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What
is coated steel?
Coated
steel is a steel that has an extra finish applied to the
natural steel surface. The extra coating or finish applied
can be tin, chrome, zinc, paint or plastic. The application
of the coating is done in order to protect the surface
of the steel from oxidation and it is also used to enhance
the appearance of the material. Coated steel can be divided
into several different categories. These include hot-dip
galvanized, galvannealed, galvalume, galfan, electrolytic
zinc (electro-galvanized) and pre-painted steel.
History
of Galvanizing
Galvanization
refers the process of coating steel with zinc. Galvanizing
was named after its founder, Italian scientist Luigi Galvani.
Galvanization was originally the process of electrical
shock. The process was discovered as Galvani touched a
frog’s leg with forceps and saw the muscles twitch. The
use of the word galvanize in today’s terms refers mainly
to the chemical process of hot dip galvanizing. In the
process steel or iron is coated with zinc. The zinc reduces
the chances of corrosion.
Hot
Dip Galvanized steel
Hot-dip
galvanized steel is carbon steel that is coated with a
thin zinc layer on both sides using a hot-dip continuous
line. The coated zinc layer is tightly bound to the steel
in a molten bath of zinc at a temperature of around 460
Celsius. Galvanized steel can be used in a wide variety
of applications where rust resistance is needed. One of
the big identifying characteristics of galvanized steel
is the spangle or visible crystallization pattern on the
steel’s surface.
Galvannealed
steel
Galvannealed
steel is carbon steel that has been coated with a zinc
coating on both sides using a hot-dip continuous line.
When the steel leaves the molten zinc bath the molten
zinc coating is sent to an in-line heat treatment. This
converts it into an iron-zinc alloy as the iron is diffused
from the steel into the coating. Galvannealed steel is
different from hot-dip galvanized steel as it has no visible
spangle and it appears matte compared to the highly metallic
appearance of galvanized steel. The reddish-orange appearance
on of the surface of the steel is because of the iron
spread over the coated layer particularly when the iron
content ranges 8 to 11 percent.
Galvalume steel
Galvalume coated steel is carbon steel that is coated
with an aluminum-zinc coating using a hot-dip continuous
line. The coating mixture consists of 45 percent zinc
and 55 percent aluminum. A small amount of silicon is
also added to the mixture and this helps with the adhesion
of the coating. The corrosion resistance is much better
than that of hot-dip galvanized steel. This makes it very
popular in environments with prolonged exposure to corrosion.
Galfan steel
Galfan
steel is carbon steel that has been coated with a mixture
of aluminum and zinc on both sides using a hot-dip continuous
line. The composition of the coating layer is 95 percent
zinc and 5 percent aluminum. It is more corrosion resistant
than normal hot-hip galvanized and generally cheaper to
produce than galvalume steel thus making it the preferred
material for use in areas that needs long term resistance
to atmospheric corrosion.
Electro-galvanized steel
Electro-galvanized
steel is carbon steel that is coated with pure zinc coating
by applying an electric current. This causes the positively
charged zinc anodes to attach to the negatively charged
steel. This process helps to smooth the surface even after
painting. The final product has a similar appearance to
cold rolled steel but it is darker and it has similar
mechanical properties to the base material of cold rolled
coil.
Pre-painted steel
Pre-painted
steel is a steel that has received a layer of paint after
a treatment. In many cases the pre-painted steel also
has a layer of primer applied. The paint can be applied
to uncoated steel or steel that has a metallic coating.
In order to achieve maximum corrosion resistance a metallic
coated steel is used. These include galvalume, galfan,
galvannealed and galvanized steel. Pre-painted steel technology
has been developing fast over the past twenty years. Advances
in fade, chalk resistance, flexibility, hardness in painting
materials and many more has widened the use of the pre-painted
steel in many industries such as the automotive, appliance,
construction and furniture industries.
Why use coated steel?
The major
benefit that the use of coated steel gives is its improved
corrosion resistance. There are many different types of
corrosion in low carbon steel. It can be localized corrosion
due to certain conditions in the environment or the corrosion
can spread over the entire surface of the steel. Uniform
corrosion is a form of corrosion that is uniformly spread
over the steel and is clearly visible. In addition there
are several other forms of corrosion of low carbon steel
including crevice corrosion, poultice corrosion, erosive
corrosion, fretting corrosion. The need for corrosive
resistant steel increased to cope with a wide variety
of corrosive factors present in daily living conditions.
With coated steel technology advancing fast, there are
many different options available to meet the needs of
various different industries.
Coated Steel Applications
Hot-dip
galvanized steel is primarily used for
applications such as automobile parts, air conditioning
equipment (ducting/culvert), electric panels, building
structures (corrugated roofing/wall panels), home appliance,
ceiling frames and many more. Its anti-corrosion ability
generally depends on the quantity of zinc coated on the
surface. G90 or less in normally used for indoor facilities
or equipments while G90 or more is use in suburban districts.
Painted G90 is used in areas within cities or in industrial
zones as well as along the seashore due to the high density
of salty atmosphere.
Galvannealed
steel
is used for automobile body panels including vehicles
such as buses, trucks and cars. It is also used for parts
that require a long lifespan.
Galvalume
and Galfan steel
has a longer lifespan than hot-dip galvanized steel and
is therefore preferred in applications that requires a
long term corrosion resistance. Both types are also used
in applications with temperatures higher than 500 degree
Fahrenheit because hot-dip galvanized steel is not able
to withstand such high temperatures. Galvalume and Galfan
applications includes industrial roofing and siding, electric
boxes and panels, furnace parts, heating devices and under-body
parts of automobile as well as many more.
Electro-galvanized
steel
can be used in home appliances that require a very smooth
surface appearance. This includes DVD, computer and electronic
equipment bodies.
Pre-painted
steel
is widely used in roofing and siding, electronic devices,
exposed body parts of automobiles, household appliances,
furniture, garage doors, indoor wall panels and containers.
The anti-corrosion service life of the pre-painted steel
depends on the surface paint used. If a polyester (PE)
paint is used the service life is around 7 to 8 years.
The service life of a product painted with silicone modified
polyester (SMP) is around 10 to 12 years and 15 years
for special modified polyester. Polyvinyl chloride painted
products usually has a service life of between 15 and
20 years and between 20 and 25 years for polyvinylidene
fluoride products.
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