Hot dip galvanizing is the process of applying a zinc coating to fabricated iron or steel material by immersing the material in a bath consisting primarily of molten zinc.
The process is described in more detail below, but the process is inherently simple. The simplicity of the galvanizing process is a distinct advantage over other methods of corrosion protection.
The recorded history of galvanizing goes back to 1742 when a French
chemist named Melouin, in a presentation to the French Royal Academy,
described a method of coating iron by dipping it in molten zinc. In 1836,
Sorel, another French chemist, obtained a patent for a means of coating
iron with zinc after first cleaning it with 9% sulfuric acid and fluxing
it with ammonium chloride. A British patent for a similar process was
granted in 1837. By 1850, the
British galvanizing industry was using 10,000 tons of zinc a year for the
protection of steel.
Galvanizing is found in almost every major application and industry where
iron or mild steel is used. The utilities, chemical process, pulp and
paper, automotive, and transportation industries, to name just a few,
historically have made extensive use of galvanizing for corrosion control.
They continue to do so today.
For over 150 years, galvanizing has had a proven history of commercial successas a method of corrosion protection in myriad applications worldwide.
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Barrier protection is perhaps the oldest and most widely used method of
corrosion protection. It acts by isolating the metal from the
electrolytes in the environment. Two important properties of barrier
protection are adhesion to the base metal and abrasion resistance. Paint
is one example of a barrier protection system.
Cathodic protection is an equally important method for preventing
corrosion. Cathodic protection requires changing an element of the
corrosion circuit, introducing a new corrosion element, and ensuring
that the base metal becomes the cathodic element of the circuit.
dip galvanizing provides excellent barrier protection as well as cathodic
are two major variations of the cathodic method of corrosion protection.
The first is called the impressed current method. In this method an
external current source is used to impress a cathodic charge on all the
iron or steel to be protected. While such systems generally do not use a
great deal of electricity, they often are very expensive to install.
The other form of cathodic protection is called the sacrificial anode method in which a metal or alloy that is anodic to the metal to be protected is placed in the circuit and becomes the anode. The protected metal becomes the cathode and does not corrode. The anode corrodes, thereby providing the desired sacrificial protection. In nearly all electrolytes encountered in everyday use, zinc is anodic to iron and steel. Thus, the galvanized coating provides cathodic corrosion protection as well as barrier protection.
The galvanizing process consists of three basic elements:
Surface preparation is the most important step in the application of any
coating. In most instances where a coating fails before the end of its
expected service life, it is due to incorrect or inadequate surface
The surface preparation step in the galvanizing process has its own
built-in means of quality control in that zinc simply will not react with
a steel surface that is not perfectly clean. Any failures or inadequacies
in surface preparation will be immediately apparent when the steel is
withdrawn from the molten zinc because the unclean areas will remain
uncoated. Immediate corrective action is taken.
On-site painting or other field-applied systems of corrosion protection may involve the use of different subcontractors and/or work groups to prepare the surface and apply the coating. This can result in problems in coordinating activities, leading to costly and time-consuming delays, errors, and disputes concerning responsibility and financial liability. Once a job has been delivered and accepted at the galvanizer’s plant, there is one point of responsibility for ensuring that the material leaves the plant properly galvanized. That point of responsibility is the galvanizer.
Surface preparation for galvanizing typically consists of three steps:
caustic cleaning, acid pickling, and fluxing.
A hot alkali solution often is used to remove organic
contaminants such as dirt, paint markings, grease, and oil from the metal
surface. Epoxies, vinyls, asphalt, or welding slag must be removed before
galvanizing by grit blasting, sand blasting, or other mechanical means.
Scale and rust normally are removed from the steel surface by
pickling in a dilute solution of hot sulfuric acid or ambient
temperature hydrochloric acid.
Surface preparation also can be accomplished using abrasive cleaning as an
alternative to or in conjunction with chemical cleaning. Abrasive cleaning
is a process whereby sand, metallic shot, or grit is propelled against the
steel material by air blasts or rapidly rotating wheels.
Fluxing is the final surface preparation step in the galvanizing
process. Fluxing removes oxides and prevents further oxides from forming
on the surface of the metal prior to galvanizing and promotes bonding of
the zinc to the steel or iron surface. The method for applying the flux
depends upon whether the particular galvanizing plant uses the wet or dry
In the dry galvanizing process (see Figure 5), the steel or iron materials are dipped or pre-fluxed in an aqueous solution of zinc ammonium chloride. The material is then thoroughly dried prior to immersion in molten zinc. In the wet galvanizing process, a blanket of liquid zinc ammonium chloride is floated on top of the molten zinc. The iron or steel being galvanized passes through the flux on its way into the molten zinc.
In this step, the material is completely immersed in a bath consisting of
a minimum of 98% pure molten zinc. The bath chemistry is specified by
the American Society
for Testing and Materials (ASTM) in Specification B 6. The bath
temperature is maintained at about 850 F (454
Fabricated items are immersed in the bath long enough to reach bath
temperature. The articles are withdrawn slowly from the galvanizing bath
and the excess zinc is removed by draining, vibrating, and/or
Art Galvanizing specializes in the Centrifuge Process for Small and
Art Galvanizing specializes in the Centrifuge Process for Small and Difficult Items.
The chemical reactions that result in the formation and structure of the galvanized coating continue after the articles are withdrawn from the bath as long as these articles are near the bath temperature. The articles are cooled in either water or ambient air immediately after withdrawal from the bath.
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The two properties of the hot dip galvanized coating that are closely
scrutinized after galvanizing are coating thickness and coating
appearance. A variety of simple physical and laboratory tests may be
performed to determine thickness, uniformity, adherence, and appearance.
Products are galvanized according to long-established, well-accepted, and
approved standards of the ASTM. the Canadian Standards Association (CSA),
and the American Association of State Highway and Transportation Officials
(AASHTO). These standards cover everything from the minimum required
coating thicknesses for various categories of galvanized items to the
composition of the zinc metal used in the process.
Testing methods and interpretation of results are covered in the publication, The Inspection of Products Hot Dip Galvanized After Fabrication, published by the AGA and available from theArt Galvanizing or AGA.
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