Chemical Engineering December 2011 - 48

Engineering Practice
tank has been erected. These tanks
may need to undergo a chemical process
to protect the coating while the
tank is being erected. Once a tank has
been erected in the field, there may be
areas of the tank that are extremely
difficult to sand blast or fully prepare
for field coating. Field applied coatings
are sprayed on using scaffolding to
reach the walls and roof. Even " missing
the smallest spots " leaves exposed
areas open to corrosion. The thickness
of coatings applied in the field cannot
be controlled as closely as is possible
under ideal factory conditions.
Weather and other environmental
factors - dust, humidity, temperature
and wind - also affect the quality and
timeliness of the curing process in the
field. Because of the variables in this
type of application, you should ensure
there are adequate quality control
measures listed in the specification by
the tank manufacturer, and consider
third party inspections.
The application process
Surface preparation is the first step
in the application process. It involves
both cleaning the surface and achieving
a profile on the steel. Regardless
of how good the coating is, it must
have a clean surface with an adequate
blast profile to perform properly, in
the same way that the most advanced
skyscraper must have a solid foundation
to rest upon.
The most common method of surface
preparation is abrasive blasting. Different
abrasives can be used, including
steel grit or shot, garnet, coal slag, and
even items such as walnut shells. The
type and size of grit should be chosen
in accordance with the coating manufacturer's
recommendations. Abrasive
blasting may either be done automatically
(typically by a centrifugal blast
machine) or manually (air blasting). In
the case of air blasting, it is critical that
the air used for blasting is dry and free
of contaminants. The surface should be
inspected prior to the coating application
to ensure that the proper cleanliness
and profile have been achieved.
Powder coatings are applied using
powder spray guns that apply an electrostatic
charge to the powder particles
when they are atomized (Figure 3).
This charge draws it to the grounded
FIGURE 4. An accurate epoxy-coating test
includes the use of a high-voltage spark
detector, which shows not only the smallest
missed spots in the coating, but also spots
where the coating does not meet minimum
thickness requirements
FIGURE 3. Epoxy powder coatings are
applied using powder spray guns, which
apply a charge to the powder particles
when they are atomized. The charge
draws particles to the grounded part.
The charged particles are preferentially
drawn to the thinnest areas and edges,
resulting in a uniform coating
part. The advantage of electrostatic
application is that the charged particles
are drawn preferentially to the
thinnest areas, which results in a
more uniform coating. They are also
drawn to edges, which are typically
difficult to coat.
In the typical porcelain enamel processes
such as glass-fused-to-steel, a
liquid-slurry of glass frit and proprietary
chemicals is applied in multiple
coats covering the sheets and exposed
edges. A chemical bond is created during
firing in the furnace where the
steel and the glass coating meet.
It is important that the coating
thickness be measured throughout the
application process to ensure that the
manufacturer's recommended thickness
is achieved. If the coating is too
thin, the steel may not be adequately
covered and pinpoint rusting can soon
occur. If the coating is excessively
thick, it may crack from internal
stresses or a loss of flexibility.
Testing the coating
Because the coating is such an important
consideration, it is equally important
that the quality-control tests
used are up to standard. A wet sponge
detector is commonly used by tank
manufacturers. A wet sponge with
an electrical charge is moved across
the surface of the cured coating. The
metal behind the coating also receives
a charge. Wherever the wet sponge
touches a " missed spot " or bare metal, a
48 CHEMICAL ENGINEERING WWW.CHE.COM DECEMBER 2011
current is completed and the " holiday "
is made known by either an audible or
visual indicator. However, this test is
accurate only for bare spots. It does not
test for thinly applied coatings.
A more accurate test - the high
voltage spark detector - also shows
where coating does not meet minimum
thickness requirements (Figure
4). Both a wand and the tank wall receive
a charge and where the coating
is bare or less than minimal thickness
required, a spark appears showing
the exact location of the holiday. Determining
thin spots in the coating is
vital because a thinly applied coating
may allow for premature corrosion of
the tank wall.
The safety factor
Coatings play a role in safety. It is unsafe
to use an old storage tank to hold
a product that it was not designed and
tested for. If you are considering recycling
a used tank for storing a new
product, be sure to first have the tank
professionally evaluated for safety and
corrosion resistance. Also keep in mind
that the coating for tanks used for storage
of food should meet U.S. Food and
Drug Administration (FDA) guidelines,
or equivalent local standards.
Moving and expansion
If you would ever consider moving or
relocating a tank, think about the coating.
Tanks featuring factory-applied
coatings usually are moveable without
a follow-up recoat, as panels are individually
coated in the factory and easily
taken apart and reassembled. Coatings
that are field applied will tear or
break when the tank is dis-assembled
and pieces are moved, creating seams
where corrosion can start. It is important
to consider whether you plan to
expand your tank before it is built if at
all possible. A bolted tank with factory
applied coating to individual panels,
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Chemical Engineering December 2011

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