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Sharp corners/edges and anodizing

Current question and answers:

Sharp edges not anodising

November 24, 2020

Q. Good evening all,

Where I work we have started see a problem that we have not had before.

We are noticing that some sharp internal edges are either not anodising correctly or not dying up correctly.

We have noticed it on various parts from multiple customers.

Our sulphuric acid percentage is currently sitting at 14% with our aluminium content in the bath is 15 g/l.

Our degreasing bath and etching bath have recently been checked and seem to be Ok.

Any help with this would be much appreciated.


leon chambers
- london england
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November 25, 2020

A. When you anodize a flat sharp edge like a knife blade or the tooth of a machine thread, you have to recognize that the coating goes 1/2 under the original surface of the sharp edge, FROM BOTH SIDES. The thinnest of the edge is completely converted to solid aluminum oxide with pores varying in location from having started on both sides of the sharp edge, then that brittle solid oxide breaks off leaving a bare spot.

robert probert

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

Previous closely related Q&A's starting in:


Q. I was assigned the task of optimizing the design of a 12" diameter vacuum chuck used in the semiconductor industry. The chuck has vacuum grooves on the top surface and it is to be hard anodized and lapped very flat.

One problem I need to solve is how to finish the vacuum groove corners. If they require a radius, what is the minimum radius. We have experienced flaking of hard anodize particulates from some plated edges. We must end up with a mirror finish surface from which no particulates can emerge.

Any suggestions on this matter will be greatly appreciated even if they come from a competitor.

Dan Jacob
- Fremont, California
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A. Sorry, I'm not familiar with the radius requirements for hard anodizing, although sharp corners pose problems in plating, powder coating, and other metal finishing processes as well.
You've done this before, so you don't need the warning, but readers should remember the dimensional changes when anodizing: for each thousandth of anodizing you specify, the dimensions of the surface only grow by about a half a thousandth because of the aluminum consumed in forming the aluminum oxide coating.

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

Ed. update Jan 2005: On thread 27462 Ken Vlach answers this radius question, quoting Mil-A-8625 [link is to free spec spec at Defense Logistics Agency, dla.mil] and The Surface Finishing and Treatment of Aluminum and its Alloys [affil. link to book on Amazon], ... on AbeBooks].

To minimize search efforts and to offer multiple viewpoints, we combined previously separate threads onto this page. Please forgive any resultant repetition, failures of chronological order, or what may look like readers disrespecting previous responses -- those other responses may not have been on the page at the time :-)

Hard anodising electrical insulation at sharp edge


Q. I am relying on hard anodising on an intricately machined 6061 aluminium part for electrical insulation purpose. The insulation always failed at the 90 degree edge. Max radii I can put in is 0.2 mm. Microsection always review a 'gap' at the anodised coating at the edge. Is there any way to solve this problem ? I had tried all coating parameters and thickness (up to 100 micron) and the phenomenon still exist.

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A. You are running into one of the drawbacks of anodizing known as edge, or corner defect. Anodizing "grows" perpendicular to the surface being coated, so your coating will always inferior at sharp corners. I believe (check Mil 8625) that a minimum radius on a 2 mil hardcoat should be at least .0625" for a decent coating on the corner. To my knowledge there is no solution to your problem, other than to increase your radii

Marc Green
Marc Green
anodizer - Boise, Idaho


A. The aluminum oxide builds from the surface outward therefore it will follow the configuration of the surface . If there is a sharp edge there will be a gap that will grow with the coating. The only way you could help this problem is to put as large a radius as possible on the part.

drew nosti
Drew Nosti, CEF
Anodize USA
supporting advertiser
Ladson, South Carolina

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