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Ted Mooney, P.E. RET

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Black spots on one side of thread after Anodize and Dichromate Seal

Q. We have an issue with small black sots that are only visible after the anodize process. We machine 6061 and send it out to plate. If this was corrosion wouldn't it show up in different areas? They only appear on one side of the thread. (through hole)

61811-1a 61811-1b 61811-1c

We sent some to be tested but I'm 10 days out from an answer. Looking for help.

Bobby Bourassa
GM - Odessa, Florida
February 25, 2025

Q. Anyone have anything for me on this?

Bobby Bourassa [returning]
GM - Odessa, Florida
March 28, 2025

A. Hi Bobby.

You probably realize that anodizing of threads is strongly discouraged for several reasons, but we assume you must do it to comply with a PO or spec rather than using stainless inserts?

The problem being only on one side of the threads, and only on the threads, suggests corrosion due to incomplete neutralization and/or rinsing.
If it is practical to incorporate a sodium bicarbonate (baking soda [in bulk on eBay or Amazon [affil links] ) dip after anodizing and before sealing, it would help. Otherwise improved rinsing will probably help. Without understanding the mechanics of your anodizer's installation it is hard to be specific, but if it is a manual installation, and low volume, simply manually spray rinsing those threaded holes may fix it.

Luck & Regards,

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

Q. Morning Ted, thank you for getting back to me. Yes our Po requires the anodize in the threads. We received back the testing. No corrosion. Do you think this could possibly be galling from an old tap and that the anodize process happened to "burn" the galled areas on the threads themselves?

Bobby Bourassa
- Odessa
March 31, 2025

A. Hi again.

I have no industrial experience with tapping; all I've ever done is tap or retap occasional threads as a D-I-Y amateur home handyman, so I can offer no experience. But it would surprise me if aluminum could cause any metal to be abraded off a a clean hardened steel tap. But if the tap were either rusty or had been used to tap steel parts, yes, that could be a problem. It probably needs to be an SOP that such taps are not shared for tapping different materials.

Luck & Regards,

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

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