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HOT DIP GALVANIZING
©Copyright1999 Mario S
Pennisi
Email: mariopen@coatfab.com
and
Derek Jones
INTODUCTION
The first use of hot dip galvanizing on steel was some
work reported by the Frenchman P J Malouin in 1742. A French
patent was issued to Sorel in 1837 and an English patent to
H V Craufurd in the same year. Very little has changed in
the process since that time. (See “The Origins of Galvanizing”
in Corrosion Management, August 1995 page 3).
Zinc is very successful as a protective coating for steel
because in most environments to which steel will be subjected,
zinc will act as the anode; ie it will dissolve in preference
to the steel. In simplistic terms, while there is zinc on
the surface the steel will be protected from corrosion.
Hot dip galvanizing is one of a number of methods available
to the surface finisher for applying a zinc coating to an
item. Other techniques include electroplating, mechanical
plating, and sherardising, painting with zinc-rich coatings
and zinc spraying or metallising.
In the hot dip galvanizing process, a uniform coating of
metallurgically bonded zinc-iron alloy layers and pure zinc
is produced.
The life expectancy of zinc coatings is independent on the
coating process – an equivalent coating of zinc will provide
the same life expectancy regardless of the coating process.
Hot dip galvanizing will provide over 80 microns of zinc coating,
while zinc electroplates are normally less than 25 micron.
The hot dip galvanizing process is widely used in a number
of applications, particularly constructional. Galvanizing
is normally carried out to AS/NZS4680: Hot-dip galvanized
(zinc) coatings on fabricated ferrous articles, AS/NZS4791:
Hot-dip galvanized (zinc) coatings on ferrous open
sections, applied by an in-line process, or AS/NZS4792:
Hot-dip galvanized (zinc) coatings on ferrous hollow
sections, applied by a continuous or a specialised process.
THE GALVANIZING PROCESS
Chemically clean items are galvanized by full immersion
in molten zinc. The total coating thickness is automatically
determined by the mass of the steel being galvanized and the
composition of the steel, particularly silicon and phosphorus.
In all other zinc coating processes there is no relationship
between mass, composition and thickness of coating. In hot
dip galvanizing the zinc covers corners; seals edges, seams
and rivets; and penetrates some recesses to give a full zinc
coating and thus protection to areas, which might be potential
corrosion spots with other coating systems. The galvanized
coating is slightly thicker at corners and narrow edges, giving
greatly increased protection compared to organic coatings,
which thin out in these critical areas. Complex shapes and
open vessels may be galvanized inside and out in one operation.
However, the process is not foolproof – bad welds may not
be covered completely and burrs, etc should be removed before
coating.
Virtually any article may be coated. Articles ranging in
size from small fasteners to structures hundreds of metres
high may be protected. Large galvanizing vats, together with
modular design techniques of construction and double-end dipping
allow almost any sized structure to be galvanized. Visual
inspection of aged galvanized products shows that items are
completely protected.
PREPARTION OF WORK FOR GALVANIZING
The first requirement is to remove oils and greases and
old paint coatings. This is normally done in hot strong alkali
solutions. If scale, rust and other surface contaminants are
present abrasive blasting may be necessary, otherwise these
contaminants are removed by acid cleaning or pickling in sulphuric
or hydrochloric acids, followed by rinsing. Hydrochloric acid
is preferred because it is more easily reclaimed. Iron and
steel castings are usually abrasive blast cleaned followed
by a brief acid dip. In some circumstances items may be cleaned
electrolytically to remove foundry sand and surface carbon.
Hot rolled steel surfaces covered by heavy mill scale may
require abrasive blast cleaning prior to acid cleaning.
FLUXING
Before a component can be offered to the hot zinc bath, all
moisture must be dried from the surface and other cavities.
The surface of acid cleaned steel is very active and will
oxidise rapidly, more quickly than the surface can be dried.
The metallurgical reactions between the molten zinc and the
steel surface will not occur if oxides are present on the
steel surface. To prevent this oxidation, the chemically clean,
highly active steel surface is immersed in a flux solution,
usually 30% zinc ammonium chloride with wetting agents, maintained
at about 65°C. The flux solution removes any oxide film
that might have formed on the highly reactive steel surface
after acid cleaning, and prevents further oxidation before
galvanizing. The work is then dried ready for galvanizing.
GALVANIZING
The galvanizing reaction takes place at between 445 and
465°C.
When the dried steel part is immersed in the galvanizing
bath the steel surface is wetted by the molten zinc and reacts
to form a series of zinc-iron alloy layers. The work remains
in the bath until its temperature reaches that of the molten
zinc, so that all the galvanizing reactions can go to completion.
After the surface of the molten zinc is skimmed to remove
the dross from the surface, the job is withdrawn from the
bath at a controlled rate. When the surplus surface metal
has drained off, the item is either air quenched or quenched
in water containing potassium dichromate. The item carries
with it an outer layer of molten zinc that solidifies to form
the relatively pure outer zinc coating. This shiny pure zinc
layer does not always form – in the presence of high levels
of silicon in the steel the pure zinc layer does not form
and the surface will be a dull grey colour.
The period of immersion in the galvanizing bath varies from
several minutes for relatively light articles, up to half
an hour or longer for massive structural members. This is
one of the reasons that hot dip galvanizing is charged by
the weight of the item being coated.
The resulting galvanized coating is tough and durable, normally
comprising a surface of relatively pure zinc covering zinc-iron
alloy layers bonded metallurgically to the underlying steel.
This coating completely covers the article and provides excellent
resistance to abrasion.
One of the advantages of the hot dip galvanizing process
is that a visual inspection can show that the coating is continuous.
Defects such as uncoated areas due to incorrect preparation,
carry over of dross (dull grey rough deposits) can be found
easily.
QUENCHING
The galvanized zinc surface is highly reactive. It will oxidise
rapidly in moist air (a relative humidity above 65% is sufficient
to cause the onset of corrosion) and white corrosion products
form rapidly when two zinc surfaces are stacked on each other
(storage corrosion). To delay this corrosion, galvanizers
normally water quench items in a solution of potassium dichromate
that applies a chromate film to the zinc surface.
The chromate film formed from this dichromate solution is
not suitable for powder coating. If the item is to be powder
coated the galvanizer should air quench the item after galvanizing.
To prevent the generation of corrosion products before powder
coating, it is essential that the powder coating be applied
within a few hours of galvanizing. If this is not possible,
then a light acid etch, followed with a zinc phosphate or
chromate conversion coating is mandatory.
GALVANIZING DEFECTS
As the item is withdrawn from the galvanizing kettle, zinc
drips off the item causing spikes, dags and a rough surface.
Tilting the parts as they emerge from the bath reduces the
incidence of these defects. Thus, any drips are found at the
drip point. These spikes are removed prior to returning the
parts to the customer. The lighter the section the quicker
the zinc will freeze so that drips and dags are possible on
thin sections.
When designing and fabricating parts, which are to be galvanized,
it is important to follow a number of design strategies that
will result in good draining and venting of closed sections.
This action will allow the galvanizer to do a good job safely.
STORAGE OF GALVANIZED PARTS
Zinc coated parts will sweat in humid environments when
stacked closely. The sweating will lead to corrosion and ugly
white corrosion products will be produced. Beneath each of
the locations of white powder, a pit will have been produced
and the coating thickness at these locations will have been
reduced so that corrosion of the steel will occur prematurely.
Separate galvanized parts during storage so that air can circulate
freely between the items. Use spacers to separate flat sheets.
Preparing galvanized coatings for powder coating Galvanized
coatings are a bright silvery grey colour. In these days of
brilliant colours a bright silver finish no longer is sufficient
in domestic applications. The coating of choice is a powder
coating that eventually will be able to provide almost any
colour that the mind can conceive. The powder coating is cathodic
to the zinc and if there is an adhesion defect or there is
a holiday in the coating, the underlying zinc will corrode
under the powder coating film (under film corrosion). The
corrosion products will ooze out through and over the powder
coating giving an ugly salt like appearance to the surface.
To reduce the possibility of this occurring it is essential
to prepare the corrosion free zinc surface with a conversion
coating, either zinc phosphate or chromate. A new Australian
Standard AS/NZS4506: Metal finishing-Thermoset powder
coatings has been issued for these types of coatings.
GALVANIZING FASTENERS AND SMALL COMPONENTS
Fasteners and small components are loaded into perforated
cylindrical steel baskets. They are degreased, acid pickled,
pre-fluxed and dried. The dry baskets containing the components
are lowered into the galvanizing bath. At the end of the galvanizing
treatment, the baskets of galvanized components are raised
from the molten zinc and immediately placed into a centrifuge
or spinner and rotated at high speed for 15 to 20 seconds.
Excess zinc is thrown off. The resultant coating is smooth
and uniform.
METALLURGY OF GALVANIZING
Galvanized layers and expected hardness
The molten zinc in the galvanizing kettle removes the flux
so that the zinc wets the steel surface. Immediately, a metallurgical
alloy is formed between the zinc and the steel – a thin molecular
layer of brittle, hard high-iron zinc alloy (the gamma layer
– 21-28% iron). Next a much thicker, hard, brittle alloy (the
delta layer 7-12% iron) forms. On top of this the zeta layer
forms containing about 6% iron and finally the surface layer
is virtually pure zinc (the eta layer).
The silicon and phosphorous content of the steel affect
the formation of these layers. If the silicon content is very
high, the eta layer may not form and a dull galvanized surface
will result. Each of these layers is metallurgically bonded
to each other so that they cannot be separated from each other
or from the steel as can a paint coating.
ABRASION RESISTANCE OF GALVANIZED COATINGS
Although the outer eta layer is soft and lacks abrasion
resistance, the zeta and delta layers are harder and more
abrasion resistant than the steel. The outer eta layer may
be removed in service. The exposed harder, more abrasion resistant
zeta and delta layers gives the galvanized coating outstanding
abrasion resistance so that mechanical damage to galvanized
coatings is minimised.
COATING THICKNESS
During the first minute of immersion in the galvanizing
bath zinc-iron alloy layers grow rapidly on the surface of
the steels, which are most commonly galvanized. The rate of
alloy layer growth then diminishes and is finally very slow.
When the work is withdrawn from the bath an outer layer of
relatively pure zinc is also carried out. The total zinc coating
mass applied depends mainly on the mass and thickness of the
steel being galvanized.
As illustrated below, galvanized coatings are slightly thicker
at corners and edges, an important advantage over most organic
coatings, which thin out in these critical areas.
The structure of the galvanized coating and the relative
thickness of its zinc-iron alloy layers have little or no
effect on the protective life of the coating. Protective life
depends basically on total coating mass.
ADVANTAGES OF GALVANIZING
- For most classes of steelwork galvanizing may provide
the lowest long-term cost. In many cases galvanizing may
also provide lowest initial cost.
- The galvanized coating becomes part of the steel surface
it protects.
- The metallurgical structure of the galvanized coating
provides good toughness and resistance to mechanical damage
in transport, erection and service.
- The galvanized coating is subject to corrosion at a predicably
slow rate, between one-seventeenth and one-eightieth that
of steel, depending on the environment to which it is exposed.
- Zinc’s anodic protection for steel ensures that small
areas of the base steel exposed through severe impacts or
abrasion are protected from corrosion by the surrounding
galvanized coating.
- An inherent advantage of the process is that a standard
minimum coating thickness is applied to a given section
and a particular steel composition.
- During galvanizing the work is completely immersed in
molten zinc and the entire surface is coated, even recesses
and returns, which often cannot be coated using other processes.
If required, internal surfaces of vessels and containers
can be coated simultaneously.
- Galvanized coatings are virtually self-inspecting because
the reaction between the steel and molten zinc in the galvanizing
bath will not occur unless the steel surface is chemically
clean. Generally, a galvanized coating, which appears sound
and continuous, is sound and continuous although there is
evidence that defects can be hidden beneath an apparently
sound coating.
- Galvanizing is a highly versatile process. Items ranging
from small fasteners and threaded components, up to massive
structural members can be coated.
- The mechanical properties of commonly galvanized steels
are not significantly affected by galvanizing.
- Galvanizing provides outstanding corrosion performance
in a wide range of environments.
- Duplex coatings of galvanizing-plus-paint are often the
most economic solution to the problem of protecting steel
in highly corrosive environments. Such systems provide a
synergistic effect in which the life of the combined coatings
exceeds the total life of the two coatings if they were
used alone.
DISADVANTAGES OF GALVANIZING
- The location of galvanizing plants is limited. In Queensland,
plants are currently located in Brisbane and Cairns.
- The limited number of plants requires that steelwork
fabricated in other centres must be transported to the plant
and then to the site adding to time and cost.
- The necessity to transport steelwork exposes it to damage
in transit not only to the galvanizing, but also to the
steelwork itself.
- The limited size of galvanizing tanks requires that steelwork
must be designed in suitable sizes or in modules.
- Steelwork must be designed to permit safety to the work
and to workers in the galvanizing plant.
- In-situ steelwork (eg existing structures) cannot be
hot dip galvanized.
OTHER GALVANIZING PROCESSES
Ferrous open sections and ferrous hollow sections are
being fabricated from pre-galvanized strip either hot dipped
or electroplated
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