Introduction To Hot Dipped Galvanising
1.Hot Dip Galvanising is a modern and technically advanced product
that protects iron and steel from rust by providing a thick metallic zinc
envelope that completely covers the steel surface and seals it from the corrosive
action of its environment. Its high quality zinc coating is welded to the
steel surface by an alloying reaction between zinc to form a tough, durable
and impervious coating to protect the steel by a remarkable electrochemical
system. Electrochemical protection is sometimes known as sacrificial protection
because where there is damage or minor discontinuity in the sealing coat of
zinc, the zinc sacrifices itself to protect the steel to which it is alloyed.
It will go on doing almost the last atom of zinc and makes sure that corrosion
does not attack to steel that you fabricate or use. Unlike organic paint coatings
that require frequent renewal, the galvanised coating cannot come of as it
is alloyed to the surface of the steel, coating every surface of the product,
inside and out. Although Hot Dip Galvanising is simple and effective in use,
the metallurgy that goes on during the application process is quite complicated.
The galvanised coating forms because iron and zinc react together to form
an alloy. But the surface of fabricated steel is often contaminated with rust
and oil, and alloying takes place only if the surface of the steel is clean
enough to be wetted by liquid zinc. The Galvaniser cleans the steel by degreasing
and pickling to obtain a chemically clean surface. The galvanising reaction
between zinc and steel takes place in liquid zinc, usually at 445-460°C. At
this temperature, iron and zinc react quickly. Typically, the steel may be
in the zinc for only a few minutes. Fabrication is taken out of the zinc when
the reaction is complete. Although the zinc coating has already formed, its
internal structure continues to change until the steel returns to normal temperature.
There is no cheating in Hot Dip Galvanising, as zinc will not alloy with the
steel if surface contaminants are incompletely removed. In these areas, the
alloying reaction cannot take immediate action and reprocess the work following
a simple visual inspection. This abrasion resistant and aesthetically pleasing
coating is then able to last for years and little or absolutely no maintenance
is required. Galvanising does have a finite life and its effectiveness as
a barrier depends directly on the thickness of the coating and the environment
2. Lowest long term cost.
3. The economic case for galvanising is still improving.
4. Long life. Even in cases where the initial cost of galvanising
is higher than a comparable alternative, galvanising is almost invariably
cheapest in the long term. There is a high labour cost component inherent
in the application of most other coatings and their cost directly reflects
increases in the cost of labour. The graph below shows relative cost trends
since 1980. The standard 610gm² galvanised coating* has a typical life without
maintenance of more than 50 years in rural environments and 20 to 25 years
in urban and coastal conditions.
5. Reliability. The galvanised coating is bonded metallurgically
to the base steel so that it becomes part of the steel surface it protects.
Coating life and performance are reliable and predictable. * British Standard
BS: 729-71 specifies a minimum coating mass of 610g/m² on steel over 5mm thick.
6. Coating toughness.
7. Elimination of maintenance for long periods.
8. Automatic protection for damaged areas. A galvanised coating
has a unique metallurgical structure which gives outstanding resistance to
mechanical damage to mechanical damage in transport, erection and service.
The use of galvanising defers maintenance for very long periods compared to
other coatings for steel. Remember, maintenance can be extremely expensive
when it entails difficult access, or when structures are located in remote
areas. Galvanised coatings corrode preferentially to steel, providing sacrificially
or cathode protection to small areas of steel exposed through damaged to the
coating. Unlike organic coatings no touch up is needed.
9. Complete Protection.
10. Ease of inspection.
11. Time savings in factory, and on site. Every part of galvanised
article is fully protected, even recesses, sharp corners and inaccessible
areas.No coating applied to a structure or fabrications after completion can
provide the same protection. Galvanised coating thickness is controlled automatically
by steel thickness and a standard, minimum coating mass results. Soundness
and continuity coatings are judged readily by eye. Galvanising is faster than
painting, and elimination of site painting, can save substantial time in site
erection or assembly.
12. Mechanical-properties Galvanising has no effect on the mechanical
properties of commonly galvanised structural steels.
Steps to galvanising
1 Soil and grease removal: Hot alkaline cleaner is used to
remove oil and grease soluble paints.
2 Rinsing: To prevent alkaline from reacting with pickling solution, preventing
unnecessary acid waste.
3 Scale removal or pickling: Subsequently the steel passes through an acid
bath to remove surface rust and mill scale to produce a clean metallic surface.
4 Rinsing: To remove acid and iron salt.
5 Prefluxing: The cleaned steel is next immersed in hot flux solution ( usually
zinc ammonium chloride) to prevent oxidation and ensure that the surface is
chemically clean before its immersion in molten zinc.
6 Hot Dip Galvanising: The dried fabrication is next immersed in molten zinc
where it immediately reacts to form the zinc-iron alloy layers on its surface.
The period of immersion depends solely on the zinc and weight of the steel
7 Finishing: The work is then withdrawn at a controlled speed. In addition
to the zinc/iron alloy layers, a coating of relatively pure zinc solidifies
at the surface when chilled in water. This total zinc coating is metallurgical
bond to steel, completely covering the whole article.
8 All materials will be transferred to the deracking section for final inspection
and touch up of any bare spots and removal of sharp edges.