Summary for black spots on thread Discussion
black spots on one side of thread are discussed as a post-anodize issue on machined 6061 parts sent out for plating. The original poster notes the marks are only visible after anodize, appear on one side of a through-hole thread, and corrosion testing came back negative. They ask whether old-tap galling could have been burned by the anodize process. Ted Mooney replies cautiously that he has no industrial tapping experience, but says it would surprise him if clean hardened steel would be abraded by aluminum; however, if the tap was rusty or had been used on steel parts, that could create a problem. He suggests a shop SOP to avoid sharing taps between different materials. The thread does not confirm a root cause, but points toward tooling contamination or tap condition rather than corrosion.
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)
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
or
Amazon [affil link]
) 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 Mooney, P.E. RET
Striving to live Aloha
finishing.com – Pine Beach, New Jersey
Need quick confidential answers? $25
Need project assistance? $100/hr.
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 Mooney, P.E. RET
Striving to live Aloha
finishing.com – Pine Beach, New Jersey
Need quick confidential answers? $25
Need project assistance? $100/hr.
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