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"Using Zinc Plated Nuts on Galvanized Bolts"

May 6, 2010

I would like to use zinc-plated nuts on a galvanized steel bolt in an outdoor application. The usual reason to avoid this, as far as I can tell, is that the bolt threads (3/8 - 16) are larger after galvanizing and therefore require a galvanized nut that is sized to fit. But that is my problem...the bolt has been manufactured so that is final (post galvanization) thread dimensions are the standard thread dimensions. Therefore, a galvanized A194 2H hex nut is too loose and a zinc plated A194 2H hex nut fits perfectly.

So my question is: Do you see any compatibility issues? This is the best compromise I have found...do you see better options?

Don Schaefer
Product Designer - Blue Springs, Missouri, United States

May 11, 2010

The main difference between zinc plated and galvanized is the thickness of the zinc. And the thickness determines the life expectancy of the protective coating. (directly proportional, double thickness, double life).

Zinc plated nuts are likely to have about 8-10 microns zinc thickness. (0.008 mm - 0.01 mm) and galvanized bolts perhaps 50 microns.
There aren't really compatibility issues where some reaction might take place, only a deficiency of protection on the nut compared to the bolt.
If you live in a higher corrosion area, or your application might suffer higher corrosion rates, you could do something simple like painting the nuts to give extension to the life expectancy.

Geoff Crowley
Crithwood Ltd.
Westfield, Scotland, UK
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