responds: There are several methods for coating steel with
zinc. The most commonly used for nails, bolts, and screws are
hot-dip galvanizing, mechanical galvanizing, and
electroplating. The coatings resulting from the different
methods have different characteristics and provide varying
levels of corrosion protection (see chart below).
Hot-dip galvanizing is the process of dipping steel
into a bath of molten zinc, creating a series of
metallurgically bonded zinc-iron alloy layers. The fasteners
are typically dipped in baskets, then placed in a spinner, or
centrifuge, to remove the excess zinc from the threads. The
minimum coating thickness for hot-dip galvanized fasteners
ranges from 2.0 to 6.0 mils.
The term "double-dipped" is somewhat misleading. It does not
mean that the zinc coating on the fastener is twice as thick
(and will therefore last twice as long). However, dipping a
fastener a second time does improve the overall quality of the
outer zinc layer, while adding a small amount of thickness.
Electroplating is the process of applying zinc to the
steel surface by electrodeposition. With electroplating, the
zinc does not metallurgically bond to the steel surface. Also,
the coating thickness is significantly less than with hot-dip
galvanizing, typically between .14 and .28 mils.
With mechanical galvanizing, the fasteners are placed
in a tumbler with zinc powder and other chemical catalysts.
Again, there is no metallurgical bond created, and the minimum
coating thickness ranges from .2 to .24 mils.
As a general rule, the thicker the zinc coating, the longer
the corrosion protection afforded. As the graph shows, hot-dip
galvanized fasteners will last longer than either mechanically
galvanized or electroplated products.
In addition to providing longer corrosion protection,
hot-dip galvanized coatings have a more uniform and slightly
thicker zinc layer on edges and corners. Hot-dip galvanized
coatings are also tougher and more durable than other zinc
In essence, not all zinc coatings are the same. Make sure
the coating you specify is suitable for the environment in
which the steel will be placed. Replacing an electroplated
fastener just a few years after installation may mean spending
more money than using a better quality product in the first
is the marketing services coordinator for the American
Galvanizers Association, in Aurora, Colo.