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1. How does galvanizing protect steel from
corrosion?
Zinc metal used in the
galvanizing process provides an impervious barrier
between the steel substrate and corrosive elements
in the atmosphere. It does not allow moisture and
corrosive chlorides and sulfides to attack the
steel. Zinc is more importantly anodic
to steel - meaning it will corrode before the
steel, until the zinc is entirely consumed.
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2. What are the steps in the galvanizing
process?
There are four steps:
1. Pre-inspection - where the
fabricated structural steel is viewed to ensure it
has, if necessary, the proper venting and draining
holes, bracing, and overall design characteristics
necessary to yield a quality galvanized
coating
2. Cleaning - steel is immersed in
a caustic solution to remove organic material such
as grease and dirt, followed by dipping in an
acid bath (hydrochloric or sulfuric) to remove
mill scale and rust, and finally lowered into
a bath of flux that promotes zinc & steel
reaction and retards further oxidation of
the steel... (steel will not react with zinc
unless it is perfectly clean)
3. Galvanizing - the clean
steel is lowered into a kettle containing 850 F
molten zinc where the steel and zinc
metallurgically react to form three zinc-iron
intermetallic layers and one pure zinc layer
4. Final inspection - the newly
galvanized steel is sight-inspected (if it looks
good, it is), followed up by measurement of
coating thickness with a magnetic thickness
gauge
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3. How does the cost of hot-dip
galvanizing compare to other corrosion protection
systems, such as paints?
When compared
with paint systems, hot-dip galvanizing after
fabrication has comparable initial application
costs and, almost always, lower life-cycle costs.
In fact, the lower life-cycle costs of a hot-dip
galvanized project make galvanizing the smart
choice for today and tomorrow.
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4. How long can I expect my galvanized
steel project to last in
service?
Hot-dip galvanized steel
resists corrosion in numerous environments
extremely well. It is not uncommon for galvanized
steel to last more than 70 years under certain
conditions. To get a good idea of how long your
project will last, see the service-life chart.
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5. Does the galvanized steel coating
of zinc resist abrasion?
The three
intermetallic layers that form during the
galvanizing process are all harder than the
substrate steel and have excellent abrasion
resistance.
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6. What causes wet storage stain and how
can it be prevented?
Zinc on newly
galvanized steel is very reactive and wants to
form zinc oxide and zinc hydroxide corrosion
products that eventually become the stable zinc
carbonate. When galvanized steel is tightly
stacked or stored in wet boxes that don't allow
for free flowing air, the zinc forms excessive
layers of zinc hydroxide, otherwise known as wet
storage stain. Most wet storage stain can be
easily removed with a cleaner or nylon
brush. To prevent wet storage stain, store
galvanized steel indoors or block it so that there
is ample free flowing air between each galvanized
article.
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7. Why do galvanized steel appearances
differ from project to project and galvanizer to
galvanizer and is there any difference in the
corrosion protection offered by the different
appearing coatings?
The steel chemistry is
the primary determinant of galvanized coating
thickness and appearance. Continuously cast
steel produced by the steel companies has a wide
variety of chemistries, thus the different coating
appearances.
There are several different additives that
galvanizers may put in their zinc kettle to
enhance the coating appearance by making it shiny,
spangled or matte gray. The appearance of the
coating (matte gray, shiny, spangled) does nothing
to change the corrosion protection of the zinc
coating.
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8. Can galvanized steel in service
withstand high temperatures for long periods of
time?
Constant exposure to temperatures
below 3900F (2000C) is a
perfectly acceptable environment for hot-dip
galvanized steel. Good performance can also
be obtained when hot-dip galvanized steel is
exposed to temperatures above 3900F
(2000C) on an intermittent basis.
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9. Why would you want to paint over
galvanized steel?
Called duplex coatings,
zinc and paint in combination (synergistic effect)
produce a corrosion protection approximately 2X
the sum of the corrosion protection that each
alone would provide. Additionally, duplex
coatings make for easy repainting, excellent
safety marking systems, and good
color-coding. Painting over galvanized steel
that has been in service for many years also
extends the life of the zinc coating.
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10. What are the specifications governing
hot-dip galvanized steel?
Structural steel
(plate, wide-flange beams, angles, channels, pipe,
tubing) are galvanized to ASTM A 123/A 123M.
Fasteners and small parts that fit into a
centrifuging basket are galvanized to ASTM A 153/A
153M. Reinforcing steel is galvanized to ASTM A
767/A 767M.
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11. Isn't galvanizing more expensive than
paint?
Depending on the product mix, square
feet per ton, and condition of the steel surface,
galvanizing is often less expensive on an
initial cost basis. However, as with
any purchase, the lifetime costs should be
considered when making a project decision on the
corrosion prevention system to utilize. And,
with galvanizing, the life cycle cost, i.e.
the cost per year to maintain, is almost always
less than a paint system. Paint systems
require maintenance, partial repainting and full
repainting several times over a 30-year project
life. The costs can be staggering, making
the decision to paint a costly one in the long
run.
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12. What if the article to be galvanized is
larger than the dimensions of the galvanizer's
kettle? Can it still be
galvanized?
Galvanizers can progressively
dip such a fabrication or article of steel.
They dip one half in the molten zinc bath, remove
it, turn it around or over and immerse the other
half in the zinc. This method is often
erroneously referred to as 'double
dipping'.
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13. What is the difference between hot-dip
galvanized fasteners and zinc-plated
fasteners?
Hot-dip fasteners generally have
about 10 times as much zinc on the surface and are
suitable for use in all exterior and interior
applications. Zinc-plated fasteners will
provide a disappointing performance if used
outside, especially when used to connect hot-dip
galvanized structural steel members.
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14. How long will hot-dip galvanizing
protect my steel from corrosion?
The
corrosion rate of zinc and how long it will
provide protection is a function of the coating
thickness and the amount of corrosive elements in
the atmosphere. For example, in rural
settings where there is less automotive/truck
exhaust and plant emissions, galvanized steel can
easily last for 100 - 150 years without
maintenance. Industrial and marine locations
contain significantly more aggressive corrosion
elements such as chlorides and sulfides and
galvanized steel may last for 50 - 100 years
in those cases. The relationship between
coating thickness and atmospheric conditions is
contained in a popular graph developed by the
AGA. Please see the publication Hot-Dip
Galvanizing for Corrosion Protection: A
Specifier's Guide on this web site.
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15. Are there any special design and
fabrication considerations required to make steel
ready for hot-dip galvanizing?
Yes.
Specifically, fabricated steel must allow for easy
flow of the cleaning chemicals and molten zinc
metal over and through it. This means that
gussets must be cropped, holes put in the proper
location for draining and venting of zinc from
tubular configurations, weld flux removed,
overlapping surfaces must be seal-welded, and
light gauge material temporarily braced. The
details of design and fabrication are contained in
the AGA publication The Design of Products to
be Hot-dip Galvanized After Fabrication,
found on this web site.
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16. Where are galvanized steel products
used?
First of all, the variety of things
galvanized is broad. Structural steel
(angles, channels, wide-flange beams, I-beams,
H-beams), grating, expanded metal, corrugated
sheets, wire, cables, plate, castings, tubing,
pipe, bolts & nuts. The industries that
utilized hot-dip galvanized steel range from
bridge & highway (reinforcing steel for decks
and column concrete, girders, stringers, light and
signposts, guardrail, fencing), water &
wastewater treatment plants (walkway
grating/expanded metal, handrails) architectural
(facades, exposed structural steel, lentils),
parking garages (reinforcing steel for concrete
decks, exposed structural steel columns and
barriers), pulp & paper plants (structural
steel, walkways, handrail), OEMs (motor housings,
electrical cabinets, frames, heat exchanger
coils), electrical utilities (transmission towers,
distribution poles, substations, wind turbine
poles), communication (cell towers), rail
transportation (poles, switchgear, miscellaneous
hardware), chemical/petro-chemical (pipeline
hardware, manufacturing buildings, storage tanks,
walkways), recreation (boat trailers, stadiums,
arenas, racetracks), and many more.
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17. What are the size limitations of
steel that is to be galvanized?
The
hot-dip galvanizing process can accommodate
various different shapes and sizes of steel.
Kettle sizes vary in dimensions from one
galvanizer to the next. You can view the online listing of all
the galvanizers in North America and their kettle
sizes.
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18. What types of products can be
galvanized?
Numerous different
fabrications for a variety of applications are
galvanized each year. To view a list of the
different types of products that have been hot-dip
galvanized click here.
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19. Sometimes, the galvanized coating
is shinier in some places than others. Why is
that?
The galvanized coating
appearance may either be bright and shiny
resulting from the presence of an outer layer of
pure zinc, or duller, matte gray as the result of
the coating's intermetallic layers being exposed.
Performance is not affected. Coating appearance
depends on the amount of zinc in the coating.
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20. Is the zinc coating's thickness
consistent over the entire
piece?
Coating thickness depends on
the thickness, roughness, chemistry, and design of
the steel being galvanized. Any or all of these
factors could produce galvanized coatings of
non-uniform thickness. Members of the American
Galvanizers Association galvanize to ASTM
standards, which define minimum average coating
thickness grades for various material categories.
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21. What can I do to minimize possible
warping & distortion? Is it possible to
determine - prior to galvanizing - which pieces
might be prone to this
occurrence?
Minimizing potential
warpage and distortion is easily done in the
project's design stages by selecting steel of
equal thicknesses for use in every separate
subassembly that is to be hot-dip galvanized,
using symmetrical designs whenever possible, and
by avoiding the use of light-gage steel (<1/16"
/ 1.6 mm). Some structures may benefit from the
use of temporary bracing to help maintain their
shape and/or alignment.
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22. Can I paint right over the
galvanized coating? If so, what procedure should
be followed?
Galvanized coatings can
be easily and effectively painted, not only for
aesthetics but also to extend the structure's
service life. The age and extent of weathering of
the galvanized coating dictate the extent of
surface preparation required to produce a quality
paint system over galvanized steel. ASTM D 6386,
Practice for Preparation of Zinc (Hot-Dip
Galvanized) Coated Iron and Steel Product and
Hardware Surfaces for Painting, should be
consulted for suggested surface preparation
methods for galvanized coatings of varying ages.
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23. How much weight will my material
gain from galvanizing?
As an average,
the weight of the article will increase by about
3.5% due to zinc picked up in the galvanizing
process. However, that figure can vary greatly
based on numerous factors. The fabrication's
shape, size, and steel chemistry all play a major
role, and other factors like the black weight, the
different types of steel that get welded together,
and the galvanizing bath chemistry can also have
an effect.
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24. Are slip-critical connections a
concern when the steel is to be
galvanized?
When galvanized parts are
used for slip-critical connections, they must
either be brushed, abrasive blasted, or painted
with zinc-silicate paint to increase the surface
roughness and, thus, the slip factor.
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25. I'm interested in specifying
hot-dip galvanizing for reinforcing steel. Are
there any concerns with fabricating rebar after
galvanizing?
Rebar is commonly
fabricated after galvanizing. In order to minimize
the possibility for coating damage, avoid bending
the rebar at a radius of more than 8 times its
radius. ASTM A 767, Specification for Zinc-Coated
(Galvanized) Steel Bars for Concrete
Reinforcement, has a table that provides maximum
bend diameters for various-sized rebar.
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26. Can I specify how much zinc to put
on the steel?
No, the steel chemistry
and surface condition are the primary determinants
of zinc coating thickness. Leaving the steel in
the molten zinc a little longer than optimal may
have one of two effects: 1) it may increase the
coating thickness, but only marginally; 2) it may
significantly increase the coating thickness and
cause a brittle coating.
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27. What does it mean to "double-dip"
steel?
"Double-dipping" is the
progressive dipping of steel that is too large to
fit into the kettle in a single dip.
Double-dipping cannot be used to produce a thicker
hot-dip galvanized coating.
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28. What is the reason for
incorporating venting & drainage holes into a
project's design?
The primary reason
for vent holes is to allow otherwise trapped air
and gases to escape; the primary reason for drain
holes is to allow cleaning solutions and molten
zinc metal to flow entirely into, over, and
throughout the part, and then back into the tank
or kettle.
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29. If I stitch-weld, will there be
uncoated areas after galvanizing?
When
stitch-welding is used, there is a possibility of
gas release between gaps, which will prevent the
galvanized coating from forming in these areas. By
leaving at least a 3/32" (2.4 mm) gap between the
contacting surfaces, gases are allowed to escape
and cleaning solutions and molten zinc are allowed
to flow in between the surfaces for a complete and
uniform coating.
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30. What is "white rust" and how can
it be avoided?
"White rust" is the
term mistakenly applied to wet storage stain,
which actually is a milder corrosion product than
white rust. Wet storage stain can be avoided by
properly stacking freshly galvanized articles,
avoiding unprotected exposure to wet or humid
climates, or by using a surface passivation
treatment after galvanizing. Wet storage stain
typically weathers away once the part is in
service. (True "white rust" is most commonly
associated with galvanized cooling towers.)
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31. Is there a way to provide for
intentionally ungalvanized areas?
Yes,
but because masking or stop-off materials may not
be 100% effective, contact your galvanizer for
suggestions.
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32. Is there any environmental impact
when the zinc coating sacrificially corrodes? Is
zinc a safe metal?
There are no known
studies to suggest zinc corrosion products cause
any harm to the environment. Zinc is a naturally
occurring element (25th most abundant element in
the earth), and necessary for all organisms to
live. It is a recommended part of our diet (RDA 15
mg) and necessary for reproduction. It is used in
baby ointments, vitamins, surgical instruments,
sunscreens and cold lozenges.
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33. Should I be concerned when
galvanized steel comes in contact with other
metals?
Zinc is a noble metal and will
sacrifice itself (i.e. corrode, give up its
electrons and create a bi-metallic couple) to
protect most metals. So, it is recommended to
insulate galvanized steel so that it doesn't come
in direct contact with dissimilar metals. Rubber
or plastic, both non-conductive, are often used to
provide this insulation.
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34. What is the difference between
hot-dip galvanizing after fabrication and
continuous hot-dip galvanized
sheet?
The process steps are similar
but the production equipment is very different.
After fabrication galvanizing is a more manual
process where structural steel (fabricated plate,
wide-flange beams, angles, channels, tube, pipe,
fasteners) is suspended by wire, chain or hook
from crane hoists and immersed in the cleaning
solutions and zinc. Continuous sheet galvanizing
involves uncoiling sheet, passing it through the
cleaning steps and molten zinc bath at speeds up
to 500 feet per minute, drying and recoiling.
The uses of after-fabrication galvanized steel
are usually exterior in nature because the zinc
coating is relatively thick (3.0 - 6 mils, 75 -
150 microns, 1.7 to 3.6 oz/sq. ft.) and will
protect steel from corrosion in most atmospheric
conditions for 50 to 100 years. Galvanized sheet
is suitable for interior applications because of
the relatively thin coating (0.45 oz on each
side), unless it is painted after galvanizing.
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35. What is a G90 or A60
coating?
G90 is a grade of galvanized
sheet produced to ASTM A653. It has 0.90 oz/sq.
ft. of zinc overall or 0.45 oz/sq. ft. per side.
A60 is also a grade, has 0.30 oz/sq. ft. per side,
and has been annealed after galvanizing to produce
a surface that promotes good adhesion of paint.
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36. Is a salt spray test in a
laboratory appropriate to estimate the corrosion
rate of zinc coated steel?
In order
for zinc to develop its protective patina of zinc
carbonate that is very stable and non-reactive, it
requires a wetting and drying cycle like that
produced by nature. Salt spray tests keep the zinc
wet and essentially wash the zinc corrosion
products off as they develop, inflating the
corrosion rate of zinc. This lab test is not
reflective of real-world performance of zinc
coatings.
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37. Can galvanized steel in service
withstand high temperatures for long periods of
time?
Constant exposure to
temperatures below 390 F (200 C) is a perfectly
acceptable environment for hot-dip galvanized
steel. Good performance can also be obtained when
hot-dip galvanized steel is exposed to
temperatures above 390 F (200 C) on an
intermittent basis.
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38. Can I specify how much zinc to put
on the steel?
No, the steel chemistry
and surface condition are the primary determinants
of zinc coating thickness. Leaving the steel in
the molten zinc a little longer than optimal may
have one of two effects: 1) it may increase the
coating thickness, but only marginally; 2) it may
significantly increase the coating thickness and
cause a brittle coating.
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39. What is "cold"
galvanizing?
There is no such thing as
cold galvanizing. The term is often used in
reference to painting with zinc-rich paint.
Galvanizing by definition means a metallurgical
reaction between zinc and iron to create a bond
between the zinc and the steel of approximately
3600 psi. There is no such reaction when zinc-rich
paints are applied and the bond strength is only
several hundred psi.
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