Galvanized Rebar
Using zinc materials is one way
to protect rebar.
By Dr. Tom Langill
Dr. Langill is the technical
director for the American Galvanizers Association
based in Englewood, Colo.
Zinc metal has a number of
characteristics that make it well suited for
use as a coating for protecting iron and steel
products from corrosion. The excellent field
performance of zinc coatings is due to its ability
to form dense, adherent corrosion product films,
and the subsequent rate of corrosion is considerably
below that of ferrous materials - some 10 to
100 times slower, depending on the atmosphere.
While a fresh zinc metal surface
is quite reactive, the zinc metal forms a thin
film of corrosion products when exposed to the
atmosphere. The film of corrosion products transforms
into a dense, transparent barrier layer that
prevents strong attack on the zinc metal, is
not water-soluble and erodes slowly over time.
In addition to creating a
barrier between the steel and the environment,
zinc also can protect the steel galvanically.
If the coating is damaged, zinc, which is anodic
to steel and iron, will preferentially corrode
and sacrificially protect the iron against rusting.
Figure 1 shows the position of zinc and steel
in the Galvanic Series. Anodic metals will sacrifice
themselves to protect cathodic materials from
corroding.
Steel that is embedded into
concrete can also be protected by a zinc coating.
The zinc surface is more resistant to chloride
ions than bare steel by a factor of 5 to 10.
The corrosion rate of zinc, when the system
goes into the active corrosion mode, is significantly
lower than bare steel. The corrosion products
of the zinc coating will migrate away from the
coated bar and will not build up around the
bar, expand and possibly cause concrete cracking
as steel corrosion products do. The zinc corrosion
products migrate into the concrete matrix.
Several zinc-coating processes
produce different coating properties. The only
coating that forms a metallurgical bond with
the steel comes from the hot-dip galvanizing
process. This is the process that is used to
coat steel reinforcing bars.
Hot-dip galvanizing
process and coating metallurgy
Unlike paint coatings
that form an adhesive bond with the underlying
steel, galvanized coatings develop a metallurgical
bond through the formation of a series of iron-zinc
alloy layers. However, in order for these intermetallic
layers to form properly, the steel must be prepared
and processed in a specific sequence. The galvanizing
process consists of three basic steps: surface
preparation, fluxing and galvanizing. Each of
these steps is important in obtaining high quality
hot-dip galvanized coatings.
Surface preparation.
It is essential
that the material surface is clean and uncontaminated
if a uniform, adherent coating is to result.
Surface preparation is usually performed in
sequence by caustic (alkaline) cleaning, water
rinsing, acid pickling and water rinsing.
The caustic cleaner is used
to clean the material of organic contaminants
such as dirt, paint markings, grease and oil,
which are not readily removed by acid pickling.
Scale and rust are normally removed by pickling
in hot sulfuric acid at 65 degrees Celsius (149
F) or hydrochloric acid at room temperature.
Water rinsing usually follows caustic cleaning
and acid pickling.
Fluxing.
The final cleaning of
the steel is performed by a flux. The method
of applying the flux to the steel depends upon
whether the "wet" or "dry"
galvanizing process is used. Dry galvanizing
requires that the steel be dipped in an aqueous
zinc ammonium chloride solution and then thoroughly
dried. This "preflux" prevents oxides
from forming on the material surface prior to
galvanizing. Wet galvanizing uses a molten flux
layer that is floated on top of the bath metal.
The final cleaning occurs as the material passes
through this flux layer before entering the
galvanizing bath.
Galvanizing. The
material to be coated is immersed in a bath
of molten zinc maintained at a temperature of
435 to 460 C (815 to 830 F). Typical bath chemistry
used in hot-dip galvanizing contains a minimum
of 98 percent zinc along with a variety of trace
elements or alloy additions. These additions,
which could include lead (up to 1.2 percent),
aluminum (up to 0.005 percent), tin (about 0.05
percent), nickel (up to 0.1 percent) and bismuth
(about 0.1 percent), can be mixed into the zinc
to enhance the appearance of the final product
or to improve the drainage of the molten zinc
as the material is withdrawn from the zinc bath.
The time of immersion in the galvanizing bath
varies, depending on the thickness and the chemical
composition of the steel being coated.
Coating structure
and appearance
The surface appearance
and the coating thickness of the galvanized
coating can be affected by a number of variables,
which include steel chemistry, immersion time,
bath temperature, steel surface roughness, rate
of withdrawal from the galvanizing bath and
control of the cooling rate by water quenching
or air cooling. However, of all these factors,
steel chemistry has the greatest influence on
the coating structure and appearance.
The two major alloying elements
in the steel that have a significant effect
on the galvanizing process are silicon and phosphorus.
Both of the elements act as catalysts during
the galvanizing process and result in rapid
growth of the iron-zinc alloy layers of the
coating.
Galvanized coating
structure
During the galvanizing
process, a series of alloy layers form as a
result of the metallurgical reaction between
the molten zinc and the steel. Figure 2 shows
the cross section of a coating developed on
steel with a low silicon content (less than
0.03 percent). The coating consists of a very
thin gamma layer next to the steel substrate,
a blocky delta layer, and a columnar growth
of zeta crystals. The various alloy layers contain
different amounts of iron, with the highest
iron content in the layers closest to the steel.
The iron-zinc intermetallic layers are covered
by a layer of pure zinc (zeta) that is formed
when the product is withdrawn from the molten
zinc bath. This outer layer gives the galvanized
product its distinctive shine and spangled appearance.
Not all galvanized coatings
contain all of the layers shown in the above
figure. Depending on the steel chemistry and
the processing conditions, the coating may contain
only one or two of the layers.
Although galvanized coatings
may have a variety of microstructures, essentially
no change occurs in the corrosion resistance
of the coating. Corrosion protection is a function
of coating thickness, not coating structure.
The service life of bright, shiny coatings is
similar to those with a matte gray appearance.
Forming and fabricating
galvanized products
Welding, cutting and
drilling of the steel should be done prior to
galvanizing to minimize the exposure of unprotected
edges and to take advantage of the protection
afforded by the zinc coating. However, there
are situations in which the galvanized products
need to be assembled or fabricated in the field.
For these situations, the contractor should
be aware of the properties and the limitations
of the coating.
Bending. Reinforcing
products that have been hot-dip galvanized in
intermittent batches after fabrication exhibit
different bending characteristics than continuously
fed sheet galvanized products. This is due primarily
to the coating thickness and the coating structure
that is developed on each of the products. The
coating thickness on sheet galvanized products
is much thinner than that on batch galvanized
products. Most galvanized sheet products have
either an entirely pure zinc coating or a coating
that is totally alloyed (galvannealed). Both
types of coating have excellent bending properties.
The pure zinc coating stretches during forming
operations, while galvannealed coatings develop
small cracks to relieve the bending stresses.
The coating structure on batch galvanized products,
such as rebar, is typically a combination of
coating structures, as shown in Figure 2. During
bending, the outer pure zinc layer tries to
stretch, while the alloy layers attempt to relieve
stresses by cracking. Flaking of the coating
can occur if the bending is too severe. Avoid
bending products that have an excessively thick
coating (greater than 10 mils).
Abrasion and impact
resistance of galvanized coatings. The
zeta and delta alloy layers are actually harder
than many base steels. These alloy layers offer
excellent abrasion resistance during heavy loading
and severe service conditions. The softer zeta
layer has good impact resistance.
Corner and edge
protection. Since
galvanizing is a total immersion process, all
areas of the product are coated, including those
that are hidden or hard to reach. Galvanized
coatings on edges and corners are at least as
thick, and sometimes thicker, as on other parts
of the product. Due to the alloy layer formation,
the coating does not thin out on edges and corners
as do paint or spray applied coatings. These
areas are where protection is typically needed
most.
Welding galvanized
steel. Welding
can be accomplished by either grinding away
the zinc coating and directly welding the base
metal, or by welding through the galvanized
coating. Materials that have been galvanized
may be welded easily by all common welding techniques.
In general, anything that can be welded before
galvanizing can be welded after galvanizing,
but some minor changes to the welding technique
need to be incorporated to ensure full weld
penetration. These changes are primarily intended
to allow the galvanized coating to burn off
at the front of the weld pool.
For normal flat welds on galvanized
steel, the welding current can remain the same
as on bare steel, but for fillet welds, the
current may need to be increased about 10 amps.
Butt welds may require a slightly wider gap
since the penetration of the weld for galvanized
steel is less than for uncoated steel. Travel
speeds on the root pass should be reduced by
10 percent to 20 percent, and the electrode
drag angle should be increased. All these items
are intended to increase the weld penetration
and to stabilize the arc that can be disturbed
by the evolving zinc vapor.
When galvanized steel is welded,
fumes of zinc oxide are produced. If inhaled
in sufficient quantity, the fumes can result
in "metal fume fever" or "zinc
chills." In severe cases, vomiting can
occur. These flu-like symptoms are of short
duration and typically pass within a 24-hour
period. Adequate ventilation or fume extraction
should be used and the welder's head should
never be in the plume. If adequate ventilation
is not possible, the welder should be fitted
with a respirator.
Repairing galvanized
coatings. Galvanized
coatings that have been damaged or welded can
be repaired using one of the following three
methods proscribed in ASTM A 780: Zinc-Based
Solder, Zinc-Rich Paint or Zinc Metallizing.
Zinc-Based Solder - The zinc-tin-copper
solder is applied in stick form after the surface
has been prepared by wire brushing. The surface
to be repaired must be free of grease and scale.
A paste or liquid flux is applied as the surface
is heated with a torch to a temperature of 200
to 300 C (362 to 572 F). The molten solder is
spread with a knife or spatula then wiped with
a wet cloth to remove flux residue. Thickness
measurements are taken to ensure that the required
coating is applied.
Zinc-Rich Paint - Zinc-rich
paint containing a minimum 65 percent zinc dry
film thickness can be used for repair. The paint
is applied by brushing or spraying over a surface
that has been prepared to a "near-white"
finish. Thickness measurements are taken to
ensure that the required coating is applied.
Zinc Metallizing - Sprayed
zinc (metallizing) should be applied to a surface
that has been cleaned to a "white metal"
finish. Zinc wire or zinc powder can be used
to feed the metal-spraying guns. The sprayed
coating should be applied as soon as possible
after surface preparation and before visible
deterioration of the surface has occurred. Thickness
measurements are needed to ensure the required
coating has been applied.
Codes of practice
and standards
The regulation of the
hot-dip galvanizing of steel reinforcing bars
is handled in the United States through specifications
developed by the American Society for Testing
and Materials (ASTM). The specific document
for hot-dip galvanizing of reinforcing bars
is Specification A 767/A 767M, "Standard
Specification for Zinc-Coated (Galvanized) Steel
Bars for Concrete Reinforcement." Assemblies
of reinforcing bars are hot-dip galvanized per
Specification A 123/A 123M, "Standard Specifications
for Zinc (Hot-Dipped Galvanized) Coatings on
Iron and Steel Products."
Work is underway through the
International Standards Organization (ISO) to
incorporate a general set of zinc coating requirements
into one standard for galvanizing steel reinforcing
bars. The standard has been drafted and is currently
in a review cycle. The ISO document is numbered
ISO/CD 14657.
Zinc performance in
concrete
Zinc coated steel has
been used in high corrosion areas for more than
50 years in Bermuda. Examination of bars and
concrete from one bridge in Bermuda during 1995
revealed that the zinc coating was still intact
and providing corrosion protection to the reinforcing
bar. Examinations of other bridges in Pennsylvania,
Vermont, Iowa and Florida have also shown that
the hot-dip galvanized coating is protecting
the steel reinforcing bar. This excellent field
performance is in contrast to the poor performance
of hot-dip galvanized steel reinforcing bars
in accelerated laboratory tests. The laboratory
tests attempt to accelerate the corrosion of
steel reinforcing bars by immersing the bars
in salt solutions and measuring the corrosion
rate. In actual rebar installations, there are
periods when the bar is exposed to a salt solution
but they are followed by a drying period when
the corrosion products stabilize and form a
barrier to further corrosion. This process cannot
occur in the accelerated testing. So, in spite
of the poor performance in accelerated tests,
hot-dip galvanized reinforcing bars perform
very well in real world applications.
