The hot dip galvanizing process applies a
continuous metallic zinc coating to steel
rebar by immersing the bars in a bath of
molten zinc at about 450°C. A metallurgical
reaction takes place between the steel and
the zinc producing a coating made up of a
series of iron-zinc alloy layers which form
at the steel/zinc interface with an outer
layer of pure zinc (the eta phase) adhering
to the outer surface as the rebar is removed
from the zinc bath. The figure below is a photomicrograph
of a galvanized steel coating cross-section
and shows a typical coating microstructure
consisting of three alloy layers and a layer
of pure metallic zinc.
View
an animation of the zinc patina growth
(flash required)
The unique structure of a hot dip
galvanized coating offers many important
advantages over other coatings. The
galvanized coating is metallurgically bonded
to the underlying steel, producing a coating
that has an order of magnitude greater
adhesion than, for example, fusion bonded
epoxy coatings. In addition, the iron-zinc
alloy layers of the coating are actually
harder than the underlying steel which,
combined with a soft outer layer of pure
zinc, produces an extremely tough and
abrasion resistant coating.
Galvanized rebar can generally be treated
in the same manner as uncoated rebar and
does not require special handling
precautions to protect the coating during
handling, transport to, and placement at,
the job site. Further, the bond strength of
galvanized rebar to concrete is no less than
that of uncoated bar, and in many cases is
somewhat better. This allows the same
reinforced concrete design specifications
(bar size, lap lengths, etc.) to be used for
galvanized rebar as unprotected rebar.
Abrasion
Resistance
