Removing anodizing from aluminum

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.

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

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

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

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
Falls Township, Pennsylvania

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.

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.

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.

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.

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.

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

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.

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.

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.

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