Metal finishing Q&As since 1989

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Mechanically Galvanized Fasteners

We have a customer who has specified using ASTM A193 B7 stud bolts with ASTM A194 Gr 2H nuts. Both of these fasteners are to be mechanically galvanized to ASTM B695-85, class 50 thickness minimum, with type II yellow chromatic treatment.The nuts are tapped oversize to allow tightening by hand.

1.Do these treatments cause any detrimental effects to the fasteners such as hydrogen embrittlement, etc?

2.Do these treatments invalidate the original specifications A-193/194 ?

3.Who might provide these treatments?

Cary White
- Nixa, Missouri
2001

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2001

Hello Cary!

Well, I can answer two of your questions.

1)-- No, the mechanical galvanizing does not cause hydrogen embrittlement; at least not if done properly per ASTM B695. Since the process is NOT electrolytic, and all you are doing is smearing zinc powder onto the surface, you do not charge the substrate with hydrogen. The only concern you have is in the preparation of the bolt or nut for coating, as acids might be used. Yet ASTM B695 addresses that issue, in paragraphs 5.2.2 and 5.2.3, by referring to Practices ASTM B242, ASTM B183, and ASTM B322, all of which mitigate hydrogen embrittlement potential.

2)-- ASTM A193 only refers to the material from which you will make the bolt; coating the material is the province of the part (bolt) specification. And ASTM A194, while not insisting on coating or no coating, notes that zinc coatings should not be used above 390 °F. Sure sounds like it's a user's choice...

Regarding who does the processing, I'd expect that one of the fine shops that support this website have this in their arsenal: but I can't speak for them.

Good luck, and I hope this helps!

Lee Gearhart
metallurgist - E. Aurora, New York