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Removal and Masking of Anodize Surfaces


Q. I have an old cast aluminum chassis with a black dye anodize finish that I need to refurbish( Mil-A-8625 [link is to free spec spec at Defense Logistics Agency, dla.mil] Type 1, Class 2). What is the best way to remove the original anodizing? It also has many press fit, threaded inserts that I would prefer not to remove when I have it re-anodized. Is there a way to mask off the inserts during the anodizing process?

Thanks for you help.

Daniel Zuczek
- Goodyear, Arizona


A. Sir,

To strip your part of anodizing may be a mistake, when you anodize you very little to the size of part, anodizing, has depth into the surface and will be removed while stripping.

In other words when you strip this part you will loose your size and in turn, the size for the inserts, and if the inserts are steel and you anodize them, they will get heavy etched to dissolve the inserts

But you can strip Black dyed anodizing with a caustic solution IE: cleaner

Chris Snyder
plater - Charlotte, North Carolina


A. Use the stripping solution for best results. This will remove the anodized coating without very much affecting the base metal. I'll try to retrieve my files if you should require the solution recipe. I'm sure it's a mixture of chromic acid and phosphoric acid.

For your masking needs, try inquiring from the sponsors of this site. We are using a rubber-like coating for this purpose.

Dado Macapagal
- Toronto, Ontario


A. Daniel Zuczek -

The best way to remove the old anodize is a combination phosphoric acid/chromic acid strip. The make up is 78.5 grams/liter of chromic acid and 162 ml/liter of phosphoric acid. This is basically a non-etch strip, although you may lose some material due to the fact that the anodize penetrates the base metal. I would not recommend a caustic strip like the gentleman above did. That would be very likely to over-etch your part and is really suitable as a strip only for racks used in processing, not for tight tolerance parts. Your inserts may be masked off using various stop-off lacquers, but I would remove them if at all possible if they are made of a dissimilar metal. The smallest pinhole in the masking material, and they may burn up or be otherwise damaged.

Hope this helps,

Peter Cox
- Newburyport, Massachusetts


A. I don't know if it is ignorance or just plain too cheap but what I have found is that most anodizing shops are quick to convince you that they can carefully strip those delicate parts that need to be stripped using caustic but it is your parts that will be ruined. The ones that do have a selective stripper use it to strip their aluminum racks because they don't want to remove any more base metal than necessary to prolong the life of the rack. You may find one of those shops if you look long enough. They should all have a selective stripper on hand just for those special jobs. There are strippers (sulfuric based with additives)that you mix 50-50 with water and heat to 160 °F. It takes about 10 minutes to strip .002" thick coating. Longer at lower temperature. 40 - 60 minutes at room temperature. The container can be 316 stainless for processing but must store chemical in a plastic container when not in use. A five gallon container cost about $50.00. That is just one of many selective strippers available. There are a few non proprietary recipes out there such as Nitric & Ammonium Bifluoride and some using chromic Acid which you should stay away from simply because it is very nasty and on the EPA's hit list as major hazardous. Find someone that is experienced working with commercial chemicals and have them help you. Greg

Greg Gillespie
- San Antonio, Texas

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