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1. Galvanized Steel
Corrosion Resistance General Properties
Electrochemical Protection
Hot-dip galvanized steel has strong corrosion
resistance. Galvanized steel consists of a zinc coating on
a steel substrate. Since zinc is anodic to iron, a break in
the coating does not lead to corrosion of the substrate. This
sacrificial anode of zinc is a form of galvanic protection.
Physical Protection
Zinc itself is an "active" metal, thus when
exposed to the atmosphere it initially reacts with oxygen
in the air to oxidize while contact with carbon gas creates
zinc carbonate. These thin films protect the steel from more
serious environmental attack. It is tenacious and serves as
a barrier between the steel and environment to prevent further
corrosion.
Corrosion Resistance to atmospheric assault
Corrosion of galvanized steel sheets depends
on the protective surface film. The thin film can provide
better protection when air is less polluted. If the air contains
harmful gases such as sulfur dioxide, a porous film forms
on the surface and accelerates corrosion. The lifespan of
steel sheets is proportionate to the amount of zinc coating,
but in the case of identical amount of zinc coating, environment
is the most important factor of corrosion. In cities and industrial
areas, steel sheets deteriorate faster than out in the country
where the air is cleaner. The corrosion resistance rate of
steel sheet in industrialized areas stand at only 25~40% of
that in rural regions. In addition, the lifespan of steel
sheets are 5-10 times longer indoors than in an outdoor environment.
Galvannealed Steel Sheet
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| To improve weldability and paintabilty
of pure zinc galvanized steel, it is reheated (500~550
°C) in a galvanneal furnace to give it a gray
Fe-Zn alloy layer. Iron consists of about 10% of
the alloy layer, making it a more noble appearing
metal and less susceptible to ionization. Galvannealed
steel sheet is optimal for preventing local corrosion.
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2. Formability
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Hot-dip galvanized steel sheet manufactured at continuous
galvanizing line has excellent zinc tenacity and the Fe-Zn
alloy layer is very thin so coating does not exfoliate
during drawing. Galvanized steel sheet produced from vertical
furnace has excellent formability compared to that of
cold-rolled steel sheet. |
Mechanical Properties
3.Paintability
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Galvanized sheets are usually painted before use. Paint
adhesiveness and corrosion resistance after painting is
heavily influenced by the type of treatment before painting.
Generally, oiling is preferred to chromate treatment.
It is important to clean the surface before painting to
have strong adhesion. Galvannealed steel sheets have a
phosphate layer due to the Fe-Zn alloy coating giving
steel sheets improved adhesiveness and corrosion resistance.
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Result of Paintability Test
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| Erichsen
test after cross-cutting (adhesive tape flaking
test) |
| DURGRIP(Galvanized) |
DURGRIP(Galvanized) |
Cold-rolled
steel sheet |
| Corrosion resistance
after painting |
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| DURGRIP(Galvanized) |
DURGRIP(Galvanized) |
Cold-rolled
steel sheet |
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Paintabilty Testing Procedure
4. Weldability
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Characteristics |
- zinc coating has better conductivity than iron,
thus reducing electrical resistance on the contact
area .This leads to lower heat generation.
-Since the melting point of zinc is lower than that
of iron, continuous welding is impaired by attachment
of zinc at the electrode.
-Zinc is more ductile than iron so electrical density
is reduced as pressure from the electrode is applied
onto large contact area of steel sheets. |
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Property of Repeated Welding |
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The graph shows optimum welding conditions of galvanized
steel sheets. Optimum range of galvanized steel sheet
is higher than that of the cold-rolled sheet. thickness:
0.8mm electrode:0.5mm (cylinder type with cut edge) time:10seconds
When continuously spot welding galvanized steel sheet,
zinc attaches onto the electrode, impairing the welding
process. This intensifies with increased zinc coating
weight.
Galvanized steel sheet with thin zinc coating can be continuously
welded more than 5000 times. In case of painted sheets,
appropriate electrode conditions must be met. Galvannealed
steel sheets can be continuously welded more than 5000
times. |
Methods for Improving Weldability
Shield Metal Arc-Welding
When shield arc welding, it advisable to use
an electrode with high basicity to obtain good fludity and
metal properties. For example, KSD 7004 E 4303 (lime titanium
type), E 4313 (high titanium oxide), E 4316 (low hydrogen
type) are good covers
Solderability
Galvanized steel sheet can be soldered without
exfoliating the surface film. Any simple solvent will remove
the film. Chromate-treated steel sheet has excellent solderability.
Non-corrosive solvent or mixture of zinc chloride (15~20%)
and ammonium chloride(3.3~5%) are favored solvents. It is
necessary to thoroughly clean and dry the steel sheet after
applying solvent.
5. Galvannealed Steel
Sheet
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Resistance Welding
Welding galvanized steel is more difficult than
welding cold-rolled steel because it has lower
electric resistance, a lower melting point, and
wider contact area under electrode pressure. Galvannealed
steel sheet, coated with a Fe-Zn alloy, has a
higher melting point and greater hardness than
pure zinc coated steel; thus weldability is enhanced.
Shield Metal Arc Welding
Has similar weldability as cold-rolled sheets.
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Change
of welded part strength and nugget diameter in accordance
to welding current |
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Raise
welding current by 10~30% 