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Black anodized parts feel like a chalkboard

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"Chromating - Anodizing - Hardcoating"
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Q. Hi--

I'm an ME with 20 years of engineering experience...I just got some black anodized (MIL-A-8625E type 2, class 2) AL 6061-T6 parts from my machinist and the finish is really odd. It feels like a brand new chalkboard. Fingernails leave a mark, pretty much anything leaves a mark. My pix don't properly show the condition, but the chalkboard description is spot on.

In my years of work, I've never seen anything like this except maybe once when some anodized parts weren't sealed, and they felt a little porous. I've had parts bead blasted before anodization and they were not like this at all.

I'm working with the shop on what went wrong, but was hoping to get some expert advice from you friendly folks. Any ideas on what might cause this surface?

Thank you!

Sandheep Surendran
- San Francisco, California
March 5, 2022

A. Lack of proper seal of a smooth anodized surface will feel more like a slight stickiness when touched with bare hands, not roughness

Unnecessarily long etch times, a rework (stripping and reanodizing), excessive pressure while bead blasting, blasting with sharp cornered media, blasting with wrong type of media -- these would be my primary suspects for the rough surface finish, there are others, but these would be the first things I'd look at

Janis Ziemelis
- Riga, Latvia
March 6, 2022

"The Metallurgy of Anodizing Aluminum"
by Runge & Kaufman

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March 6, 2022

thumbs up sign Thanks for the response--and yes, I do remember now that the unsealed parts had a weird sticky feeling.

Q. I'd hate to scrap these parts--do you think that they could be reworked? Maybe stripped, reblasted, and reanodized?


Sandheep Surendran [returning]
- San Francisco, California

A. It is impossible to give you that answer not knowing what dimensional tolerances are there on the part, every inside dimension will become slightly larger and the outside dimensions - smaller, that much is certain. By how much? It will depend on how much they will need to be etched to get rid of the oxide that is on there now, then the blasting will somewhat change the surface again, then etching (for cleaning up embedded blasting FOD) again will remove some material

But if you plan using blasting, I very much recommend finding someone that does stainless steel media blasting (round shot), stay away from anyone that is using coarse sharp edged media, it will chop up the soft aluminum and create the "chalk board" surface.

Glass bead blasting, if done correctly, will most likely work, but it is quite a bit more difficult to get very consistent results with it compared to stainless shot. Glass beads always break down no matter how careful with pressure the operator is, and this silicone dust gets embedded into aluminum and causes all sorts of problems.

Wet blasting might be good, I personally have no experience with it, but corrosion inhibitor for aluminum should be used or parts should be dried quickly (including all blind holes) before packing/shipping, I've had parts delivered to me that have sat for few hours in wet paper towels that then develop spot corrosion on them, tiny dark dots - small pits all over the places where the parts were in contact with the wet paper just because the operator was too lazy to blow out water from blind holes...

Janis Ziemelis
- Riga, Latvia
March 8, 2022

thumbs up sign Thanks again, Janis--this is very helpful!

Sandheep Surendran [returning]
- San Francisco, California
March 8, 2022

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