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4340 steel part has become etched

Q. Hello,
last month we observed severe etching on a steel part.
I tested bunch of specimens under same parameters, but nothing could replicate the defect. I am curious to know what can cause etching on part?

Any info will be very helpful.

Thanks,

Olivia Benson
- Ontario, Canada
November 16, 2023

A. There's a universe of possibilities, Olivia, and I can't eliminate any yet :-)

This is a machined part? No finishing was involved at all? You simply machined it and then for no apparent reason at all it just started "etching" itself? What do you mean by "severe etching"? Is the surface full of pits?

If it's been exposed to no processing, then defective material or environmental factors caused it. Have you checked the machining fluid in that machine? Have you checked its storage conditions? Have you checked with the machinist as to whether the "etching" was apparent to him/her immediately upon machining? Thanks! Luck & Regards,

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

Q. My Bad. here's the history on the part.

Part is after finishing all the machining parameters. It has been shotpeened as well. We plate the part, but due to some issues in plating, we had to strip the coating as part of the rework. Our stripping solutions are very basic - sodium hydroxide and ammonium nitrate. Part stayed in the strip solution for about 15 hours. No etching defect observed, except for some sludgish buildup. 12 days later we found etching defect in the same locations as the buildup.

My SME state that our stripping solutions are incapable to create chemical etching.

I am trying to understand the root cause of this.

Olivia benson [returning]
- Ontario, Canada
November 20, 2023

A. Hi again.
Okay, stripping of plating on shot-peened high strength aerospace components caused delayed etching and pitting in an area where sludge built up. I can't think of a good answer but at least I clearly understand the question, which means others readers will hopefully have theories to offer :-)
Luck & Regards,

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

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