TPT November 2012

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Blasting and coating
November 2012 95
The advantages of galvanising
THE tube and pipe industries are, like
many sectors worldwide, constantly
looking for more efficient, effective
and commercially viable ways of
operating. As a process, galvanising
is incredibly flexible, long-lasting and
environmentally friendly, and one which
can be incorporated into a variety of
projects, both large and small-scale. So,
how does the process work, what is the
technology behind it and what sorts of
projects within the industry is the process
being used on?
Hot dip galvanising is essentially the
process of coating clean steel with a
layer of molten zinc to protect the item
from corrosion and provide a long-
lasting, durable covering. Because it
forms a series of zinc alloy layers with
the iron in the steel it is more robust than
other coatings that only bond chemically
or mechanically; it also has the added
advantage of fully coating the steel,
inside and out.
The galvanising process starts
with cleaning the steel to prepare it
for galvanising. Cleaning involves the
complete removal of light grease, scale
and dirt using a variety of techniques.
Commonly, the steel product is dipped
into an alkaline or acidic degreaser
and then rinsed before being dipped
in a hydrochloric acid rinse at room
temperature. After cleaning the product
is rinsed and then usually dipped in a
flux solution, which is typically made
up from zinc ammonium chloride held
in a temperature range between 65°C
and 80°C. This final stage removes the
last traces of oxide from the surface
and coats the product with a thin film
of flux. After the steel has dried, it is
dipped into the molten zinc which is
around 450°C. Finally the work may be
immersed in water to cool it, or allowed
to cool in air.
When the steel is dipped into the
molten zinc, a series of zinc-iron alloy
layers are formed. The main thickness of
the coating is created at this time, after
which the metallurgical reaction slows
down so even if the steel remains in the
galvanising bath for a considerable time,
it will not form an overly thick protective
layer. Typically items will be dipped for
four to five minutes, and when they are
removed a layer of pure molten zinc
will be taken out on top of the alloy. It is
this that cools to show the bright, shiny
appearance generally associated with
newly galvanising products.
Zinc is an essential mineral, which is
needed to sustain life and is found in
many places including rocks and soil, as
well as plants, animals and the air that
we breathe. It’s widely abundant and
can be indefinitely recycled. It reacts
with air to create a surface film that is
insoluble to rainwater and acts as a
barrier to completely prevent moisture
and oxygen reaching the steel itself.
Not only does Zinc lengthen the
lifespan of steel, but it is energy-efficient
throughout its production and whole
lifecycle. The galvanising process uses
resources considerately to ensure a
relatively low environmental burden, and
galvanised steel can easily be recycled
with steel scrap or it can be removed, re-
galvanised and then reused.
The past twelvemonths alone has seen
Wedge Group Galvanizing undertake
a number of national and international
projects of all shapes and sizes.
One such project involved the
galvanising of steel used to create a
number of pipe flanges connecting
pipelines carrying oil as part of
petrochemical giant BP’s (Floating
Production Storage and Offloading)
FPSO PSVM scheme in waters off the
west coast ofAfrica. The galvanised pipes
were installed as part of a system used
by the offshore industry to receive and
process hydrocarbons, which then store
oil until it can be offloaded onto a tanker
or transported via pipelines. The FPSO
PSVM project is aiming to tap into four
oil fields off Angola with waters ranging
from 1,500 to 2,500m deep. Galvanising
the steel was essential in helping reduce
the risk of rust and corrosion.
A project closer to home has seen
the Wedge Group galvanise 120 tons of
steel tubes, spanning almost 19 miles, as
part of the production of more than 300
animal pens at the North West Auctions
cattle market in Cumbria, which officially
opened in August. A series of 48mm-
wide tubes were manufactured to put
together all the fences and gates needed
to produce 2.7m² pens, which were then
galvanised to ensure long-termprotection
against the highly corrosive environment.
Wedge Group Galvanizing – UK
Website:
The cattle market