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topic 3694

What are Mechanical Galvanized Fasteners

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Q. I have a customer who is requesting Mechanically Galvanized Fasteners for a job. I am having a problem trying to determine if these are the same as Hot Dipped Fasteners, Zinc Plated Fasteners or something totally different. Anyone know?

DONALD T [last name deleted for privacy by Editor]
-Brewer, Maine

October 11, 2012

Q. Hi

Please tell me the difference between mechanical galvanize and hot dip galvanize.


Ghulam Mujtaba Mirza
- Guang Zhou, China

October 11, 2012

A. Hi Donald; hi Ghulam

Steel fasteners are coated with zinc and zinc alloys for decorative value sometimes, but always to retard rust via sacrificial protection.

There are many different ways to apply the zinc including zinc electroplating, mechanical plating, sherardizing, hot dip galvanizing, zinc-rich dip-spin coatings, etc. The protective value of the zinc depends to some extent on the exact composition of the alloy because plain zinc, zinc-nickel, zinc-iron, and zinc-cobalt have somewhat different electromotive potentials. But more important is that, because the coating is sacrificial, the protection is largely proportional to the thickness of the zinc coating.

While it might be theoretically possible to apply a given thickness coating via any of these technologies, each technology tends to be most suited and most economical at a different thickness. For example, for thin and inexpensive coatings, zinc electroplating can't be matched. For very heavy coatings capable of lasting many decades in severe outdoor exposure, nothing matches hot dip galvanizing. In between, are the other technologies like mechanical plating, sherardizing, and dip-spin coatings.

There are also secondary considerations like zinc electroplating offering the most decorative look but presenting the danger of hydrogen embrittlement. And, of course, fasteners have threads, and threads can become distorted and out of tolerance with thick coatings.

To address the mechanical galvanizing process specifically, it is a process where zinc powders and very tiny glass beads are tumbled with the fasteners in a way that one part falls upon another with the beads and powder between them, with the beads pounding the powder onto the substrate. It is often specified where the required coating thickness is a little too high to be practical for zinc electroplating, or where the risk of hydrogen embrittlement of the fastener is considered unacceptable, and where hot dip galvanizing would foul the threads.

Luck and regards,

Ted Mooney, Teds signature
Ted Mooney, P.E.
Pine Beach, New Jersey
Striving to live "Aloha"

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