-------
II.8.3 COIL COATING
II.8.3.1 INDUSTRY DESCRIPTION
II.8.3.1.1 General Description [2-18]
The coil coating industry consists of at least 73 plants pro-
cessing approximately 1.2 billion square meters of painted coil
each year. Facilities vary in size and corporate structure,
ranging from independent shops to captive operations. Indepen-
dent shops obtain untreated coil, conversion coating chemicals,
and paints, and produce a wide variety of coated coil. Typical-
ly, the annual production at these plants is low compared to that
from the captive coating operations. The captive coil coating
operation is usually an integral part of a large corporation
engaged in many other kinds of metal production and finishing.
The coil coating sequence, regardless of basis material or con-
version coating process used, consists of three functional steps:
cleaning, conversion coating, and painting. Basically, there
are three types of cleaning operations used in coil coating, and
they can be used alone or in combinations. These are: mild
alkaline cleaning, strong alkaline cleaning, and acid cleaning.
There are four basic types of conversion coating operations, and
the use of one precludes the use of the others on the same coil.
These are: chromating, phosphating, use of complex oxides, -and
no-rinse conversion coating. Some of these conversion coating
operations are designed for use on specific basis materials. The
painting operation is performed by roll coating and is independent
of the basis material and conversion coating. Some specialized
coatings are supplied without conversion-coating the basis" ma-
terial. For example, Zincrometal is a specialized coating con-
sisting of two coats of special paints that do not require con-
version coating. In this process, coils are cleaned and dryed,
and then receive two coats of the special paints.
The selection of basis material, conversion coating, and paint
formulation is an art based upon experience. The variables that
are typically involved in the selection are appearance, color,
gloss, corrosion resistance, abrasion resistance, process line
capability, availability of raw materials, customer preference,
and cost. Some basis materials inherently work better with cer-
tain conversion coatings, and some conversion coatings work
Date: 9/25/81 II.8.3-1
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better with certain paint formulations. On the whole, however,
the choice of which combination to use on a basis material is
limited only by plant and customer preferences.
The following subsections describe the coil coating processes in
more detail.
Cleaning
Coil coating requires that the basis material be clean. A
thoroughly clean coil assures efficient conversion coating and a
resulting uniform surface for painting. The soils, oils, and
oxide coatings found on a typical coil originate from rolling
mill operations and storage conditions prior to coil coating.
Such substances can stop the conversion coating reaction, cause a
coating void on part of the basis material, and cause the produc-
tion of a nonuniform coating. Cleaning operations must chemi-
cally and physically remove these interfering substances without
degrading the surface of the basis material. Excessive cleaning
can roughen a basically smooth surface to a point where a paint
film will not provide optimum protective properties.
Aluminum and galvanized steel are prone to develop an oxide coat-
ing that acts as a barrier to chemical conversion coatings.
However, these oxide films are easier to remove than rust and,
therefore, require a less vigorous cleaning process. A mild
alkaline cleaner is usually applied with power spray equipment to
remove the oxide coating and other interfering substances. The
cleaning solutions normally used consist of combinations of sodi-
um carbonates, phosphates, silicates, and hydroxides. These com-
pounds give the solution its alkaline character and emulsify the
removed soils. Soap and detergents may be added to the solution
to lower the surface and interfacial tension. A good cleaning
solution also rinses easily. Solutions may be made stronger with
the addition of more sodium hydroxide.
Steel, unless adequately protected with a film of oil subsequent
to rolling mill operations, has a tendency to form surface rust
rather quickly. This rust on the surface of the metal prevents
proper conversion coating. A traditional method of removing rust
is an acid applied by power spray equipment. The spraying action
cleans both by physical impingement and the etching action of the
acid. The power spray action is followed by a brush scrub which
further removes soil loosened by the acid. The brush scrub is
followed by a strong alkaline spray wash which removes all traces
of the acid and neutralizes the surface.
A spray rinse follows the alkaline cleaning step. Spray rinsing
is conducive to the fast line speeds which make coil coating an
economical coating procedure. The spray rinse physically removes
Date: 9/25/81 II.8.3-2
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alkaline cleaning residues and soil by both the physical impinge-
ment of the water and the diluting action of the water. The
rinse water is usually maintained at approximately 66°C (150°F)
to keep the coil warm for the subsequent conversion coating
reactions and to help the rinsing action. The rinsing action
prevents contamination of the conversion coating bath with clean-
ing residues which are dragged out on the strip and that could be
subsequently deposited in the conversion coating solutions. The
rinsing step also keeps the surface of the metal wet and active,
which permits faster conversion coating film formation.
The no-rinse conversion coating and the Zincrometal processes
require a coil that is clean, warm, and dry. For these proces-
ses, a squeegee roll and forced air drying are used to assure a
clean, dry coil following alkaline cleaning and rinsing.
Conversion Coatings
The basic objective of the conversion coating process is to
provide a corrosion-resistant film that is integrally bonded
chemically and physically to the base metal and that provides a
smooth and chemically inert surface for subsequent application of
a variety of paint films. The conversion coating processes
effectively render the surface of the basis material electrically
neutral and immune to galvanic corrosion. Conversion coating on
basis material coils does not involve the use of applied electric
current to coat the basis material. The coating mechanisms are
chemical reactions that occur between solution and basis ma-
terial.
Four types of conversion coatings are normally used in coil
coating:
• Chromate conversion coatings,
• Phosphate conversion coating,
• Complex oxides conversion coatings, and
• No-rinse conversion coatings.
Chromate conversion coatings, phosphate conversion coatings, and
complex oxide conversion coatings are applied in basically the
same manner. No-rinse conversion coatings are roll applied and
use quite different chemical solutions than phosphating, chromat-
ing, or complex oxides solutions. However, the dried film is
used as basis for paint application similar to phosphating,
chromating, and complex oxide conversion coating films.
Chromate conversion coatings can be applied to both aluminum and
galvanized surfaces but are generally applied only to aluminum
surfaces. These coatings produce an amorphous layer of chromium
Date: 9/25/81 II.8.3-3
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chromate complexes and aluminum ions. The coatings offer unusu-
ally good corrosion-inhibiting properties but are not as abrasion
resistant as phosphate coatings. Scratched or abraded films
retain a great deal of protective value because the hexavalent
chromium content of the film is slowly leached by moisture, pro-
viding a self-healing effect. Under limited applications, these
coatings can serve as the finished surface without being painted.
If further finishing is required, it is necessary to select an
organic finishing system that has good adhesive properties.
Chromate conversion coatings are extremely smooth, electrically
neutral, and quite resistant to chemical attack.
Chromate conversion coatings for aluminum are carried out in
acidic solutions. These solutions usually contain one chromium
salt, such as sodium chromate, or chromic acid and a strong oxi-
dizing agent such as hydrofluoric acid or nitric acid. The final
film usually contains both products and reactants and water of
hydration. Chromate films are formed by the chemical reaction of
hexavalent chromium with a metal surface in the presence of
"accelerators", such as cyanides, acetates, formates, sulfate,
chlorides, fluorides, nitrates, phosphates, and sulfamates.
Chromate conversion coating requires that the basis material be
alkaline-cleaned and spray-rinsed with warm water. The cleaning
and rinsing assures a clean, warm, wet surface on which the con-
version coating process takes place. Once the film is formed, it
is rinsed with water followed by a chromic acid sealing rinse.
This latter rinse seals the free pore area of the coating by
forming a chromium chromate gel. Also, the sealing rinse more
thoroughly removes precipitated deposits that may have been
formed by hard water in previous operations. The coil is then
subjected to a forced air drying step to assure a uniformly dry
surface for the following painting operation.
Phosphate conversion coatings provide a highly crystalline, elec-
trically neutral bond between a base metal and paint film. The
most widespread use of phosphate coatings is to prolong the
useful life of paint finishes. Phosphate coatings are primarily
used on steel and galvanized surfaces but also can be applied to
aluminum. Basically, there are three types of phosphate coatings;
iron, zinc, and manganese. Manganese coatings are not used in
coil coating operations because they are relatively slow in
forming and, as such, are not amenable to the high production
speeds of coil coaters.
The remaining two phosphate coatings are applied by spraying or
immersing the coil, with the major difference between them being
the weight and thickness of the dried coating. Iron phosphate
coatings are the thinnest and lightest and generally the cheap-
est. Iron phosphate solutions are applied chiefly as a base for
Date: 9/25/81 II.8.3-4
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paint films. Spray application of iron phosphating solutions is
most commonly used. The coating weights range from 0.22 to
0.86 g/m2.
Zinc phosphate coatings are quite versatile and can be used as a
base for paint or oil, as an aid to cold forming, to increase
wear resistance, and to provide rustproofing. Zinc phosphate
coatings can be applied by spray or immersion with applied coat-
ing weights ranging from 1.08 to 10.8 g/m2 for spray coating and
from 1.61 to 43.1 g/m2 for immersion coating.
Phosphate coatings are formed in the metal surface, incorporating
metal ions dissolved from the surface. This creates a coating
that is integrally bonded to the base metal. In this respect,
phosphate coatings differ from electrodeposited coatings, which
are superimposed on the metal. Most metal phosphates are insol-
uble in water but soluble in mineral acids. Phosphating solu-
tions consist of metal phosphates dissolved in carefully balanced
solutions of phosphoric acid. As long as the acid concentration
of the bath remains above a critical point, the metal ions remain
in solution. Accelerators speed up film formation and prevent
the polarization effect of hydrogen on the surface of the metal.
Accelerators commonly used include nitrites, nitrates, chlorates,
and peroxides. Cobalt and nickel nitrite accelerators are the
most widely used and develop a coarse crystalline structure. The
peroxides are relatively unstable and difficult to control, while
chlorate accelerators generate a fine sludge that may cause dusty
or powdery deposits.
After phosphating, the coil is passed through a recirculating hot
water spray rinse. The rinsing action removes excess acid and
unreacted products, thereby stopping the conversion coating reac-
tion. Insufficient rinsing could cause blistering under the sub-
sequent paint film from the galvanic action of the residual acid
and metal salts.
The basis material is then passed through an acid sealing rinse
comprised of up to 0.1% by volume of phosphoric acid, chromic
acid, and various metallic conditioning agents, notably zinc.
This solution seals the free pore area of the coating by forming
a chromium chromate gel. Also, this acidic sealing rinse more
thoroughly removes precipitated deposits formed by hard water in
the previous rinses. Modified chromic acid rinses have been used
extensively in the industry. These rinses are prepared by re-
ducing chromic acid with an organic reductant to form a mixture
of trivalent chromium and hexavalent chromium in the form of a
complex chromium chromate.
Complex oxide conversion coatings can be applied to aluminum and
galvanized surfaces but are generally applied to only galvanized
surfaces. The nature of the film and the chemical and physical
Date: 9/25/81 II.8.3-5
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reactions of its formation are a function and a reinforcement of
the naturally occurring protective oxide coating that is found on
galvanized surfaces. The physical properties of the complex
oxide conversion coating film are comparable to those of chromate
conversion coating films and phosphate conversion coating films.
Complex oxide film is formed in a basic solution while the films
described earlier are formed in an acidic solution. Complex
oxide conversion coating reactions do not contain either hexa-
valent or trivalent chromium ions. However, the sealing rinse
contains much greater quantities of hexavalent and trivalent
chromium ions than do the sealing rinses associated with phos-
phate conversion coatings and chromate conversion coatings.
Recent developments in chromate conversion coating solutions have
resulted in a solution that can be applied to cold rolled steel,
galvanized steel, or aluminum without the need for any rinsing
after the coating has formed on the basis material. The basis
material must first be alkaline cleaned, thoroughly rinsed, and
forced-air dried prior to conversion coating. The conversion
coating solution is applied with a roll mechanism used in roll
coating paint. Once the solution is roll coated onto the basis
material, the coil is forced-air dried at approximately 66°C.
The no-rinse solutions are formulated in such a way that once a
film is formed and dried, there are no residual or detrimental
products left on the coating that could interfere with normal
coil coating paint formulations.
Although no-rinse conversion coatings currently represent a small
proportion of the conversion coating techniques that are used,
they offer several advantages, including fewer process steps in a
physically smaller process line, higher line speeds, application
of a very uniform thickness by roll coating rather than spray or
dip coating, and reduction of waste treatment requirements
because of the reduced use of chromium compounds. Disadvantages
include roll coating mechanism wear possibly reducing quality,
the closer coordination of entire line that is needed, difficulty
in adaptation, and the hazardous organic acids content of the
no-rinse conversion coating chemicals.
Painting
Roll coating of paint is the final process in a coil coating
line. Roll coating is an economical method to paint large areas
of metal with a variety of finishes and to produce a uniform and
high quality coating. The reverse roll procedure for coils is
used by the coil coating industry, and allows both sides of the
coil to be painted simultaneously.
The paint formulations used in the coil coating industry have
high pigmentation levels (providing hiding power), adhesion, and
Date: 9/25/81 II.8.3-6
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flexibility. Most coatings of this type are thermosetting and
are based on vinyl, acrylic, and epoxy functional aromatic poly-
ethers, and some reactive monomer or other resin with reactive
functions, such as melamine formaldehyde resins. Also, a variety
of copolymers of butadiene with styrene or maleic anhydride are
used in coating formulations. These coatings are cured by oxida-
tion mechanisms during baking, similar to those that harden
drying oils.
After paint application, all coils are cured in an oven. Curing
temperatures depend upon basis material, conversion coating,
paint formulation, and line speed. Typical temperatures range
from about 93°C to a maximum of about 454°C. Upon leaving the
oven, the coils are quenched with water to induce rapid cooling
prior to rewinding.
The quench is necessary for all basis materials, conversion coat-
ings, and paint formulations. A coil that is rewound when too
warm will develop internal and external stresses, causing a pos-
sible degradation of the appearance of the paint film and of the
forming properties of the coil. The volume of water used in the
quench often has the largest flow rate of all of the coil-coating
processes. However, the water is often circulated to a cooling
tower for heat.dissipation and reuse.
The finished coils are used in a variety of industries. The
building products industry utilizes prefinished coils to fabri-
cate exterior siding, window and door frames, storm windows and
storm gutters, and various other trim and accessory building
products. The food and beverage industries utilize various types
of coils and finishes to safely and economically package and ship
a wide variety of food and beverage products. Until recently,
the automotive and appliance industries have made limited use of
prefinished coils. These industries have relied on post assembly
finishing of their products. Recently, the automotive industry
has begun using a cold rolled steel coil coated on one side with.
a finish called Zincrometal. This coating is applied to the
under surfaces of the exterior automobile sheet metal to protect
them from corrosion. The appliance industry uses prefinished
coils in constructing certain models of refrigerator exteriors to
provide a finished product that minimizes the costly and labor-
intensive painting operation after forming.
Coil coating operations are located throughout the country,
usually in well established industrial centers. Compared to some
other industries, coil coating operations are not physically
large. Coil coating operations use large quantities of water and
are often a significant contributor to municipal waste treatment
systems or surface waters. In addition, the curing ovens from
coil coating operations are a source of air pollution in the form
of reactive hydrocarbons.
Date: 9/25/81 II.8.3-7
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Table 8.3-1 presents an industry summary of the coil coating
industry.
TABLE 8.3-1. INDUSTRY SUMMARY [2-1]
Industry: Coil Coating
Total Number of Subcategories: 3
Number of Subcategories Studied: 3
Number of Dischargers in Industry:
• Direct: 36
• Indirect: 54
• Zero dischargers: 0
II.8.3.1.2 Subcategory Description [2-18]
The primary purpose of subcategorization is to establish group-
ings, within the coil coating industry, such that each group has
a uniform set of effluent limitations. While subcategorization
is based on wastewater characteristics, a review of the other
subcategorization factors reveals that the basis material used
and the processes performed on these basis materials are the
principal factors affecting the wastewater characteristics of
plants in the coil coating industry. The coil coating industry
is therefore divided into the following three Subcategories:
• Coil coating on steel,
• Coil coating on zinc coated steel (galvanized), and
• Coil coating on aluminum or aluminized steel.
The following subsections describe the above Subcategories.
Coil Coating on Steel
Fifty-nine facilities in the coil coating industry were surveyed
for process type and pollutant levels. Of these, 38 plants are
in the coil coating on steel subcategory. Ten facilities coat
steel alone and the remaining 28 coat a combination of steel
coils and coils from the other Subcategories. The production
rate is approximately 85,000 m2/hr. Operations used at these
facilities include acid cleaning, strong alkaline cleaning,
phosphating, no-rinse conversion coating, roll coating, and
Zincrometal coating. Water usage rates for the general opera-
tions at steel coating facilities are listed in Table 8.3-2.
Date: 9/25/81 II.8.3-8
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TABLE 8.3-2.
SUMMARY OF WATER USAGE RATES FOR THE COIL
COATING INDUSTRY BY SUBCATEGORY [2-18]
Operation
Number
of
plants sampled
Water use, L/m2
Range
Mean
Steel
Cleaning
Conversion coating
Quenching
All operations
11
8
4
13
0.04
0.04
2.0
0.37
7.3
0.76
5.7
13
1.9
0.43
4.0
4.5
Galvanized
Cleaning
Conversion coating
Quenching
All operations
10
10
5
12
0.17 -
0.03 -
0.44 -
0.65 -
8.8
0.98
5.1
8.4
1.9
0.49
2.7
3.6
Aluminum
Cleaning
Conversion coating
Quenching
All operations
12
12
9
15
0.21 -
0.18 -
1.2 -
0.26 -
2.0
1.8
3.5
5.8
0.97
0.56
2.3
2.5
Coil Coating on Zinc Coated Steel (Galvanized)
Within the 59 plants surveyed, 17 coil coat on galvanized steel
with a production of approximately 60 x 103 m2/hr. Only two
facilities produce coated galvanized steel alone. Operations
used at the galvanized coating facilities include mild alkaline
cleaning, phosphating, chromating, complex oxide treatment,
no-rinse conversion coating, roll coating, and Zincrometal
coating. Table 8.3-2 above also presents water usage data for
the general operations at galvanized coating facilities.
Coil Coating on Aluminum
Thirty-nine of the facilities coil coat on aluminum with a pro-
duction rate of 90 x 103 m2/hr. Nineteen facilities coat only
aluminum coils. The aluminum coating facilities use mild alka-
line cleaning, phosphating, chromating, complex oxide treatment,
no-rinse conversion coating, and roll coating. Water usage rates
for the general processes in this subcategory are listed in Table
8.3-2 above.
Date: 9/25/81
II.8.3-9
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II.8.3.2 WASTEWATER CHARACTERIZATION [2-18]
Water is used in virtually all coil coating operations. It pro-
vides the mechanism for removing undesirable compounds from the
basis material, is the medium for the chemical reactions that
occur on the basis material, and cools the basis material follow-
ing baking. Water is the medium that permits the high degree of
automation associated with coil coating and the high quality of
the finished product. The nature of coil coating operations, the
large amount of basis material processed, and the quantity and
type of chemicals used produces a large volume of wastewater that
requires treatment before discharge.
Wastewater generation occurs for each basis material (steel, gal-
vanized and aluminum) and for each functional operation (clean-
ing, conversion coating, and painting). The wastewater generated
by the three functional operations may (1) flow directly to a
municipal sewage treatment system or surface water, (2) flow
directly to an on-site waste treatment system and then to a muni-
cipal sewage treatment system or surface water, (3) be reused
directly or following intermediate treatment, or (4) undergo a
combination of the above processes.
Coil coating operations that produce wastewater are characterized
by the pollutant constituents associated with respective basis
materials. The constituents in the raw wastewaters include ions
of the basis material, oil and grease found on the basis mate-
rial, components of the cleaning and conversion coating solu-
tions, and the paints and solvents used in roll coating the basis
materials. The following tables present wastewater characteri-
zation data for each subcategory. The data presented are the
results of verification analysis of the industry. Prior to veri-
fication sampling, a screening program was conducted to identify
the presence or absence of the 129 priority pollutants. Those
pollutants detected in screening at a concentration greater than
10 yg/L were further studied in the verification analysis. The
minimum detection limit in the verification analysis for pesti-
cides was 5 yg/L and for all other toxic pollutants, 10 yg/L.
Any value below its detection limit is presented in the follow-
ing tables as BDL, below detection limit.
Tables 8.3-3 through 8.3-6 present raw wastewater characteriza-
tion data for each general process in each subcategory and for
the wastewater in each subcategory when combined into a single
representative stream as a whole. Table 8.3-7 presents raw
wastewater flow data for each subcategory.
Date: 9/25/81 II.8.3-10
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