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Can plating weaken a screw?

2007

Is there something in the plating process that might have a chance of weakening a part, e.g. zinc plated M12 screws? Somehow they're breaking in the field under vacuum environment.
We use M12 grade 12.9 zn plated socket hd cap to bolt down top and bottom plates of a chamber. Maybe pressure is too much for the screws? What would you recommend?

Jon

Jon Cerezo
Mech Designer - Santa Clara, California, USA

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Absolutely! Yours is a high strength fastener where proper hydrogen embrittlement relief after plating is critical.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2007

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Good afternoon:

Reading this post got me to dusting off some 20 year old files I had pigeonholed away.

When SAE J1199 was first approved in February 1978, there were entries for class 12.8 and 12.9 fasteners, with the 12.9s having a footnote, part of which read, "Caution is advised when considering the use of class 12.9 bolts and screws....some environments may cause stress corrosion cracking of non plated as well as electroplated products."

When J1199 was revised in September 1983, class 12.8 and 12.9 disappeared from the standard. Apparently there were some cases where these fasteners were misused and abused. If you go to the latest revision of J1199, still no mention of 12.8 or 12.9 fasteners. Now probably in this application stress corrosion cracking isn't an issue, but I usually get nervous when these fasteners are specified, because they have been so misused.

If you can get your hands on SAE Technical Paper 820112, "Delayed Fracture of Class 12.8 Bolts in Automotive Rear Suspensions", by Thomas J. Hughel, it might make for some good reading on the history of this subject.

Steve Bizub
- St Louis, Missouri
2007