Did you know?
Zinc shares the same chemical family as Cadmium & Mercury! Yet unlike its family members, zinc is a required dietary mineral.
Reference: Universalium
Many people use “galvanizing” as a misnomer, or a generic term for all types of zinc coatings. All zinc coatings are not the same, in fact their physical, chemical and corrosion resistance ability can be extremely different.
An AGA photomicrograph of various zinc coatings.
The first micrograph depicts a Metallized coating.
Metallizing (or Zinc Spraying) is accomplished by feeding zinc in
either wire or powder form to a spray gun, where it is melted and
sprayed onto the steel surface by using the combustion gases and/or
auxiliary compressed air to provide the necessary velocity. Metallizing
allows coating of fabricated items which cannot be galvanized
because of their size or because the coating must be performed on
the job site. Abrasive cleaning of the steel to white metal is
required before metallizing. Once applied, the zinc coating is
normally sealed with a thin coating of a low viscosity polyurethane,
epoxy or vinyl resin. There are some limitations as to the thorough
coverage due to structurally inaccessible areas such as recesses,
hollows and cavities. This is a similar limitation as with painting.
Coating consistency is dependent on operator experience and coating
variation is always a possibility especially around corners and edges.
Metallizing does provide cathodic protection, but there are no
zinc-iron alloy layers in the coating. Metallizing is approximately
85% as dense as hot-dip galvanizing and, in fact, the black areas
on the photomicrograph are actually voids in the coating. If
the voids were removed, the coating thickness would in reality be
about 15% thinner.
The next micrograph is a Hot-Dip Galvanized
coating. The characteristics of hot-dip coatings are described in further detail
here.
The next micrograph shows Zinc Paint which
consists of zinc dust bound in organic or inorganic binders. The white
particles in this photomicrograph are zinc oxides while the black areas
are the binders. Zinc-rich paints are primarily barrier coatings but
also provide some limited cathodic protection. The binder must be
conductive or the zinc particles must be in contact in order to provide
the cathodic protection. Suitable zinc-rich paints actually provide a
useful repair coating for damaged hot-dip galvanized coatings. Uneven
film coats may develop if the paint is applied by brush or roller,
and “mud cracking” may occur if the paint is applied too thick.
Galvanized Sheet is produced via a
continuous galvanizing process which is also a hot-dip process,
although usually limited to steel mill operations. The process
consists of coating sheet steel, strip or wire on machines that
can be over 500 feet in length and run material at speeds of
over 300 feet per minute. The coating thickness is minimal
compared to that of batch hot-dip galvanized coatings and has minimal
zinc-iron alloy layers, however, barrier and cathodic protection is
still provided.
The last micrograph shows an Electroplated (or Electrogalvanized) coating.
These terms generally refer to zinc coatings that are applied to steel
sheet or strip by electrodeposition in a steel mill facility. There are
no zinc-iron alloy layers, however, barrier and cathodic protection
is still provided.
With any of these coatings, the most important aspect to remember is that the service life is directly proportional to the thickness and amount of zinc in the coating. Simply put, the thicker the coating the longer it will last.
More detailed information on the various types of zinc coatings can be
found in the American Galvanizers publication titled ‘Zinc Coatings’.
Specific comparisons between Hot-Dip Galvanized coatings and Metallized coatings can be found
here
and between Hot-Dip Galvanized coatings and Zinc Rich Paint
here.
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