Unlike organic paint systems, a hot dip galvanized coating is
produced by a metallurgical reaction between iron and the coating
material i.e. molten zinc. A series of hard abrasion resistant
iron/zinc alloys are formed and these are overcoated with relatively
pure zinc as the product is withdrawn from the galvanizing bath.
The various layers all play a significant role in the provision
of corrosion protection. For the coating to form, the steel surface
is required to be totally free from all contaminants such as
mill scale, rust, grease and oil.
As the name implies, the hot dip galvanizing process entails
dipping or immersion into a series of cleaning and pre-treatment
chemicals prior to immersion in the molten zinc. The advantage
of this method is that all product surfaces are wetted uniformly,
including areas which would be inaccessible for cleaning and
coating by other methods.
Galvanizing entails the following operations:
1.Inspection prior to galvanizing
This important pre-requisite to processing is to ensure that the design of
structures, welding and fabricating standards as well as the surface condition
of material, is acceptable for galvanizing to the relevant specification. Slag
from stick welding (as opposed to shielded arc welding) is not readily removed
by the cleaning chemicals while the presence of some paints (e.g. enamel) are
also difficult to remove except by abrasive blasting, grinding or the use of
a paint remover. Water borne paints or the appropriate marker should always
be used for identification or marking during fabrication of products which
are to be galvanized. Inspection prior to galvanizing is also necessary to
ensure that products can be galvanized without the danger of an explosion.
Sealed cavities where vent holes are not provided can result in the formation
of superheated steam which may cause injury to personnel and damage to the
product.
2.Degreasing (dirt, paint and oil removal)
Components which are appropriately suspended from materials handling devices
are immersed in a degreasing chemical. This is to ensure that steel surfaces
are not masked from the subsequent and most important acid pickling process.
If an alkaline caustic soda based degreaser is used, subsequent rinsing in
water is essential in order to avoid a neutralising effect on acid during pickling.
Acid degreasers normally contain hydrochloric acid but phosphoric acid based
degreasers are also effective.
3.Acid Cleaning (rust and mill-scale removal)
Either hydrochloric or sulphuric acid is used to prepare steel for galvanizing.
Hydrochloric acid is used unheated at a concentration of about 15%. Sulphuric
acid is heated to a temperature of about 70ºC with a concentration of
10%. In conventional plants, material is rinsed in water prior to being transferred
to a flux solution. In some plants where hydrochloric acid is used, rinsing
is dispensed with. The acid carried over is converted into either ammonium
chloride by additions of ammonium hydroxide or zinc chloride by adding zinc
dust. Other contaminants are filtered out.
4.Flux
After pickling in acid, material is transferred to a flux solution which
consists of ammonium chloride and zinc chloride. This solution is normally
heated to
a temperature of 70ºC. The flux plays an important role in that it provides
barrier protection to prevent flash rusting during the period between acid
pickling and immersion in the zinc. It also has a final cleansing effect
on steel surfaces as they enter the zinc. The flux also facilitates the formation
of a uniform coating which is free from discontinuities.
5.Hot Dip Galvanizing
After fluxing and further drying (ideally in a drying oven to avoid excessive
splashing of zinc due to the presence of moisture), the products are dipped
into the molten zinc which is heated to a temperature of about 450ºC.
This is more or less 31ºC above the zinc melting temperature. The steel
is immersed at a fairly rapid speed and once the coating has formed, withdrawal
is at a slow speed (ideally < 1m per minute) to ensure uniform drainage
and a smooth finish. In automatic or semi-automatic plants, where products
such as small bore tubing are hot dip galvanized, withdrawal speeds are much
faster with the ultimate coating thickness and finish controlled by external
air wiping and steam jet blasting of the internal bore. In most plants, the
zinc contains small quantities of aluminium (typically 0.005%). Apart from
enhancing the initial appearance of the coating, the aluminium provides added
benefits which include a slight reduction in coating thickness on excessively
reactive steels and a lower degree of wasteful zinc oxide formation on the
molten zinc surface in the bath.
6.Quenching
This process solidifies the zinc coating to ensure easy handling. It also arrests
the alloying reaction in the case of reactive steels, which continues well
below the melting temperature of zinc. The quench water normally contains a
passivating chemical which retards the formation of white rust (wet storage
stain) until such time as the freshly applied reactive zinc surface has developed
a stable and protective basic zinc carbonate film. Products which may be prone
to distortion are air-cooled and not quenched in order to avoid the effects
of thermal shock.
7.Quality Control and Final Inspection
In order to achieve the required coating standards, routine analyses of the
various chemicals used in the process are essential. Failure to do this results
not only in substandard coatings but also additional processing costs for the
galvanizer. Final Inspection of Hot Dip Galvanized coatings is relatively simple.
Visual examination ensures that no coating discontinuities are present and
the coating is uniform. Unlike other coatings, hot dip galvanizing will not
coat contaminated steel surfaces. A suitably calibrated coating thickness measurement
instrument is used to ensure that coating thicknesses conform to the requirements
of the relevant specification.
