Sound technical content, curated with aloha by
Ted Mooney, P.E. RET
Pine Beach, NJ
The authoritative public forum
for Metal Finishing since 1989
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Removing anodization without spoiling the aluminum
Q. I want to strip the Anodized layer from some Aluminum parts without Spoiling the Aluminum layer. I may try 32 g/l NaOH @ 50 °C for 5 minutes. Any better ideas? I look forward to hearing from you soon.
Chris Mantle- Kendal, Cumbria, UK
2003
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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  2003 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 2003 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 2003 |
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.
William Beeson- Phoenix, Arizona, USA
2003
Q. I have a All Aluminum RC car and I'm getting ready for the big car show in a few months. The purple is old and whack and I want a mirror finish. I have a spare chassis I tried this on but it took a while. I took 4 sandpapers and oven cleaner ⇦this on eBay or Amazon [affil links] , then Mothers Mag & Aluminum Polish ⇦this on eBay or Amazon [affil links] and did got a fine mirror look in my chassis? is there any easier way of taking the color out?
Michael Fox- New Rochelle, New York, United States
2005
Q. SYNTHESIS OF CHROMIC ACID FOR ETCHING ALUMINUM OXIDE? Hi,
In many journals I have seen that aluminum oxide is removed by a mixture of phosphoric and chromic acid.
But there are many methods for producing chromic acid. Which method can produce chromic acid that is used for etching aluminum oxide?
Please help me.
Sekhar.
student - Ruston, Louisiana
July 11, 2008
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. Is there something I am not understanding about your situation? Thanks.
Regards,
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
July 2008
Etching problem while stripping anodize from high purity aluminum
Q. Hi,
I am anodizing and then stripping the anodize from high purity (99.99%) aluminum foil. I am anodizing in either oxalic acid or sulfuric acid baths and am stripping the anodize in a heated phosphoric acid/chromic acid strip at 60 °C. The problem is that, in the strip bath, if the high purity Al foil is touching titanium or even (6061) alloy aluminum, the aluminum foil etches during anodize stripping. It seems to be some sort of galvanic reaction. It is causing us to make custom fixtures using high purity aluminum or re-rack onto plastic racks before going into the strip. Any suggestions on how to prevent this etch reaction so we can use 6061 aluminum fixturing?
Thank you,
Ken
NanoMotif, LLC - St. Paul, Minnesota USA
October 24, 2018
RoHS compliant Stripping solution for Aluminium anodising?
Q. We are into anodising of aluminium business. We are keen to remove hexavalent chrome in all states.
We need a solution to strip anodising aluminium oxide without removing base aluminium. This is we are looking for substitute for mixture of Phosphoric acid and chromic acid.
If anyone has developed either Generic or Proprietary solution please let us know.
Please note again it should not have any Hazardous substance including hexavalent chrome.
This is to find out thickness of aluminium oxide coating on anodic surface
metal finishers - Bangalore. India
November 13, 2018
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