What are Mechanical Galvanized Fasteners

A. Hi Donald

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/galvanizing, 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 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,

A. It may be mechanical galvanizing, or possibly thermal diffusion (sort of a successor to Sherardizing) or maybe even a zinc-rich dip-spin coating, Mario. Unfortunately, the person or company who is offering a different coating always knows more about it than the outsider and is always a step ahead. This means one will usually comes off poorly if they try to demonstrate their knowledgable of someone else's coatings.

For that reason I think the best approach in these cases is to test which one is superior in some way. And there is certainly nothing wrong with you deciding what those tests should be to present hot dip centrifuged coatings in the best light.

If you think the other coating is softer, prescribe a scribe test with a hardened knife. If you think your coating has greater salt spray resistance, try that. Better freedom from stick-slip when torquing up, try that. If you think the other coating is not easily strippable, try that. I know you are already very familiar with hot dip galvanizing, but go to some of the trade association websites and read the advantages again and see if you can formulate tests where hot dipping will come out on top. Good luck.