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Anodize remover
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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 toxcicity.
Chris Francis- Hampton VA
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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
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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 (I note this because no company was listed with your name) 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 and)
Matthew Stiltner- Toledo, Ohio
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Wont the caustic soda mixture damage the aluminum...that is destroy the machined finish and possibly pit it.
chris francis- HAMPTON VA
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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
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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 45sec-1min and it works fairly well 99% of the time.
The surface comes out with a semi-matted 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 its got to be used in excess of 150 Deg F, so its 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 very high.
Matthew Stiltner- Toledo, Ohio
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I'm looking for a method to measure a thin layer Aluminum anodic coating (2-3 g/sqm), avoiding the acid dichromate solutions. Have you got any suggestion for a destructive or non destructive method?
Francesco Cicchetti
- Sulmona, Italy
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My first response would be an Eddy Current Meter, better known as a thickness gauge. These handy little, in most cases portable items will allow for nondestructive thickness measurements on an aluminum anodized substrate, as a matter of fact, they do work with steel too, the one I have (Elcometer 300 series) has a port of both types of probes.
Other than this, a benchtop x-ray system (kinda expensive, upwards of 10K+ in most cases). If you're looking for a way to measure the coating weight, I know of no other way than to weight beforehand, strip off the coating, weigh afterwards to get a semi-accurate reading. Maybe someone out there has an item of this type, anyone...anyone?
Matthew Stiltner- Toledo, Ohio
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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 custic soda at 110 degrees for 40 seconds to a minute, rinse parts.
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Raymond Hendrix H&H Equipment  Troy, Tennessee |
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I believe we should take a step back here and look a little closer at this question. I don't know Chris's 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 relativley inert, chemically-resistant oxide. This means that removing it when necessary is not an easy task, and is almost impossible without handling something hazardous.
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Phil Johnson |
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CHRIS,
I BOUGHT SOME ANODIZE REMOVER AT A CAR SHOW A COUPLE OF WEEKS AGO, IT IS CALLED SIMPLY ANODIZE REMOVER. THIS PRODUCT WORKS VERY WELL BUT PRECAUTIONS NEED TO BE TAKEN AS WITH ANY CHEMICAL. I USED IT ON ALUMINUM TRIM ON A 1960 CHEVY AND WAS AMAZED AT THE RESULTS. YOU SHOULD BE ABLE TO FIND IT AT ANY CAR SHOW WHERE VENDORS ARE PRESENT.
Brian Allison- Shelby, North Carolina
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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 spec at TechStreet], 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 reccomend 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, AZ, USA
October 1, 2008
I Tried stripping a clear anodize with KOH (I didn't heat the water though?) It seemed to strip it ok 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.
- Rochester, New York
October 27, 2008
Hi, Jeff. It would go a long way if you explained your situation and experience because we don't know the alloy, and whether it's a casting or extrusion or whatever, and we don't know your previous experience. It would be good if we had some idea whether this one particular part was a fluke that you encountered after previously stripping hundreds of parts, or whether you have no idea what to expect.
But a principle here is that KOH will dissolve aluminum but it will not dissolve the copper, silicon, and other alloying materials that are usually present to greatly varying degrees in aluminum parts. They remain 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.
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. Good luck!
Regards,
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 Ted Mooney, P.E. finishing.com Brick, New Jersey |