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A356 Tapped Hole "Blowout" During Type II Anodizing

(-----) February 6, 2008
47760 anodizing blow-out
John Kuenzly
Mechanical Engineer - Ojai, California, USA

simultaneous February 8, 2008

M8-1.5 stainless Heli-coils

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A. Didn't the plater tell you "do not anodize aluminum with heli-coils inserted, or who did not listen.
A blow out would not be expected, but severe erosion of the hole would be expected along with the heli-coil being worthless if it was still there.
Bad plan!

James Watts
- Navarre, Florida

February 8, 2008

A. John

The heli-coils should be installed after anodizing. Masking helicoils is challenging, to say the least.

If the part was anodized with the helicoils installed and not masked, I would expect to see most if not all of the helicoil dissolved, and if the process was hard anodize, I would expect to see a significant amount of aluminum burned away- no visible threads, the hole larger near the surface, and little or no anodize present in the surrounding areas. A catastrophic failure.

Willie Alexander
- Colorado Springs CO

February 8, 2008

A. Most definitely this would be the cause of your problem. Only aluminum should be anodized. Steel dowel pins, or helicoils are incompatible with the anodizing process, and won't survive the anodizing process unless they are adequately masked (which can be tricky). Ideally you'll want to install any hardware, that is not aluminum, after the anodizing process.

Marc Green
Marc Green
anodizer - Boise, Idaho

February 9, 2008

A. Whoa! You cannot anodize aluminum that has any other metal attached. Immediately after the aluminum becomes slightly anodized, and therefore slightly non-conductive on the surface, the current goes to the easiest place which is the stainless steel and starts "un-plating it" until it is completely dissolved and all gone.

robert probert
Robert H Probert
Robert H Probert Technical Services
supporting advertiser
Garner, North Carolina

July 20, 2011 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. I'm designing an aluminum part that needs to be teflon anodized, and also has several holes that need to maintain a very tight tolerance for a precise press fit. Based on the tolerances of the pins used for the press fit and the machining tolerances of the aluminum part, the added thickness of the anodizing layer would have to be held to ± .001mm (approximately ± .00004 in).

If the stainless steel pins are pressed into place before anodizing, will the anodizing process have any effect on the pins or will they simply act as plugs?


Jayson Moore
design engineer - Cincinnati, Ohio, USA

June 2011

A. Hi, Jayson.

As you see, we appended your inquiry to a thread that answers it. You can't do that. Robert Probert's reply is the most applicable to your situation.


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

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