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.
