Galvanizing in Concrete
How Zinc Protects Rebar in
Concrete:
Higher Chloride Corrosion Initiation
Threshold
The corrosion protection
afforded by galvanized rebar in concrete is
due to a combination of beneficial effects.
Of primary importance is the substantially
higher chloride threshold (2-4 times) for
zinc coatings to start corroding compared to
uncoated steel. In addition, zinc has a much
greater pH passivation range than steel,
making galvanized rebar resistant to the pH
lowering effects of carbonation as the
concrete ages. Even when the zinc coating
does start to corrode, its corrosion rate is
considerably less than that of uncoated
steel.
Why Galvanized
Rebar Maintains Concrete's Integrity: Corrosion
Product Migration and Concrete Matrix Densification
Zinc’s corrosion products
are loose, powdery minerals that are less
voluminous than iron corrosion products and
are able to migrate away from the galvanized
rebar surface into the adjacent concrete
matrix. As a result, corrosion of the zinc
coating causes very little physical
disruption to the surrounding
concrete. The elemental map (left) is
evidence of this migration. The white
spots in the concrete indicate zinc oxide
which has migrated away from the galvanized
rebar/concrete interface.
There is also evidence to
suggest that the diffusion of zinc’s
corrosion products helps fill pore spaces at
the concrete/rebar interface, making this
area less permeable and helps to reduce the
transport of aggressive species such as
chlorides through this interface zone to the
zinc coating. The reactions between
zinc and concrete, and the resulting
corrosion product diffusion, also explains
why galvanized rebar has such good bond
strength with concrete.
Tolerance of Lower pH due to Carbonation
Zinc remains passive at
significantly lower pH levels than for black
steel (9.5 versus 11.5) making galvanized
rebar far less susceptible to corrosion due
to carbonation of the concrete.
Zinc's Initial Reaction in
Fresh Concrete
Zinc reacts with wet
concrete to form calcium hydroxyzincate
accompanied by the evolution of hydrogen.
This corrosion product is insoluble and
protective of the underlying zinc (provided
that the surrounding concrete mixture is
below a pH of about 13.3). Research has
shown that during this initial
reaction period until coating passivation
and concrete hardening occurs, some of the
pure zinc layer of the coating is dissolved.
However, this initial reaction ceases once
the concrete hardens and the hydroxyzincate
coating has formed. Studies of galvanized
rebar recovered from field structures
indicate
that the coating remains in this passive
state for extended periods of time, even
when exposed to high chloride levels in the
surrounding concrete.
For concretes of high pH, or
where some background chlorides are
expected, the zinc surface can be passivated,
using a range of proprietary post
treatments, as a safeguard against excessive
hydrogen
evolution that may, in serious cases, reduce
the pullout strength of the bar. For normal
concrete conditions, research has shown no
statistical difference in bond strength
between galvanized rebar that was passivated
and not passivated.
Corrosion
Model
