Which I will not go into now. I restore parts professionally, including the re-anodized process. I recommend anodizing the part. I put the hard protective coating back on the part for years of trouble free cleaning.

A. Fast and easy, maybe. Dangerous, though. Just a reminder that every year trained professionals are seriously injured when something goes wrong making up solutions of sodium hydroxide. It has an enormous "heat of solution" / "heat of dilution". This means when sodium hydroxide is mixed with water, it heats up the water. Not a big problem if a small amount is well mixed into a large amount of cold water. But if a small amount of water mixes with a large amount of sodium hydroxide, that small amount of water can be instantaneously turned into steam and erupt the contents of the tank all over the operator.

So be careful, and be sure to wear at least goggles ⇦ this on eBay or Amazon [affil links] , although a full face shield ⇦ this on eBay or Amazon [affil links] and apron ⇦ this on eBay or Amazon [affil links] is actually the proper gear. Good luck.

A. In my time using NaOH (Which is very extensive!) I know for a fact about the heat which can be created when adding to water, but I have yet to ever see it steam off. Many of the strengths I worked with were 50% or higher w/w. Not to make light of the dangers, but I don't believe in driving fear when caution should be used. Mill Johnson - Jefferson Heights, Tennessee September 13, 2008 I agree that crying wolf is a very bad idea, Mill. I used to teach "Safety in the Plating Shop" for the American Electroplaters & Surface Finishers Society but I gave it up when we had to start issuing dire warnings for stuff like 10 percent sulfuric acid such that it became impossible to get people to appreciate how truly dangerous concentrated hydrofluoric acid and cyanide are :-) Still, your experience will cover only a limited range of situations. I don't just think that NaOH can erupt, I know for a fact that a number of people have been killed by very hot caustic soda (black oxide process), having been recruited as an expert witness for a case myself; and I know people who have been doused with it (fortunately, smaller eruptions while wearing apron ⇦ this on eBay or Amazon [affil links] and face shield ⇦ this on eBay or Amazon [affil links] , so the injuries were minor). The 1st Rule of Safety is "never work alone", which hobbyists ignore constantly, so the other rules become more important. A splash of caustic in the eyes when working alone could easily cause permanent blindness. Regards, Ted Mooney, P.E. Striving to live Aloha finishing.com - Pine Beach, New Jersey

A. Just for general information, most oven cleaner ⇦ this on eBay or Amazon [affil links] is simply jelled sodium hydroxide dispensed as a foam from the can, thus the dire warnings written on the cans. Immersion in a sodium hydroxide solution then is not necessary. I have worked with both; oven cleaner is usually easier to work with since it can be most wiped off before rinsing.

Hi, Joe. Bad news. Although a lot of people don't know it, caustic (lye) is incompatible with aluminum. Caustic will dissolve anodizing, but rather quickly dissolve bare aluminum. The cause of the black smut is that you have dissolved some aluminum while leaving the copper and silicon alloying materials behind.

A simple acid like muriatic will not remove the black. You will need nasty stuff like a mix of nitric & hydrofluoric acid. Even still the cylinder bore may be oversize because the anodizing is gone, and if so will be too soft to serve as a wear surface.

Even if it is re-anodized it may remain oversize. I hope you can find an anodizing shop to attempt repair, or that a replacement is cheap. Sorry.

Regards,

A. Ok, I've been looking for something to remove anodizing from some fittings that my engine builder installed in blue and red; these are small and not easily sanded. After researching this on the web I came up with Greased Lightning ⇦ this on eBay or Amazon [affil links] and Oven Cleaner as a solution. So I tried the oven cleaner and after about 20 minutes I only saw small splotches of the anodizing being affected not a good sign. So I have used a cleaner called Super Clean [affil links] that was at first produced by Castrol and the sold to a independent co. I poured enough into a stainless bowl to cover the fitting straight from the bottle and within a 30 seconds the part started bubbling like the aftermath of a alka seltzer, Within 15 minutes and light scrubbing with a nylon brush 99.9% of the anodizing was removed
This stuff is strong it needs to be kept in the original plastic bottle or a glass bottle because it will eat some plastics and leak. Use Caution Gloves And Glasses, Wash with Water

A. Hi, Sekhar. Why do you feel you must formulate this chemical? You can simply buy chromic acid like you buy your glassware and other laboratory equipment. Chrome platers buy whole drums of it. Maybe you are misunderstanding that chromic acid is sold as a dry powder/crystal? Or maybe I am not understanding something about your situation? Thanks!

Regards,

A. Hi Dr. Holavanahalli

I don't know how accurately you need to know but ...

• If you only need a general thickness number, eddy current thickness testers will tell you how thick the anodizing is.
• A second general check is the "720 rule" for anodizing, where you utilize Faraday's Law to estimate anodizing thickness.

Until the accepted standards are revised to allow a different stripping method, your results by stripping with a different method would not be considered valid by anyone else anyway.

Luck & Regards,


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