coating materials and application methods

CONVERTING TO WATERBORNES
BY RONALD KONIECZYNSKI
NORDSON CORP., AMHERST, OHIO

Just as government regulations concerning gasoline mileage in the ‘70s helped

spur today’s improved automobile engines, so government regulations concern-
ing VOC emissions from coating lines have spurred the development of some
remarkable waterborne and water-based coating materials. Now manufactur-
ers need to know how to convert their operations to use these new materials
efficiently. This means that they need to know what application equipment is
suitable and how best to use it to apply waterbornes.
A likely first question is “What is the best way to apply a waterborne material?”
The answer is “There is no best way.” The process that works best in a particular
application with a solvent-based coating will usually continue to be the best after
converting the application to a waterborne coating. The dynamics of getting
material from the applicator to the part are similar whether the material contains
mostly solvents or mostly water.

ATOMIZER SELECTION
Most coating material applicators break the material up into fine droplets or
particles, which are then carried through the air to the part being coated. The
process of breaking up the material is called atomization, and the equipment
that does the atomization is called an atomizer. Typical atomizers are air spray
guns, rotary atomizers, and disks. None of these is “best” for all waterborne
applications. Instead, the shape of the part being coated, the coverage required,
and the production rate determine the best atomizer for a particular application
using waterbornes, just as they do with solvent-based materials.
To illustrate the importance of part configuration on atomizer selection,
consider a simple box-like part, open on one end, requiring paint on both the
inside and outside surfaces. The outside of the box might best be painted with a
soft spray using electrostatics to get good part coverage, high transfer efficiency
(TE), and good wrap. A rotary atomizer with electrostatics would be a good
choice for the outside of the box.
The rotary atomizer and electrostatics would be a poor choice, however, for
the inside of the same box because the Faraday cage effect caused by the electro-
statics and the walls of the box would actually keep most of the paint out of the
box. A better choice for the inside of the box would be an airless spray atomizer.
Airless spray uses the momentum of the paint particles to get the paint to the
part, rather than electrostatic attraction.
The point is that a manufacturer who has spent a lot of time perfecting the
coating application process for solvent-based material should stick with that
process when converting to waterbornes, if the latest technology and good
equipment are already in place and good TE is being obtained.
Sometimes a particular waterborne coating formulation may need to be
modified slightly to accommodate the atomizer. For example, an emulsion
may tend to separate when subjected to severe centrifugal force on a spinning
rotary atomizer cup.
Does this mean that you don’t have to change anything in order to convert
to waterbornes? No, it doesn’t. Even though the basic application process may
not change, some of the specific pieces of equipment used for that process
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may not be suitable for waterborne materials. Metal parts may corrode. Seals
may swell or leak.

CONSTRUCTION MATERIALS
Waterbornes rust plain steel and in some cases attack aluminum. Even stain-
less steel parts can be damaged by some formulations. For example, 400 series
stainless steel can dissolve over time in contact with a highly acidic formula-
tion. On the other hand, parts made from 316 stainless steel hold up well with
most waterbornes.
This means that at least some of the application equipment will need to be
replaced when a system is converted to waterbornes. Piston pumps made of plain
or alloy steel have to be replaced with pumps made of stainless steel. Pipes and
distribution systems need to be made of corrosion-resistant materials such as
stainless steel. Atomizers should contain only stainless steel or plastic wetted
parts. Parts made from aluminum will perform satisfactorily for some water-
borne materials, but will corrode quickly in contact with others. Some water-
borne formulations can even become “explosive” in contact with aluminum.
Seals in atomizers and pumps need to be changed if they are not compatible
with the waterborne material. There is no single best seal material for water-
bornes because the formulations vary so much. In some cases, the seals in equip-
ment used for solvent-based coating materials are also suitable for waterbornes.
For example, Buna-N is suitable for some solvent-based paints and is also a good
choice for many waterbornes.
One caution about reusing equipment from a solvent-based coating operation
for waterbornes, a surprising amount of “dirt” from the old coating material can
turn up in the new coating material after the conversion to waterbornes. A few
filters in the fluid lines can prevent a lot of downtime due to plugged nozzles
and orifices.
As with the coating application process and most of the equipment, the physi-
cal plant does not necessarily need to change in order to convert to waterbornes.
Often the formulation of the waterborne material can be tweaked a little to
accommodate the facility. For example, the drying time for a waterborne primer
may need to be adjusted for the time available before the color coat is applied.
The TE can drop after converting to waterbornes even though the application
process is the same and much of the equipment is unchanged. This is especially
true if the application process includes atomizing the material.

ELECTROSTATICS
All spray guns and centrifugal applicators, like rotaries and disks, atomize the coat-
ing materials and propel the atomized particles toward the part being coated. With
these devices, all the particles that are not aimed directly at the part will miss it and
be wasted. The waste can be minimized and TE improved if the atomized coating
material is given an electrical charge that is opposite in polarity to the charge on
the part being coated. Opposite electrical charges attract and some of the material
that would miss the part entirely instead gets drawn to it by the electrical forces.
The technique is called electrostatics and has been used for years by painters and
coating applicators to improve the TE of their operations.
Until recently, most coating materials were solvent based and did not conduct
electricity readily. This made it easy to use electrostatics with these materials by
simply placing a high-voltage electrode in the coating material at the atomizer
nozzle. The coating material picks up a static charge of electricity as it is atomized.
Waterborne coating materials conduct electricity much more readily than
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solvent-based materials, however, and the electrical charge drains off down
the paint hoses. None of the charge gets into the particles of atomized coating
material, so there is no electrostatic attraction between the particles and the
part being coated. Without electrostatics, TE drops to unacceptably low levels.
The challenge in converting to waterbornes is getting comparable TE with
these conductive materials as with the nonconductive solvent-based materials
they replace. This means finding a way to get the electrostatic voltage into the
atomized particles rather than letting it drain away through hoses and equip-
ment made conductive by the waterborne coating material. Any equipment that
contacts an electrical ground, such as a pipe or a damp concrete floor, provides
an electrical pathway that drains off the electrostatics. This means that the elec-
trostatics won’t work in a waterborne system unless all the wetted equipment is
isolated from potential grounds.

SYSTEM ISOLATION
Waterborne systems are commonly isolated in one of three ways. (1) Complete
isolation of all equipment that contacts wet coating material. (2) Isolation of
the charging electrode from the wet coating material. (3) Isolation of only the
atomizer and its feed hose by using a voltage blocking device. Each method has
advantages and disadvantages.

Complete Isolation
The advantage of completely isolating the entire application system is that the
coating material can be directly charged with electrostatic voltage. If isolation is
successful, the resulting TE will be the highest possible for the specific applica-
tion. The exact TE that can be achieved in a specific application depends on the
part geometry, line speed, application equipment, and other factors, the same
as it would with a solvent-based coating material.
To isolate a complete waterborne system, every pump, tank, pipe, atomizer, or
other piece of equipment that sees wet coating material must be set on a plastic
table or hung from a plastic rod or stuffed in a plastic pipe sleeve. Suitable com-
mon and inexpensive plastic materials for this purpose include polyvinyl chlo-
ride (PVC), polyethylene, and polypropylene. Teflon, some nylons, and Delrin
are also good isolation materials for high voltage, but are relatively expensive.
Dry air is one of the best isolation materials. A 12-in. air gap will isolate equip-
ment charged with electrostatic voltage, except in cases of extreme humidity. Air
has some advantages over plastic as an isolator for an electrostatic system. A paint
spill down a plastic table leg can make it conductive. Humidity in a thick coating of
dust on a plastic pipe can make it conductive. The disadvantage of air as an isola-
tor is that it is an easily penetrated barrier between a charged part and a grounded
part — too easily penetrated by personnel or by loose hoses or other equipment.
Unfortunately, it’s almost impossible to design an isolated system that won’t
accidentally ground out, and a system that grounds out is less efficient. In one
case, for example, an 18-in. long, hollow PVC table leg provided a direct path to
ground because it was set on a concrete floor and humidity from the concrete
made the inside of the leg conductive. That particular short took a full week of
troubleshooting to find and correct.
Besides being inefficient, isolated systems can be dangerous because they
can store too much electrical energy. All the equipment that gets wetted with
electrically charged coating material stores electrical energy, much like a giant
capacitor. All that stored energy gets discharged if the system gets shorted out. If
the system is big enough, and stores enough electrical energy, an operator can get
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injured by shorting it out accidentally by touching a charged hose or atomizer.
It is impossible to draw a definite line that says, “A system this small is safe,
and a system that big is dangerous.” Trying to define a safe electrical shock is
like trying to define a safe height from which to fall. For example, a shock itself
might only be annoying, but the victim might be so startled by it that it results
in a bump on the head or an injury in some other way. Although a safe system,
with regard to storage of electrical energy, may be a contradiction in terms,
some guidance regarding the size of a “probably unsafe” system would be use-
ful. Unfortunately, no regulations directly applicable to electrically charged
waterborne systems are available.
By making some assumptions about the meager data that is published,
extrapolating to the 70,000\100,000 V range used for electrostatics, and plug-
ging the resulting voltage and capacitance values into the standard equation for
storing electrical energy in a capacitor, the following can be developed:

Maximum\Energy = 3.5 Joules = CV2

where C=Capacitance (farads)
Rearranging: CMAX =7/V2

where the voltage is the maximum available from the power pack.
This equation can be used as an indicator of the potential for a given isolated
system to pose a serious shock hazard. The capacitance of the system, as mea-
sured with a capacitance bridge or a suitable capacitance meter, must be less
than the value of CMAX if there is a possibility of accidental human contact, which
could result in an electrical shock. For example, if a 100,000-V electrostatic
paint system has a capacitance G700 picofarads, caging and interlocks should
be considered for operator protection.
For comparison purposes, a single 55-gal. drum and 200 ft of 3/8-in. inner
diameter hose, all set 12 in. above a ground, can have between 450 and 900 pico-
farads of capacitance. This means that a typical paint system, which has much
more hardware, would almost certainly exceed the maximum capacitance value
and could store potentially dangerous levels of electrical energy.
The storage of electrical energy can be reduced by lowering the electrostatic volt-
age. The voltage term is squared in the equation for energy storage in a capacitor.
This means that a given system at 100,000 V will store four times the energy that it
would at 50,000 V. At the lower voltage, not only will the system be safer, but guns
and cables will last much longer before breaking down electrically. Perhaps an even
more compelling reason for lowering the voltage is to maximize TE. The maximum
TE for most waterborne coating materials occurs between 40,000 and 60,000 V. By
comparison, the maximum TE for a less conductive solvent-based material can be
90,000 V or more. Handguns present a special problem when a coating application
system is converted from solvent-based materials to waterbornes.
An isolated electrostatic system for waterbornes can have multiple auto-
matic atomizers or it can have a single handgun. It cannot have both, nor can
it have more than one handgun. National Fire Prevention Association (NFPA)
regulations dictate that the electrostatic voltage to any handgun must turn off
when the trigger is released. Since all the atomizers in a waterborne system are
connected electrically by their fluid hoses, the voltage remains “on” to an idle
handgun as long as it is “on” to any atomizer in the system. This means that a
handgun cannot be used with electrostatics if there are other atomizers in the
system, and without electrostatics it is impossible to achieve the maximum TE.
To summarize, completely isolated systems have the potential to allow the
maximum TE for a given application because they allow the coating material
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to be directly charged with electrostatics. In practice, that potential is rarely
achieved unless the application system is very small because it is difficult to keep
the electrostatic charge isolated.
Fully isolated systems can store too much electrical energy and become dan-
gerous. To prevent operator injury, such systems need to be caged and equipped
with interlocks to prevent access while the system is operating. Unfortunately,
this means that even minor maintenance to the equipment is impossible while
any part of the system is operating at high voltage because all the equipment is
electrically connected by the fluid hoses. This is also why only one handgun can
be permitted in a completely isolated system.

Indirect Charging
Indirect charging avoids many of the problems of completely isolated systems,
but at a price. Indirect charging systems charge the coating material between the
nozzle of the atomizer and the part, rather than at the atomizer. This is done by
placing the high-voltage electrode in the air stream near the nozzle but not in
direct contact.The coating material particles pick up a charge after they leave the
atomizer.
Because the high voltage never directly contacts the application equipment,
there is no opportunity for the charge to drain away down paint hoses. On the
other hand, any charge inadvertently imparted to the application system drains
away harmlessly to ground because the system is not isolated from ground. In
fact these systems can, and should, be intentionally grounded to prevent storage
of electrical energy.
There is little capacitive storage of electrical energy in an indirect charged
electrostatic system so any electrical hazard is greatly reduced. This means that
safety caging and interlocks can be less intrusive, or eliminated completely. With
handguns no longer connected electrically by their hoses, there is no need to limit
the number of handguns in a particular application system.
Several coating application equipment manufacturers offer atomizers specially
designed for indirect charging. These devices position the electrostatic electrode
away from the coating material stream so that there is no direct electrical contact
between the application equipment and components charged with high voltage.
Some conventional atomizers can also be retrofit with indirect charging apparatus,
making the conversion to waterbornes easy and relatively inexpensive.
The downside of indirect charging is lower than optimum TE. Indirect charging
does improve TE over comparable nonelectrostatic systems. Unfortunately, tests
prove that the TE with indirect charging is less than the TE that can be achieved by
direct charging in any given application — that means using the same applicators,
coating material, part shape, etc.
The difference can be considerable, up to 40% improvement in TE in extreme
cases. Even the best indirect charged systems rarely achieve TEs within 10 percent-
age points of what is possible for the same application but using direct charging
for the electrostatics.
To maximize TE, the coating material should be directly charged with electro-
static voltage, but to minimize shock hazards and operational problems, the size
of the charged parts of the system should be minimized. This can be achieved
by using a voltage block at each atomizer. Each atomizer then becomes a mini-
isolated system with no electrical connection to any other atomizer in the system.

Voltage Blocks
Voltage blocks are devices that allow coating material to pass through to the
atomizer but prevent voltage from leaking back the other way. They allow coat-
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ing material to flow from the grounded pumps or kitchen to charged atomizers,
yet block voltage from leaking back from the atomizers to the pumps or kitchen.
This means that the hardware in the pump house and distribution system can be
virtually the same as for a conventional solvent-based system, or for an indirect
charged waterborne system.
Since the primary advantage of voltage block technology is that it limits the
amount of hardware at high voltage, it is important to install these devices as close
to the atomizer as possible. The connecting hoses between the voltage block and
the atomizer are at high voltage, so keeping them short minimizes both the capaci-
tance and the opportunity for accidental grounding. A voltage block for one atom-
izer is compact, requiring about as much space as a small electrical control box,
so it can be mounted inside the spray or ventilation booth close to the atomizer.
The mini-isolated systems created by voltage blocks do not have the problems
found in large isolated systems because less hardware is charged with electrical
energy. Capacitance is greatly reduced, making the system inherently safer. Safer
systems mean easier access to the inside of the spray booths. Often a simple
guard rail and warning sign can replace elaborate caging and interlocks. Voltage
leakage problems are minimized, since only the atomizer and a short hose are
charged, making it easy to keep the TEs up to a high level. By isolating atomizers
from each other, mini-isolated systems have some unexpected advantages. First,
the NFPA limitation concerning handguns no longer applies. Each handgun is
independently isolated from every other handgun so the voltage to idled guns
can be turned “off.”
In fact, spraying waterbornes with a handgun and voltage block can be easier
than spraying the old solvent-based material with the handgun. Solvent-based
material is charged at the gun barrel so a high-voltage cable to the gun is required.
Since waterborne coating material conducts electricity, however, it can be directly
charged at the voltage block and the cable to the gun can be eliminated. With the
cable gone, the gun feels lighter and the hose bundle flexes more easily.
Even automatic atomizers, such as rotaries or disks, require less maintenance
if the coating material is charged at the voltage block rather than at the atomizer,
as it was when spraying solvent-based material. This is because the high-voltage
cables last longer when they don’t get flexed over and over by the motion of the
gun mover or robot.
A second unexpected advantage of making each atomizer into a mini-isolated
system is that every atomizer can operate at a different voltage, or at zero voltage,
as desired. For example, in-plant experience might show that the rotary atom-
izers in a paint line run best at 60,000 V, but the handguns perform better at
45,000 V. With voltage blocks, the handguns and the rotary atomizers can run
at different voltages, yet all can be supplied from a common paint distribution
system.
Finally, in an application system for waterbornes and using voltage blocks,
any atomizer in the system can be shut down and repaired or changed out, even
though the other atomizers are operating at high voltage. The ease of access to
production application equipment is comparable between a voltage-blocked
waterborne system and the solvent-based coating material system it replaces.

CONCLUSION
An often unstated goal when converting a coating application system to water-
bornes is to disrupt “the way it’s done now” as little as possible, particularly if
the existing system has good equipment and is performing well. That goal is not
out of reach because the existing process, and much of the existing equipment,
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can often be used for waterbornes. Usually only the atomizers will need to be
modified for waterbornes, or replaced with atomizers specifically designed to
handle waterborne materials. The remaining equipment and distribution system
can be reused unless made of materials that will corrode in waterbornes or be
damaged by exposure to them.
Well-engineered conversions from solvent-based coating materials to water-
bornes result in the highest possible operating efficiency at low cost and with
maximum operator safety. The operating cost, in terms of TE, should be about
the same as that of a good solvent-based paint system. To get high TE, electro-
statics must operate at peak efficiency. This means directly charging the material
with electrostatic voltage, but limiting the hardware that gets charged.
Voltage-blocking devices confine high electrostatic voltage to only the atom-
izer and hoses to the atomizer. This means that the rest of the coating material
application system can be the same or similar to the system before the conver-
sion is made. The electrostatics will still operate at high efficiency because the
coating material can be directly charged so the TE will be comparable before and
after conversion. Because system capacitance, or the capacity to store electrical
energy, can be controlled to “safe”’ levels, safety issues with the converted sys-
tems are not prohibitive. In other words, a voltage-blocked waterborne system is
as close as possible to the solvent-based material system it replaces with a coating
material that conducts electricity.
To summarize, here is how to convert an application system from solvent-based
coating materials to waterbornes: (1) Reuse the existing process and hardware if
it is up to date and performing well for the existing solvent-based system. Change
components where materials are not compatible with waterbornes. (2) Turn each
atomizer into a mini-isolated system by installing a voltage block in the coating
material hose, as close to the atomizer as possible. Directly charge the material
for maximum TE. (3) Lower the voltage to maximize TE, extend equipment life,
and reduce shock hazard. (4) Take advantage of the fact that waterbornes conduct
electricity. Remove the electrostatic cables from the atomizers and charge at the
voltage blocks. Cables will last longer and the guns will move easier.

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coating materials and application methods
AUTODEPOSITION OF ORGANIC
COATINGS
BY THOMAS C. JONES
HENKEL SURFACE TECHNOLOGIES, MADISON HEIGHTS, MICH.

Auto deposition of organic coatings refers to a chemical process for depositing

paint coatings onto metal surfaces. This process resembles electrodeposition in
that the objects to be coated are submerged in an aqueous bath containing ion-
ized organic materials; however, in autodeposition, the coating is produced by a
chemical reaction between the metal surface and the bath constituents as opposed
to electrodeposition where the coating is produced by the electrolysis of water due
to an imposed electric current. After autodeposition, the wet film is water rinsed
and cured by application of heat to produce an inert, corrosion-resistant coating.

COATING MECHANISM
The first step in the coating mechanism for the autodeposition process is the
chemical reaction between the metallic surface and the inorganic constituents
of the coating bath. The coating deposition step of the autodeposition process
involves the controlled destabilization of an aqueous polymer latex dispersion,
which is negatively charged, by the positive ions generated at the surface of the
metal by the inorganic chemical reaction.
The components of an autodeposition coating bath include a weak acid
(hydrofluoric acid, HF) in the range of 0.2% to 0.3% by volume, an anionically
stabilized latex and pigment dispersion (latex/pigment), and chelated ferric ion
in solution (FeF3). The total solids content of the bath is less than 10%, and the
coating solution has the viscosity of water.
The chemical reactions that result in an auto deposition coating are as fol-
lows. For iron dissolution, the major contributor is

2 Fe F3 + Fe 3 Fe+2 + 6 F-1,

and a minor contributor is

2 HF + Fe Fe+2 + H2(g) +2 F-1;

deposition occurs when

Fe+2 + (latex/pigment) Fe(latex/pigment).

Iron that is not entrapped in the wet film is converted to FeF4-1 by the addition
of an oxidizing agent. Since FeF4-1 does not react with the anionically stabilized
latex/pigment dispersion, the process is sludge-free.
As the autodeposited film builds, the diffusion of reactants to the surface is
slowed, and the rate of film deposition decreases. This self-limiting mechanism
results in a final coating that is extremely uniform and conforms to the underly-
ing surface. All areas exposed to the coating bath become coated. This feature
of the autodeposition process is important since even enclosed areas will be
protected against corrosion, as long as the solution has wet the surface. Typical
coating thicknesses are about 15 to 30 mm (0.6 to 1.2 mils).
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FILM PROPERTIES
Analysis of a cured autodeposited film shows the presence of iron throughout
the organic layer. It is believed that the positively charged iron reacts with anionic
sites (e.g., sulfonate groups and carboxyl groups), formulated into the polymer
backbone, to effect the deposition. No additional cross-linking agents are required.
Although many organic emulsion polymers can be autodeposited, there
actions are the same; however, the properties of the cured film depend on the
chemical nature of the polymer. For example, polyvinylidene chloride (PVDC)
latex emulsions are the most widely used at present in the commercial practice
of autodeposition. Films deposited show excellent resistance to the penetration
of moisture and oxygen to the base metal and, hence, offer superior corrosion
resistance, as well as the excellent hardness, formability, and adhesion charac-
teristic of paint films using this type of resin.
Another commercial autodeposition process utilizes acrylic resin polymers to
produce films that are resistant to high temperatures (>400°F) in the presence
of aggressive fluids (e.g., alkaline polyglycols).
Carbon black pigments, which are effectively encapsulated by the polymers and
thus deposit simultaneously with the resin, are highly effective. A nonpigmented
version of the polyvinylidene chloride latex is commercially available as a primer
for subsequent top coating. While other colored pigments have been successfully
evaluated on a laboratory basis, none are at present commercially available.
Zinc and zinc-alloy coated steels can be effectively painted by autodeposition
by varying the process chemistry.

PROCESS SEQUENCE
Commercial autodeposition systems employ movement/transfer of the work
package from stage to stage by either a continuous conveyor or by an indexing
hoist. Conveyorized systems offer the advantage of assured agitation (to bring
fresh reactants to the metal surfaces) due to the movement of the work through
the coating tank (as well as the cleaning and rinsing tanks). Hoist-operated
systems conserve space due to decreased transfer distance.
A typical process sequence is shown below. (Contact times in each stage vary
from 30 seconds to 2 minutes, with the exception of the oven where 15 to 30
minutes is common).
Stage 1: Spray alkaline cleaning
Stage 2: Immersion alkaline cleaning
Stage 3: Immersion water rinse (plant water)
Stage 4: Immersion (or spray) deionized water rinse
Stage 5: Autodeposition (immersion)
Stage 6: Immersion water rinse (plant water)
Stage 7: Immersion sealing rinse
Stage 8: Cure

Cleaning
Good cleaning is essential to successful autodeposition. Any residual soils, which
hinder the solubilization of metal ions, can prevent or reduce coating formation.
Although most organic soils (e.g., drawing compounds and rust preventive oils)
are readily removable by alkaline cleaners, inorganic soils (e.g., weld spatter, scale,
and rust) often require cleaning in an acidic material.
Immersion cleaning is usually required to ensure adequate soil removal from
recessed areas such as tube interiors and box sections, which are inaccessible by
spray. To protect the chemicals in the tank from excessive buildup of soils, a
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(smaller) spray cleaner stage is used beforehand. A slight overflow of this tank
results in decreased cleaner loss.

Rinsing
Plant water rinsing is employed to remove residual cleaner carried through on
the workpieces (and racks). This is followed by a rinse with deionized water to
remove any hard water salts. The effect of salt buildup over time results in gradu-
ally decreasing coating film thickness per unit of immersion time.

Autodeposition
Autodeposition occurs by the reactions given above. The control parameters are
paint solids (gravimetric determination), acid level (free fluoride concentration),
and oxidation/ reduction potential (in millivolts), which is proportional to the
ratio of FeF3 to Fe+2.

Postcoating Rinsing
A plant water rinse (usually immersion, but low-pressure spray applications
have been used) removes traces of unreacted latex. A final sealing rinse contains
chemicals that react with any soluble iron in the wet film to eliminate porosity
after the film is cured.

Cure
The drying of a polyvinylidene chloride autodeposited coating is simply the
removal of water from a coalesced wet film. No solvents are present. Commonly
practiced parameters are 15 to 30 minutes at convection oven temperatures
of 210 to 230°F. Shorter times (5 to 10 minutes) may be achieved on simple
(i.e., line-of-sight to all surfaces) parts by the use of medium-intensity infrared
radiation.
Acrylic coatings require a higher temperature (320–350°F) range for complete
cross-linking.

FEATURES OF AUTODEPOSITION
A phosphate pretreatment process is not required for autodeposition, minimiz-
ing requirements for capital and floor space. Dragout is also minimized because
of low paint bath viscosity. There is no coating buildup on hangers because a
cured autodeposited film is inert to further reaction. Furthermore, since the
coating process relies on chemical reaction, coating of all hidden or recessed
areas occurs with even coverage. The coating does not pull away from sharp
edges, coats evenly over machined surfaces (e.g., threaded fasteners), and is free
from runs, sags, orange peel, and similar defects. This effect is enhanced by the
low redispersibility of the wet film, which allows water rinsing to remove excess
supernatant prior to oven cure. Very low maintenance is required and energy use
is reduced because of the elimination of the phosphate pretreatment process.
Finally, autodeposition is environmentally benign with low or no VOC emission
or heavy metal effluent. No fire hazards are present.
Since the autodeposition reaction is diffusion controlled, the supernatant film
on freshly coated pieces lifted from the bath continues to deposit paint solids.
As a result, there is minimal loss (i.e., transfer efficiency averaging 95%) of solids
to the water rinse following the coating bath. This effect is further enhanced by
the absence of any external force (e.g., electric current), which would increase the
concentration of solids at the immersed surface of the work. Since bath solids are
maintained at only 5% to 7% by weight, any dragout effects are minimal.
Autodeposition is a versatile means of coating complex parts and assem-
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blies. For example, in the automotive industry, the following parts are coated
by autodeposition: jacks, brake backing plates, fan blower housings, suspen-
sion components, headlamp mounting assemblies, intermediate steering shafts,
leaf springs (individual and as assemblies), power brake booster housings, seat
frames, seat tracks, and miscellaneous brackets, connectors, and fasteners. Some
nonautomotive examples include office furniture components (e.g., drawer slides,
file frames), appliance accessories, hand tools, exercise equipment, and patio
furniture.

EQUIPMENT CONSIDERATIONS
All of the stages for an autodeposition process, with the exception of the coat-
ing tank, are identical to those employed in other finishing processes and are
not discussed here. For this reason, retrofit of an existing coating system to
autodeposition can be relatively simple. The following comments pertain to
the coating tank alone:
Materials of Construction: The coating tank is a mild steel tank lined with an
acid-resistant material. To avoid damage to the liner by parts or racks fall-
ing into the tank, it is advisable to use materials with high impact resistance.
Traditionally, three-ply (soft, hard, soft) rubbers have been employed.
Agitation: A gentle agitation of the coating bath is provided by properly spaced
mixers with AC variable frequency drive (0.5-hp motors typical). Agitation is
required only when workpieces are in the paint tank and may be decreased or
stopped during downtime since no paint settling will occur in short periods (e.g.,
2–3 weeks).
Cooling Equipment: The temperature of the bath should be controlled in the range
of 68 to 72°F. Heating/cooling coils in the bath are required (1) as a safeguard
against accidental heat/cold carry-in; or (2) if ambient plant conditions warrant
(heating or cooling). Even when a bath is used to process metal at a high rate,
there is no measurable temperature increase due to exothermic chemical reactions.

207
coating materials and application methods
POWDER COATING EQUIPMENT
BY NICHOLAS P. LIBERTO
POWDER COATING CONSULTANTS DIV. OF NINAN INC., BRIDGEPORT, CONN.

There are many ways to apply powder coating materials; however, the material
that is to be applied must be of a compatible type. For instance, if the application
method is fluidized bed, the powder coating material must be a fluidized bed
grade. Conversely, if the method of application is electrostatic spray, the powder
material must be an electrostatic spray grade.
Once the material is correctly selected, the application method is chosen by
part design and production goals. There are two forms of application methods:
fluidized bed application and spray application. These vary as widely as the
applications they suit.

FLUIDIZED BED
This application method was the first one used to apply powder coating materi-
als. It is still used on many applications where the cured-film thickness is above
5.0 mils. Typical items are wire products, electrical bus bars, etc.
The fluidized bed application method can be performed in two ways. One way
is the nonelectrostatic fluidized bed. This process requires preheating the part
so that powder will melt and adhere to it. The hot part is placed into a fluidized
bed of powder for coating. The amount of powder that is applied to the part
is a function of how hot the part is and how long it is in the bed. It should be
obvious that tight film-thickness control is not of primary concern when this
method is used, as the total coating thickness often exceeds 10 mils.
To gain more control of film thickness on the part, with a fluidized bed sys-
tem, the principles of electrostatics are introduced. As shown in Fig. 1, the part

Fig. 1. Electrostatic fluidized bed for powder coating.

208
is transported above the fluidized bed and the powder is attracted to it. The part
requires no preheating prior to being placed above the bed. Powder is attracted
to the part by an electrostatic charge on the powder particle. This electrostatic
charge is developed in an electrostatic field either above or in the fluidized bed.
Film thickness on the part now is controlled within tighter tolerances not
only by the amount of time the part is in the fluidized bed but also according
to how much electrostatic charge is on the powder particle. Sometimes, heat
still is used in this process to overcome Faraday cage problems caused by part
configuration. This process routinely applies powder from 5 to 10 mils thick.
Electrostatic fluid bed application is used for coating electrical motor arma-
tures. These require a high dielectric strength coating with close film-thickness
control to allow the wire to be wound properly.

SPRAY APPLICATION
Applying powder coating with electrostatic spray equipment is broken down
into two types. In each case electrostatics must be used to attract powder to the
part. There is no mechanical attraction or adhesion to hold powder to the part
as seen in liquid spray systems. The two types of electrostatic spray equipment
are corona-charged spray guns and tribo-charged spray guns.

Corona Guns
This device uses an electrostatic generator to create an electrostatic field between
the gun and a grounded part. Powder is sprayed through the field, picks up an
electrostatic charge, and is attracted to the part. The amount of charge that
is transferred on the surface of the powder is a function of electrostatic field
strength and the amount of time the powder particle is in the field. Also of
importance is the surface area of the powder particle, as finer powder particles
hold less electrostatic charge. The following equations (see Fig. 2) best explain
how the powder is charged:

Field Strength: E=V/d

Charge on Particle: Q= 1/2 CEt2
Notice that some factors are more important that others. For instance, elec-
trostatic field strength is directly proportional to applicator electrode voltage.
Also, the distance between the part and the applicator (sometimes called the
target distance) will directly affect electrostatic field strength.
The charge on the powder particle (which causes the attraction) is most affect-
ed by the amount of time the particle is in the field (by its square). The time and
field strength will determine how much powder is attracted to the part (i.e., first
pass transfer efficiency). The time the powder particle is within the electrostatic
field is most easily controlled by adjusting the velocity of the powder pumped
through the gun, or applicator, and reducing the speed of the applicator motion.
It is a known fact that systems that use reduced powder velocity and slow gun
motion will provide the best coating efficiency with the least effort.
The powder coating process is most often used to apply a charged dielectric
material (powder coating) and onto a conductive (grounded) part. However,
electrostatic powder coating on nonconductive materials (i.e. plastics, rubber,
glass, etc.) can be performed using a conductive primer or aiding powder attrac-
tion by heating the surface to be coated. Additionally, electrostatic charging of

209
conductive materials (i.e., blended metallic powders) can be difficult since they
can short-circuit the applicator’s charging circuit. However, most equipment
manufacturers provide electrodes outside the powder path to overcome this
problem.
Both positive and negative polarity electrostatic guns are available from
most manufacturers to provide efficient charging of widely divergent coating
materials. It is worth noting that 98% of all applicators used in powder coating
operations are negative polarity devices.
Code requirements insist that certain protection circuits be part of the system.
Among these are current limitation to control arcing and grounding of all equip-
ment and products that are coated to dissipate stored charges. System interlocks
are required for automatic equipment. Guidelines for this equipment are listed
in National Fire Protection Association Code (NFPA) 33.

Tribo Guns
Tribo-charged spray equipment uses the principle of frictional electrostatic
charging. This type of charging is best explained by the following analogy: When
you shuffle your shoes on a carpet in the winter, you create an electrostatic
charge that is stored in your body. This charge is usually dissipated when you
come into contact with a ground, such as a light switch. This phenomenon will
only occur in a dry (not humid) environment. This is why we are not bothered
by static electricity in the humid summertime, but only in the dry air of winter.
Tribo-charge spray equipment will direct the powder stream through a path
that it will tumble and rub against a dielectric surface within the applicator,
yielding a frictional electrostatic charge on the powder particle. This path is
accomplished by lengthening the powder route through the spray equipment in
either a straight, radial, or oscillating path. The amount of electrostatic charge
that builds up on the surface of the powder particle is a function of several vari-
ables, including (1) the amount of time the powder particle is subjected to the
frictional charging apparatus; (2) surface area of the powder particle; (3) dryness
of air the powder is transported with or comes into contact with; and (4) the
type of resin material from which the powder is made.

Fig. 2. Principle of corona charging.

210
 Fig. 3. Powder delivery system.

Controlling these variables is important to assure that the powder particle will
be properly charged. Remember: if the powder is not charged, it will not adhere
to the part unless the part is hot enough for the powder to stick on contact.
The amount of electrostatic charge that typically is developed by this appa-
ratus is less than that produced by corona equipment. The polarity of the tribo
charge is a function of the material being sprayed and the material that it is
rubbed against. If the same two materials are used, the polarity will always be
the same.
Tribo-charge applicators can often be used to overcome Faraday areas on
difficult-to-coat parts, as there is no electrostatic field used to charge the pow-
der. This flexibility, however, is often overshadowed by the additional process
and coating materials controls that are required to ensure successful coating.

Powder Bells
This device uses an air turbine to rotate a conical cup used to atomize the
powder coating. Powder is pumped to the cup where the rotational forces cause
complete powder atomization. The feed system used to support this device is
similar to that of spray guns. These devices employ the corona charging method,
described earlier in this article.
Powder bells are capable of dispersing a large quantity of powder coating
over a large area. Therefore, the typical applications for this device are large flat
components, such as appliances and automobile bodies.

POWDER DELIVERY
All spray application equipment requires a delivery system (see Fig. 3). This
211
 Fig. 4. Hopper designs.

delivery system consists of a feed hopper, a powder pump, and a powder feed hose.
The feed hopper can be one of two types (see Fig. 4). The first type is called a
gravity feed hopper. As the name suggests, this feed hopper uses gravity to move
powder to the powder pump located at the bottom. This hopper usually is coni-
cal in shape to funnel powder to the pump. Sometimes a mechanical stirrer or
vibratory assist is used to maintain an even powder flow. Frequently, without a
mechanical assist, powder will bridge across the bottom of the funnel causing
uneven feed to the pump. Since there is no air mixed with the powder in the hop-
per, this device is often employed when spraying blended metallic powders that
can be stratified within a fluidized hopper.
The second type of powder feed hopper uses a fluidized bed. It is the same as the
fluidized bed system described previously. A compressed-air supply is connected
to the plenum chamber below the fluidizing plate. The fluidizing plate causes the
air to fluff powder in the hopper to a state resembling water. Now the powder can
be drawn out by the powder pump. Since powder is mixed with compressed air
from the plenum, the powder within this device is very homogeneous in nature.
Powder pumps are mounted on the hopper and are connected to a pick-up tube
to draw powder out of the hopper. These pick-up tubes usually are positioned an
appropriate distance into the fluidized bed to assure that the turbulence usually
present on the surface of the fluidized plate is not drawn up into the powder
pump. This turbulence can cause inconsistent powder feed to the applicators.

Box Feeders
Powder equipment manufacturers also provide methods of pumping powder
coatings directly from their shipping containers (box or bag) to the spray gun.
This method is called the box feeder and utilizes a tilted vibrating table to sup-
port the box of powder. A powder pump connected to a pick-up tube is inserted
in the lowest portion of the box. A compressed air jet is employed at the end of
212
 Fig. 5. Powder pump.

this tube to assist powder flow into the tube. Powder is then pumped directly
from the box to the spray gun without the need of a feed hopper. This approach
makes color change cleanup quick and easy, as only the pick-up tube, pump,
and hose need to be cleaned. Changing the powder box completes the color
change task.

PUMPS
Most powder pumps are designed to work by the venturi principle. Compressed
air is directed perpendicular to the venturi pickup, causing a differential in
pressure, or vacuum, that siphons powder out of the feed hopper or box feeder.
When the powder enters the compressed air stream, it is pushed through the
powder hose toward the applicator. An additional compressed air supply is
introduced at the point where the powder enters this air stream (see Fig. 5) to
dilute the powder and increase its velocity. Increasing powder velocity ensures
that the powder stays within the air stream as it proceeds through the hose,
reducing surging or pulsing problems. Surging occurs when the powder lays at
the bottom of the hose until enough air pressure builds behind it to push it out
with a burst. Both air supplies have check valves to force the air to go through
the powder hose, allowing independent control of both powder quantity and
speed through the feed hose.
Powder hose can be made from several materials, including urethane, vinyl,
and certain rubber compounds. Hose diameter and length are critical. Diameter
is dictated by the powder pump used; it always should match the manufacturer’s
recommendation. Length always should be as short as possible to reduce back
pressure to the powder pump. This reduces surging of the powder stream to the
gun. Avoid bends and kinks in the hose routing.
The more powder you pump using venturi style pumps the faster it travels
through the electrostatic field. Consequently, transfer efficiency will be lower
at higher feed rates. Applications requiring highly controlled powder flow at
a wide range of output rates use high density - low pressure (HDLP) powder
213
 Fig. 6. Gun motion devices.

pumps. These devices deliver a column of powder to the applicator without

having to mix it with compressed air. Reducing the compressed air within the
powder stream decreases the velocity of the powder delivered from the applica-
tor, slowing powder speed, increasing powder density, and eliminating aerody-
namic issues that may cause coating difficulties on box-shaped parts. Since these
pumps employ significantly smaller diameter feed hose, the hose is much easier
to clean with compressed air purging, making these pump the preferred choice
for “fast color change” systems.

GUN MOTION
Automatic spray devices are often accompanied by some ancillary equipment
used to produce spray gun motion. Gun-motion equipment can be broken down
into four general categories: oscillators, reciprocators, multi-axis machines and
robots.
Each of these gun-motion systems has a different design and is used to fill a
specific coating requirement; however, all have one common feature. They are
designed to move the spray gun(s) in one or more planes to coat a larger area
than a fixed spray gun. Thus, the number of spray guns required to coat a given
area can be reduced. This makes for a more efficient and economical system
design

Oscillators & Wagglers

One type of gun-motion device is called an oscillator. This design is different
from other movers in that it usually has a fixed stroke and speed. Some units
have adjustment of these parameters, but they cannot be used while the machine
is running. The main component of this type of equipment is an eccentric wheel
214
 Fig. 7. Multi-axis movement.

and lever as shown in Fig. 6. The motor rotates the eccentric wheel. The lever,
which is attached to the wheel at some distance from the center, will translate
this rotary motion to a vertical motion.
Stroke length is determined by the position at which the lever is attached to
the eccentric wheel and by the diameter of the wheel itself. It can be adjusted by
locating the lever at different points on the wheel radius. Speed is dictated by
the motor and gear reducer used in the design. Sometimes, there are clutches
and adjustable belt sheaves that will provide some speed adjustment; however,
neither speed nor stroke adjustment can be changed while the unit is running.
Wagglers (radial oscillators) pivot the gun through an arc, where straight
oscillators provide vertical gun motion in a straight line. Gun-to-part target
distance is affected with radial oscillators, while straight oscillators will not
have this problem.

Reciprocators
Reciprocators (see Fig. 6) use a variety of electronics to control both stroke and
speed. In these machines, the mechanical linkage between the motor and guns
is fixed; therefore, speed and stroke control must be adjusted electrically. These
adjustments are sometimes made at the control panel and sometimes at the
unit itself. For instance, stroke adjustment can be made by moving electrical
limit switches in the unit or by adjusting an electronic feedback loop variable
in the control panel.
Speed control is accomplished by a variety of methods depending upon the
type of motor used. For instance, those designs that use a DC motor will provide
speed control by varying voltage to the motor. Reciprocators that use AC motors
have variable speed-control circuits to adjust speed. Both types allow adjustment
during operation. This offers some flexibility over the oscillator design when
different stroke lengths and speeds are required to coat different parts during
the production cycle.
215
 Fig. 8. Conventional powder booth system.

Multi-Axis Machines
Both oscillators and reciprocators provide movement in one plane only. Multi-
axis machines were developed to provide increased coating flexibility and meet
a demand for total automation. Multi-axis machines have been successful in
eliminating some or all of the manual touch-up necessary on some products.
Though costly, this increased automation often will pay for itself by providing
consistent part coating with minimal, if any, touch-up.
The multi-axis machine design is made up of two or three reciprocators that
will move the gun(s) in two or three planes. The convention used to label the
three axes of motion is as follows (see Fig. 7).

X = parallel to the conveyor travel

Y = up and down
Z = in and out

The design of these units is the same as reciprocators with respect to the
control of speed and stroke adjustment; however, because the units must track
parts moving along the conveyor, the addition of a programmable logic control-
ler (PLC) is required.
The PLC will accept inputs from encoders (that determine conveyor speed)
and photo cells or limit switches (that determine part position). This informa-
tion is used to determine at what speed the multi-axis machine must run to
track the part and when the multi-axis motion program is to be executed. The
purpose of this complex tracking and motion system is to provide gun dwell
time and powder pattern direction.

Robots
Most robots provide six axes of gun motion by adding wrist movement. Robotic
machines can be electrically or hydraulically driven. Because of their cost and
complexity, these units are rarely used in powder coating systems. Another
detriment to these units is that hydraulic fluid is not something you want to
have around powder. Also, powder coating material is very abrasive and can play
havoc with hydraulic seals and pistons.

POWDER RECOVERY
A powder booth/recovery system must accomplish two specific goals: contain
the powder overspray within the booth and remove the powder from this con-
tainment air so that it can be reused or disposed of properly. Powder booths are
designed using several filtration techniques to separate the overspray powder
216
 Fig. 9. Cartridge booth system.

from this containment air stream depending upon if the system will reclaim this
powder or employ a spray-to-waste strategy, the number of reclaimed powders,
and the time available to perform the color change.

Cyclone Booth System

A cyclone powder booth system, as pictured in Fig. 8, is made up of a spray
booth, cyclone(s), a cartridge collector, and possibly ductwork.
The spray booth can be made of metal, plastic, or composite sandwich
designs. Metal booths provides strength and durability but attract more powder
that will prolong color change time. Plastic will allow more light into the booth
and will attract less powder, reducing color change time. Composite sandwich
designs offer strength and attract the least power, significantly improving color
change time. All powder booths should provide a smooth interior to facilitate
easy and thorough cleaning.
Ductwork connection(s) can be at one of several locations. The preferred
method is to locate the ductwork connection in the base of the booth as this
provides a down-draft air flow inside the booth helping to keep it clean.
The booth may have devices, such as baffles, to help control air flow within
the booth, touch-up openings to provide access for manual spraying, and gun
slots to provide access for automatic equipment.
The cyclone is designed to separate most of the powder from the airflow
before entering the filtration section. This has several benefits. First, air entering
the filter is “precleaned,” which will lower the loading on the filter media. This
translates to longer filter life. Second, the powder collected in the cyclone can
be easily recycled. Since the cyclone is a cleanable device, color change is attain-
able without additional equipment. Multiple cyclones are used when air flow
is so high that one cyclone isn’t practical for a given plant ceiling height. Twin
cyclones are used in parallel before the filtration section. Cyclone efficiency can
vary by manufacturer and design with some systems delivering in excess of 90%
of the powder into the reclaim device.

217
 The filtration section used with a cyclone booth is a cartridge collector, given
its name for the cartridges used to separate powder from the air flow. These
paper cartridges are cleaned with a “back pulse” of compressed air to shock
the powder from the cartridge surface. The cartridges will separate most of the
powder out of the air flow from the booth (up to 99% efficiency). These are not
cleanable devices for color change. The blower fan that produces the air flow in
the booth typically is located on the clean air side of the filtration device. Final
filters are used after the fan to remove powder particles, down to 0.3 micron in
size, before the air is returned to the work environment.
All of these devices—booth, cyclone, collector, fans, and absolute filters—can
be connected by ductwork. The velocity of air within this ductwork usually is
above 4,000 fpm and the ductwork is designed to promote laminar flow to
assure “self-cleaning” during operation.
Some powder booth manufacturers have taken the approach of reducing
the ductwork in this type of booth. This design has numerous smaller cyclones
attached directly to the powder booth wall. The booth airflow enters the cyclones
directly and without ductwork. These cyclones are much smaller than those used
in standard cyclone booths, allowing for simpler cleanup. The blower, filter pack,
and final filters are downstream from, and attached to, the cyclones, allowing
the air to be returned directly to the plant.

Cartridge Booth System

The cartridge booth system (see Fig. 9) answers the same technical needs that
all powder recovery systems must address: safe containment and separation of
powder coating overspray. In a cartridge booth system, this is accomplished
by filtration of powder from the containment air using a cartridge collector
attached to the booth. There are no external filtration devices (or ductwork to
connect them) with this system.
The cartridge collector is usually located in the wall of the booth (side draft)
or in the base of the booth (down draft). The powder-laden air flow enters
the collector. The air passes through the cartridge filter and the powder is
deposited on the filter surface. Periodically, cartridges are back-pulsed with
compressed air to shock the powder from their surface and deposit it in the
collector base. Powder in the base is pumped to a reclaim stand for reuse or
to a container for disposal.
The cartridge filter pack can be removed from the blower pack for col-
or change. A separate cartridge pack is required for each recoverable color.
Cartridges are made of a paper filter media. The blower pack houses the blower
fan and filter assembly. The blower is on the clean-air side of the cartridges. Air
from this powder booth system is returned to the plant.
The booth may have touch-up openings and/or gun slots depending upon
the application for which it is used. The booth is typically of metal construc-
tion, though some manufacturers prefer plastic. This type of powder booth
system is known for its compactness. Safety is another important benefit to
this design. Since there are no “enclosed” devices the need for explosion vent-
ing is eliminated.

218
coating materials and application methods
POWDER SPRAY GUNS
BY ALAN J. KNOBBE
NORDSON CORP., AMHERST, OHIO

Two basic types of electrostatic powder guns are used for the spray application
of powder coating materials. They are corona guns and tribo guns, where corona
and tribo refer to the predominant process used in the guns for electrostatically
charging the powder particles.

CORONA GUNS
Corona charging guns work by bombarding powder particles sprayed from
the gun with charged particles called ions. The corona charging process is
illustrated in Figure 1.
The corona charging process begins with a potential (or voltage) applied to
one or more electrodes at or near the front of the gun. A high-voltage generator
is used to produce this voltage of up to 100,000 V. As the voltage on the electrode
is increased, an electric field is produced between the gun and the grounded
workpiece. When the electric field in the vicinity of the electrode reaches a
strength of about 30,000 V/cm, the field is strong enough to break down the
air in the vicinity of the electrode. This electrical breakdown of air results in the
creation of charged molecules or ions in the form of a continuous discharge
known as a corona discharge. Powder particles exiting the gun travel near the
electrode where they are bombarded by these ions and accumulate a charge.
Both the charged ions and the charged powder particles are influenced by the
electric field between the gun and the workpiece and tend to follow the electric
field to the part, as illustrated by lines in Figure 1. Ions that do not become
attached to powder particles in flight are known as excess ions or free ions.
Typically, only a few percent of the ions generated actually become attached to
powder particles in flight. Some powder particles may be shielded from other
particles in the charging zone and, therefore, do not accumulate a charge. For
these particles, aerodynamic forces resulting from the powder conveying air
might propel them toward the workpiece.

Fig. 1. Corona charging process.

219
 Charged powder particles and excess ions are both deposited on the grounded
workpiece. The charged powder particles are held onto the workpiece electro-
statically until it is transported into an oven for curing. Heat causes the powder
particles to flow together and fuse into a continuous film.
The Pauthenier equation describes the charge, over time, accumulated by a
powder particle exposed to a corona discharge:
Q(t) =Ar2Et/(t+t)
where, A = a constant, which depends on the particle composition, r = particle
radius, E = electric field strength = electrode voltage/gun-to-workpiece distance,
t = time, and t = charging time constant.
The charging performance of a corona gun can be affected by the gun-
to-workpiece distance. Today, corona guns are available that use specially
designed high-voltage generators or gun-control modules to reduce or elimi-
nate this dependency.
The high-voltage generator may be located remotely from the gun in the
gun-control module or, alternatively, part of it may be located within the gun
body itself. When the high-voltage generator is located in the gun-control
module, a high-voltage cable is used to transmit the power to the gun. When
a portion of the high-voltage generator is located within the gun body, a low-
voltage cable is used.
For spraying most types of finishing powders, a negative-polarity voltage is
produced in the high-voltage generator. This results in the powder particles
accumulating a negative charge. Positive-polarity generators are also typically
available as an option and are used primarily for charging nylon powders.
Two basic types of spray heads are available for shaping the powder particles
into a cloud as they exit a corona gun. They are called conical deflectors and flat-
spray nozzles. Conical deflectors shape the powder cloud into a circular, hollow,
dome-shaped pattern. These spray heads can produce a large, low-velocity spray,
360° SD in circumference. They are best for simple-shaped workpieces and can
produce a very high transfer efficiency. Flat-spray nozzles typically have a single
slot through which the powder particles exit. The resulting powder cloud is fan-
shaped from the side, but has a narrow width. These nozzles may have a higher
velocity than a conical spray head and are, therefore, best for spraying parts with
deep recesses and corners.
Many equipment manufacturers design their electrostatic corona powder
spray guns to comply with the different codes governing the manufacture and
use of these products worldwide. Some of the worldwide agencies that test
and issue approvals on these spray guns are Factory Mutual (United States),
Canadian Standards Association, European Committee for Electrotechnical
Standardization (CENELEC), and the Research Institute of Industrial Safety
(RIIS, Japan). In the United States, a local fire marshal would typically look for
“Approved” equipment or compliance with National Fire Protection Association
(N.F.P.A.) Standard 33, Spray Application Using Flammable and Combustible
Materials, before permitting an installation to start production.

TRIBO GUNS
Tribo or triboelectric charging guns charge powder particles as a result of the
intimate contact and subsequent separation of the powder particles from the
gun walls. The word “tribo” comes from the Greek word tribein meaning to rub.
The tribo charging process is illustrated in Figure 2.
220
 Fig. 2 Tribo charging process.

When two different materials are brought into contact, there will be a transfer
of charge from one material to the other in order to eliminate the imbalance of
charge. The magnitude and direction of the charge transfer depends on many
factors including the chemical and electronic structure of both materials. Over
the years, a lot of testing has been done contacting one material against another
and measuring the resulting magnitude and polarity of the charge attained on
each of the materials. The results of one such study, presented in the form of
a so-called “triboelectric series,” are shown in Table I. The farther two materi-
als are away from each other in the series, the greater the triboelectric charge
that should be produced when these two materials come in contact. Note that
polytetrafluoroethylene (PTFE) is shown at one end of the series as being the
strongest electron acceptor and nylon 6/6 is shown at the other end as being
the strongest electron donor. According to this study, all materials that contact

Electron Donor (+)

Nylon 6/6
Cellulose
Cellulose acetate
Polymethyl methacrylate
Polyacetal
Polyethylene terephthalate
Polyacrylonitrile
Polyvinyl chloride
Polybisphenol carbonate
Polychloroether
Polyvinylidene chloride
Polystyrene
Polyethylene
Polyproplene
Polytetrafluoroethylene
Electron Acceptor (-)
Table I. Triboelectric Series

221
PTFE should become positively charged.
PTFE is typically used for the powder contact walls in a tribo gun. Powder par-
ticles, of course, are a composition of resin, pigment, fillers, and possibly other
additives. Experience agrees well with this series in that most finishing powders
become positively charged as a result of their contact with PTFE. Today, most
powder manufacturers formulate powders specifically for tribo guns.
Tribo guns charge powder particles as long as the powder particles contact
the PTFE gun walls. Compared with corona charging, tribo charging is a highly
efficient charging process. The more contacts a powder particle makes with
the walls and the harder it hits them, the greater the charge on the particle.
Theoretically, the gun walls will be left with a charge equal in magnitude but
opposite in polarity to the charge accumulated on the powder particles. This
charge on the gun walls must be conducted away or else it will build up inside
the gun and the gun will stop charging.
Tribo guns are also available with optional powder-contact parts made out
of nylon 6/6. Because of its location in the triboelectric series, nylon parts are
ideal for charging PTFE powders. In this case, the PTFE powder particles become
negatively charged and the nylon gun walls become positively charged.
Since a tribo gun does not rely on a high-voltage generator or an electrode
at high potential, there is only a weak electric field between a tribo gun and the
workpiece. The airflow from a tribo gun thus plays a significant role in trans-
porting the powder particles to the workpiece.
The spray head can also play a significant role in the performance of a tribo
gun. Since the powder particles are already highly charged by the time they
enter the spray head, many spray head designs are possible for optimizing the
transport of the particles in just the right quantity, in the right direction, and
at the right velocity, onto a workpiece.Tribocharging guns can have a very high
transfer efficiency and they can effectively coat the widest variety of workpieces.
They are particularly good at coating difficult-to-coat workpieces, which have
deep recesses and many Faraday cage areas.

222
plating processes, procedures & solutions
ANODIZING WITH ONE UNIVERSAL
ELECTROLYTE USING PULSE-STEP-RAMP
AND RUN PROCEDURES—A GREEN
CHANGE FOR PROBLEM-SOLVING
SITUATIONS
BY FRED CHARLES SCHAEDEL, ANODIC TECHNICAL SERVICES, AFFILIATE OF
ALPHA PROCESS SYSTEMS, WESTMINSTER, CALIF.

Over the years, numerous problem-solving situations and major environmental

considerations have necessitated the development of chrome-free mixed electro-
lytes. When used in conjunction with modified pulse – step – ramp procedures,
these mixed electrolytes (organic sulfuric) are able to meet more demanding Type
IC, II and III anodize requirements, and are very important in these major areas:

QUALITY, EFFICIENCY, ENERGY SAVINGS PLUS ENVIRONMENTAL

AWARENESS
There are some concentrated additives and/or modifiers available along with
specific organic acids which improve the sulfuric acid electrolyte. Many facilities
are now using mixed electrolytes and/or stronger additives because in produc-
tion problem-solving situations their performance characteristics are far supe-
rior to conventional sulfuric acid only baths. Problem-solving improvements
were realized in smut prevention along with increased wear resistance and
hardness, even in thinner films. They can also be used in the same Type II-III
anodize tank over broad concentration and temperature ranges. This was the
reason for the well-known MAE (Multi-Purpose Anodize Electrolyte) Process
developed by Reynolds Metals during the 1960’s in the performance of work
done under a NASA contract (final patent Issued 1970)1. The MAE process
also became the basis for most of the additives being marketed today, with the
exception of those dependent upon coal tar and/or wood products including
lignin wood sulfonates.
Highly concentrated versions of the MAE process were developed represent-
ing major breakthroughs and are being used with sulfuric acid as one universal
electrolyte for type II and III anodize. Pore structure developments from these
concentrated carboxylic acid additives and modifiers contributed to major
improvements in micro finish, color anodize and hardness for type III anodic
coatings. Several patent applications made in 1962-68-69,79, 89 and 2003 refer
to these improvements.2
Research, developments and improvements made for the universal electrolyte
as presented here started in 1960, prompted by problem-solving situations.
These problem-solving situations centered around three areas:

Chemistry : Sulfuric Organic Mixed Electrolytes

Power Supply Electronics : Slow and Fast Pulse
(For difficult alloys _ 2024, 2219, 7050)
Procedures : Pulse-Step-Ramp and Run Methodology

The first problem-solving modifications to the electrolyte were (and still are)
based on endothermic polycarboxylic complex ion chemistry. Later, numerous
223
modifications were made which were manifested in pore structure development.
Finally, these modifications became the major part of the electrolyte—secondary
only in some cases to sulfuric acid—which is in many cases only required due to
older specifications where updates may be long overdue.
Anodize process procedures using pulse anodizing methodology were
investigated and modified by anodizers in the USA and other countries. Several
facilities in the USA and Japan provided information which made this univer-
sal electrolyte perform better in production. Two different types of pulse gave
optimum results:

Variable 0.5-2.0-4.0 sec pulses@ 10 -25% Max Current Density

Faster Anodize – Pore structure development
Prevent pitting and blisters in some cases
Reduces burning for Type II and III on difficult alloys
Promotes dye penetration
Variable 20-30-40 sec pulse @ 25-75% max Current Density
(For commercial and architectural anodize where time and
energy savings are priorities utilizing the recovery effect)

Variable pulse was a major contribution for problem-solving and trouble-

shooting situations involving galvanic pitting on 7000 series alloys.
Pulse anodize methodology was integrated into the system as pulse-step-
ramp with numerous procedural requirements from 1975-2003. Later, extensive
research and development for type IC, including capacitance shunt discharge,
was performed3. Eight of these major procedural requirements are presented
here.
The final development came with the integration of type IC into the uni-
versal chrome-free electrolyte, with step-ramp procedure modifications. Pulse
capacitance shunt discharge made it possible to achieve anodize pore structure
development early during the ramp cycle at 2–3 volts. This final pore structure
development initiated at 2–3 volts became a major factor for type IC Anodize.
Tartaric, Oxalic, Citric and Boric became the preferred acids for all type IC, II
III and the combination Type 123 anodize. Some of the mixed electrolytes used
by anodizers have the following formulation:

2 – 10% Sulfuric Acid

2 – 10% Tartaric/Boric
2 – 10% Concentrated Additives or Modifiers
(Including Amino Polycarboxylic Acids)

This universal electrolyte

and variable pulse along with
eight key procedural require-
ments came to be known as
the “complete spectrum
approach,” beginning in
1990 and continuing to the
present. Finally, selective
brush anodizing was intro-
duced using a super concen-
trated type IC-IIB- III and
Figure 1. One Universal Electrolyte Formulation.
type 123 Anodize.

224
 Numerous anodizers and production facilities have made this electrolyte
possible through their sincere professional technical support over more than
50 years of research in problem-solving situations. The following formulation
represents only one condensed version of one universal mixed electrolyte for
Type IC, II, IIB and III anodize. This electrolyte was also used for selective brush
for all three types of anodize (Type 123).
This universal formulation was developed after reviewing several different
process tanks in production. The ranges marked in red represent a formulation
used for Type IC and heavy thickness hard anodize (10 Mils). The low sulfuric
acid concentration and high tartaric acid concentration made this possible.
The higher concentration Type II – 23 – III HA formulation was used for high
production Type II and III anodize in the same anodize tank. The ACEA is an
Amino Polycarboxylic Electrolyte Additive/Modifier based on a concentrated
MAE Type additive along with amino acids.

PROCESS PROCEDURAL REQUIREMENTS: KEY SECRETS FOR

SUCCESSFUL ANODIZING
After working on numerous problem-solving situations and troubleshooting
for various anodize facilities, various procedural requirements were developed4.
These procedural requirements should be used for preparing Pulse – Step –
Ramp and Run procedures for all types of anodize. Here is how they were pre-
sented at anodize workshops:
Actual process procedures and/or procedural parameters are the real heart of
quality anodize. The key secrets included in this workshop date back 50 years,
in some cases. They still remain the basis for most of the finest hard anodize
procedures available. ATS was probably the first group to bring them all together
for anodize seminars, training programs and workshops.
The process procedures presented in this workshop depend upon the strategic
use of critical factors, requirements and technology which, when used together
as a Complete Spectrum Package (in conjunction with a good electrolyte) dur-
ing the ramp and run cycles will produce consistently excellent results. Critical
requirements developed in part from problem-solving situations are listed below
in the order of precedence and can be seen in real-time graphs at the end of this
discussion.

1. Activation – Voltage Pulse Early During Ramp

(Detailed 3 Stage Pulse – 2012 Paper)
2. Pulse-Step-Ramp (PSR)
3. Increased Dwell Times (3-7)
(For Proper Pore Structure Conditioning)
4. Amperage Decay or Drop Off (ADO)
5. Constant Current Density Ranging (CCDR)
6. Process Time vs. Ampere Hours
7. Real-Time Graphic Observation (monitoring for reproducibility)
8. Current/Voltage Spikes or Deviations/CSD Discovery Leading to APCD

Activation – Voltage Pulse Early During Ramp:

Activating and maintaining activation is very important at the start of (and
early in) the ramp cycle. This is accomplished by increasing the current density
(5 - 10 ASF for Hard Anodize) within 10–30 seconds after positive bus bar con-
tact in the anodize tank. The current should be slow pulsed 25–50% using anodic
discharged surface activation (APCD – if available). Field-assisted dissolution is

225
increased, producing an electropolish action activation at the surface, which can
be helpful for all alloys, but is extremely critical on 7000 series. These alloys tend
to set up corrosion cells very quickly before anodize is even initiated during the
ramp cycle. This manual pulse early in the run is considered a secret technique
and used by many top anodizers.

Pulse – Step – Ramp (Slow PSR Preferred):

Pulse-Ramp technology should be mandatory on all type II – III – 23 and 123
anodizing, in order to achieve maximum quality and efficiency along with
additional energy savings. Pulse-Ramp should always be applied as Slow Pulse –
Step – Ramp initiated during the ramp cycle. Slow pulse specifications have been
proven acceptable for more than 30 years on all aluminum alloys with improved
quality (hardness), efficiency (anodize time) and energy savings (KWH). An addi-
tional slower pulse system, which gives even greater time and energy savings, can
be used on most aluminum alloys, after the specified slow – pulse – step – ramp
and for the duration of the anodize run cycle. The current (amperage) is pulse
ramped in the voltage mode, in 0.1–0.3 volt increments and in 2–15 second steps,
which cannot be confused with the dwell periods. They may be referred to as
small ramp steps and longer dwell periods or steps.

Increased Dwell Times (3-7):

Slow Pulse – Step – Ramp procedures must have dwell times or periods while
running to constant current density. These dwell periods have been overlooked
or thought not to be important due to the lack of a full understanding of the
anodize pore structure development. They are, however, very important during
the ramp cycle as related to final quality, efficiency and energy savings. The
number and length of dwell periods for type II, 23 and type III anodize are as
follows:

Type II Clear Anodize: 2 – 4 Dwell Periods 30 sec – 3 min dwell

Dyed Black: 3 – 5 Dwell Periods 30 sec – 3 min dwell
Type 23 Hard Type II: 3 – 5 Dwell Periods 30 sec – 3 min dwell
Type III Hard Anodize: 4 – 10 Dwell Periods 45 sec – 5 min dwell
Type III Hard 3 – 10 mils: 4 – 10 Dwell Periods 1 – 10 – 20 min dwell

The number of dwell periods along with their times of duration play an
important role in Amperage Decay (ADO), which is the secret to actual coating
formation.

Amperage Decay / Drop Off (ADO):

The Amperage Decay or (ADO) is one of the most important factors/require-
ments necessary for maximum quality, efficiency and energy savings. It must
be used on all Slow Pulse – Step – Ramp – Dwell Periods and throughout the
anodize run cycle. The secret is: amperage must be controlled in the voltage
mode in order to develop amperage decay; do not use the CC control knob!

Constant Current Density Ranging (CCDR):

The current is Pulse – Step – Ramped by increasing voltage (voltage mode) until
a calculated current (amperage) is reached, which represents a running constant
current density dependent upon the square ft area in the tank and the required
current density (ASF) for the alloy. For example – A tank load of 25 sq feet to be
run at 40 ASF would be calculated and set for 1000 Amps (25 x 40 ASF) for the
duration of the run. A steady constant current density should never be used in
226
anodizing. The amper-
age must be allowed
to drop off as it pulses
within a range. To bet-
ter define the proce-
dure process, we will
use the term Constant
Current Density
Ranging (CCDR). For
example – If we are
anodizing at 40 ASF,
we may set the (ADO)
at 10%. If the amperage
is calculated at 1000
(representing 40 ASF)
and set plus 5% (1050)
and allowed to delay
(drop off) 10% to 950 Figure 2. ATS Training Graph—2000 Series Aerospace.
amps before manual or
automatic reset to 1050 Amps, then we are anodizing at an average of 1000
Amps (40 ASF). Also, with the addition of pulse, we may open the CCDR by 25%
while still anodizing at an average Current Density of 40 ASF, with increased
energy savings. This CCD Ranging, (CCDR) or average CCD ranging (AVCDRG)
is extremely important for the production of all type II – III – 23 and 123 anodic
coatings. CCDR run in the voltage mode is a key secret sometimes overlooked
by many anodizers.

Process Time vs. Ampere Hours:

There are at least three ways to process anodize. These include Time, Final
Voltage and Ampere Hours. Most loads are run strictly by time. However,
Ampere hours is actually the best way to run the process, especially when
pulse is used and may vary during the process run. When Pulse Ramp and
Constant Current Density Ranging are used along with time, the addition of
ampere hours will give very accurate final thickness results:
(± 0.0001mil), allowing for the extremely tight tolerances of today’s critical

1. Voltage Ramp to Control Current

Activation pulse can be added indicated in RED
2. Initial Current (25% or 10ASF) – 5 Min.
Activation pulse can be added indicated in RED
3 – 4. Manual Control – Dwell Steps
ADO (Amp Drop off) occurs during dwell periods
5. Pulse – Step – Ramp – Dwell
Automatic control if available
6 – 7. CCDR – Constant Current Density Ranging
Automatic control if available
Plus 5 – 10% ADO (Amp Drop off – Decay)
8 – Slow Pulse Reset during CCDR Run Cycle
Amp Hour Meter should be used with Pulse
Table 1. Training Graph notes (See Figure 2).

227
 components. The
secret here is a
consistent current
density ramp with
guaranteed repeat-
ability.
Real Time Graphic
Observation :
It cannot be over-
emphasized how
important real-time
graphic data log-
gers (see Figures
3 & 4) are for
improving quality,
efficiency (reduced
time) and energy
savings (reduced
KWH). The
operator can imme-
diately see any
problem that might
occur during the
process run. There
is a permanent
record for future
evaluation along
with a means for
continued improve-
ment on the next
process run. No
load should be run
without at least
a portable unit
Figures 3 & 4. Sonabuoy Oceanographic Tubes-PSR 6061 Alloy (1.5-2.0
Mils) -52Min without APCD vs. Sonabuoy Oceanographic Tubes APCD on line. They are
6061 Alloy (1.5-2.0Mils) 40Min. inexpensive and the
pay-off is fast with
assured results. The key secret here is to find a person who really wants to
learn anodizing, train him/her properly and give him/her a personal computer
so he/she can access all of his/her production runs. This person will make and
save you money!

Current/Voltage Spikes and Deviations/CSD Discovery Leading to APCD:

Capacitance shunt discharge (CSD) was first used to neutralize current and/or
voltage spikes. This eventually led to the development of anodic pulse capaci-
tance discharge (APCD) which is very unique. Please refer to the data logger
graph, which illustrates it in operation, as more current is developed with no
increase in voltage. Current and Voltage Spikes and/or Deviations can cause
serious procedural problems and must be evaluated by good anodize training,
experience, and technology. For example — A current increase during a ramp
dwell period could be a potential burn. Voltage Spikes could be a malfunction in
the power supply due to SCR Radical Misfiring. A definite ADO during a dwell

228
period indicates good initial anodize coating formation during the ramp cycle.
Please compare the voltage and current values with APCD and without APCD
on the data logger graphs (see Figures 3 & 4).
Voltage Without APCD With APCD
15V 25–50 Amps 300 Amps (min)
20V 150 Amps 400 Amps
30V 340 Amps 540 Amps (min)
(APCD disconnected after 30 Volts)

PROBLEM-SOLVING SITUATIONS/TROUBLESHOOTING DEFECTS

The following represent defects, properties and control factors which were
involved in problem-solving situations. Procedural requirements along with
concentrated organic additive/modifiers also represent improvements and solu-
tions to problem-solving and troubleshooting situations as follows:

1. Smut, Powder and Burning

Reduced by Pulse – Step – Ramp plus concentrated additives/ modifiers
2. Burning: Prevented by the Complete Spectrum Approach
Endothermic properties of concentrated additives/modifiers
Pulse – Step – Ramp and ADO
ADO during Constant Current Density Ranging CCDR
3. Pits/Blisters (7000 Series)
Tendency to Pit Reduced by PSR Activation @ 3–5 Volts
4. Corrosion Resistance
Increased by RSR and Amperage Decay (ADO)
(Due to better pore structure for sealing)
5. Slow Anodize Formation Rate (Speed)
Faster Anodize using PSR and CCDR
Controlled using amp hours during pulsed current
6. Anodize Tank Temperature Range
Wider Range to 100º F+ using concentrated Chemistry and PSR
(Concentrated additives & modifiers increase heat absorption in pore
structure)
7. Poor quality (dull) color Anodize
Increased Dye Penetration using PSR +Dwell
8. Pore Structure Development/Bonding Properties
PSR/Dwell times are keys to proper formation and development
PSR for Superior Adhesive Bonding Properties @ 5-10 Volt
9. Final Microfinish
Improved due to PSR – Temperature
Increased concentration of additives and modifiers
10. Power Savings (Lower KWH)
Low Voltage / Higher Current / PSR, ADO, Dwell

THE BALANCING ACT

The following balancing act5 found in another anodize workshop/paper can be
very beneficial in troubleshooting situations. Please note how the many chemical
and electrical control parameters affect the anodize formation process.
There are many different chemical products which can be used as electrolytes,
but we will only consider sulfuric acid since it is the most common. However,
the control parameters we will consider would be applicable to any electrolyte.

229
The basic electrolyte is composed of a certain concentration of sulfuric acid. The
amount is determined by two important considerations: 1.) The ability of the
solution to conduct electricity efficiently; and 2.) The ability of the electrolyte
to dissolve the aluminum oxide coating. The balance that must be determined
and maintained for consistency is that between formation and dissolution of
the anodic coating. To do that, we must carefully control the parameters that
affect that balance.

Chemical Parameters. The following diagram demonstrates how the different

parameters affect the balance between formation and dissolution. By exercising
strict control over these forces, we gain control of the type of coating we wish to
achieve. We are then able to generate a coating from the simplest thin decorative
film to the heaviest hard coat film.
Table 2. Illustration of how the different parameters affect the balance between formation and dissolution.

Anodize Formation <---- Favors --- Chemical Parameter --- Favors ----> Anodize Dissolution

Lowering <------------------------- Temperature -------------------------------> Raising

Lowering <--------------------- Acid Concentration ----------------------------> Raising
Raising <-------------------------- Acid Agitation -----------------------------> Lowering
Lowering <------------------ Aluminum Concentration ------------------------> Raising
Raising <----------------------- Additives/Modifiers --------------------------> Lowering
Heat absorption in pore structure
(Promotes anodize formation rate at higher temperatures)
Lowering <------------------------ Contaminants -------------------------------> Raising

Electrical parameters. The formation of the coating is dependent upon the flow
of electrons, which is represented by current (NOT VOLTAGE). The chemical
factors influence the flow of current and the chemical dissolution simultane-
ously as noted above. The electrical parameters primarily determine the for-
mation of the anodic coating, but if the coating is not forming efficiently it is
dissolving as a consequence of the chemical parameters.
Table 3. Illustration of how electrical parameters determine formation of the anodic coating.

Anodize Formation <---- Favors --- Electrical Parameter --- Favors ----> Anodize Dissolution

Increasing <-------------------------- Contact ------------------------------> Decreasing

Increasing <---------------------- Current Density -------------------------> Decreasing
Increasing <------------------- Current Distribution -----------------------> Decreasing
Increasing <---------------------Pulse Step Ramp --------------------------> Decreasing
(Promotes formation at low voltage)
Increasing <-------------------------- Voltage ------------------------------> Decreasing
Increasing <----------------------------Time --------------------------------> Decreasing
Increasing <-------------------- Metal Conductivity ------------------------> Decreasing

There will be other universal electrolytes that will be developed as they become
necessary to meet present and future specifications. However, the problem-
solving procedural requirements presented herein should become a part of all
anodize processing.

230
 For more information, contact:
Fred C. Schaedel
Senior Technical Director
Anodic Technical Services
Affiliate of Alpha Plus Systems
14600 Golden West St, Ste. A-206
Westminster, CA 92683
Phone: (714) 894-0109
Fax: (714) 894-0179
Cell: (714) 728-5639
Email: fcsaps@yahoo.com

REFERENCES/ FOOTNOTES
1. Reynolds, U.S. Patent 2,743, 221 (1970)
2. Working, K.C., U.S. Patent 3,434,943 (1969)
F.C. Schaedel, U.S. Patent 3,418,222 (1968)
F.C. Schaedel, U.S. Patent 4,152,221 (1979)
Kjucaricek et al., U.S. Patent 4,879,018 (1989)
3. F.C. Schaedel, proc NASF-2008
Realizing New Limits Using Anodic Discharged
Surface Activation and Conditioning for
Type III Anodize on All Alloys
4. F.C. Schaedel, proc NASF 2011
The Leading Edge Guide to Top Quality Anodizing
Using the Complete Spectrum Approach with
One Universal Type I-II-III-(123) Mixed Electrolyte
5. Richard Mahn proc AAC 2002
Anodizing Aluminum (The Balancing Act)

231
plating processes, procedures & solutions
GOLD POST-DIP TO IMPROVE
CORROSION-RESISTANCE PROPERTIES
BY OLAF KURTZ, JÜRGEN BARTHELMES, FLORENCE LAGORCE-BROC,
TAYBET BILKAY, MICHAEL DANKER, AND ROBERT RÜTHER,
ATOTECH DEUTSCHLAND GMBH, BERLIN, GERMANY

Electrolytic or electroless gold depositions are used to provide a conductive and

corrosion-resistant final coating for electronic applications. Because copper
or copper alloys are the predominant choice for the base material, a nickel or
nickel–phosphorus layer is often used as a diffusion barrier.1,2
Cost efficiency and reduced precious metal content are the main criteria to
satisfy the ever-increasing technical and quality requirements for components
used in the electronic, telecommunications and automotive industries. The
dramatic increase in the price of gold within the last decade reached its highest
position in March 2008, amounting to $1,000 per ounce.3
The gold coating thickness must be adequate to ensure pore freedom and
provide sufficient corrosion resistance. Each defect or pore formation within
the gold or barrier layer can lead to corrosion, resulting in both a reduced con-
tact area and an increase in contact resistance. In general, a pore-free deposit
is normally achieved at a deposit thickness of 0.5 to 2 μm,4,5 depending on the
substrate type and pretreatment process used.
For cost efficiency, the combination of palladium–nickel and ultra-thin gold
coatings are used as an alternative to conventional gold deposits.1,6,7
As an alternative route, special pre-treatment such as electropolishing and
the use of “inexpensive” electrolytic nickel–phosphorus barrier layers increase
corrosion resistance at the expense of a significant reduction in gold thickness.8,9
Post-treatments may also help. However, care must be taken to ensure that
they do not impair the essential surface properties (i.e., contact resistance and
solder or bonding functions needed for most connector applications). Lubricants
are often used as post-dips because they improve lubrication and abrasion prop-

Process Parameters Optimum Process

pH 5.8 5.7–6.0
Temperature 50 C
o
20–60oC
Agitation Required Required
Immersion time Reel to Reel Reel to Reel
5 sec 3–10 sec

Rack and Barrel Rack and Barrel

2 min 1–3 min

Table 1: Process Parameters for Betatec Gold Post-Dip

232
Figure 1: Contact angle determination on a solid Figure 3: Wetting of an acid-activated copper surface
surface according to Young.12 by water after treatment with Betatec gold post-dip,
u = 76°.

Figure 2: Wetting of an acid-activated copper

surface by water measured and recorded with a
tensiometer.
Figure 4: Wetting of an acid-activated nickel surface
by water after treatment with Betatec gold post-dip,
u = 92°.

erties while simultaneously avoiding fretting corrosion. Nonetheless, because

most lubricants, chemically speaking, are insulators a negative influence on
the surface contact properties is observed.
This article will describe such an aqueous post-dip treatment, providing
corrosion resistance for various types of metal surfaces without impairing
the contact resistance and other technical properties, while free from toxic
or hazardous substances that may cause harm to the environment and living
organisms.

BETATEC*: POST-DIP TO IMPROVE CORROSION RESISTANCE

The innovative Betatec aqueous gold post-dip requires a treatment time of only
a few seconds after applying the gold deposit. Table 1 summarizes the process
parameters. Properties of the post-dip include:

• Imparting hydrophobic (water repelling) surface behavior

• Repelling the action of corrosive vapor
• Improved corrosion resistance.

CONTACT ANGLE MEASUREMENT

Contact angle measurements were carried out for wetted gold surfaces, before
and after treatment, to determine the hydrophobic properties of Betatec post-
dip. Measurements were carried out using a contact angle measuring device (ten-
siometer). A mean of 30 individual measurements was used to minimize errors.
The wetting of a solid surface with a liquid depends on the respective surface

233
energies. The surface energy consists of the potential energy of the molecules or
atoms on a surface (specific surface energy). The energy results from the ratio of
work per surface increase DW to the surface growth DA. For liquids, this surface
energy equals the surface tension.10,11

s = ΔW/ΔA [N/m or J/m2]

Thomas Young established the relationship between the free surface energy
ss of a solid, the interfacial energy gsl of the solid and the suspended drop, the
surface tension sl of the liquid and the contact angle (u) between the vectors sl
and gsl (see Fig. 1).12
Young’s formula can be described as follows:

cosu = ss – gsl/sl

(Indices s and l represent “solid” and “liquid,” respectively).

The most stable thermodynamic state of a system is that of lowest (free)
energy. Therefore, each system strives to avoid surfaces possessing high surface
energy or tending to reduce surface contact. It is for this reason that materials
are slightly wetted with materials of a low surface energy. The wetting angle can
be within the following limits: 0°<u <180°. A solid can be wetted by a liquid if
the contact angle is u <90°.
A pure copper surface is nearly completely wetted by water. Figure 3 shows
such a wetting with a very small contact angle that lies outside measurement
accuracy. In this case, the surface energy of the copper (1.85 J/m2) is significantly
higher than the energy of the water (0.05 J/m2): sl << ss.13
Surface energy is also influenced by surface preparation. The sample shown
in Figure 2 was activated (i.e., all oxides were removed from the surface prior to
measurement). For an inactivated copper surface the contact angle increases
to approximately 60°. An oxide layer, therefore, leads to a more hydrophobic
copper surface.
For meaningful measurements and to remove the aforementioned strong
influence, all post-dip treated metal combinations on top of the copper sub-
strate (including copper, nickel, gold, and a combined nickel/gold layer) were
activated (removal of the oxide layer) prior to treatment with Betatec.
The copper substrate standard immersed in Betatec post-dip provided an
increased contact angle of approximately 76° (see Fig. 3).
Tests were also carried out for a nickel surface (surface energy of nickel is
2.45 J/m2) with a measured contact angle of approximately 92° after treatment
with the post-dip.14 Hence, despite nickel possessing considerably higher surface
energy, thereby making it more hydrophilic, Betatec post-dip treatment was very
effective at imparting hydrophobic surface properties (see Fig. 4).
Similar testing was undertaken with pure gold surfaces (surface energy of 1.5
J/m2) that were also made water-repellent, achieving a contact angle of 87°.15 In
these cases, an almost complete wetting of the gold surface was achieved with-
out the post-dip treatment. For the final contact angle measurements, copper
substrate was plated with 1.5 μm nickel (followed by 0.3 μm gold). The non-
treated sample showed close to 100% complete wetting effect after activation.

234
 These same samples after post-dip treatment provided a contact angle of 76°.
The results from all these tests highlight the strong hydrophobic properties of
Betatec post-dip for both pure metal surfaces and electrodeposited nickel/gold.

CORROSION RESISTANCE STUDY

Reference standards were used to assist corrosion inspection and sample
assessment.15 Typical laboratory corrosion test conditions utilize an artificial
atmosphere provided by a climate chamber. The atmosphere can be carefully
controlled with respect to humidity, temperature, and the desired concen-
tration of corrosive gas or gas mixtures
(e.g., sulphur dioxide, chlorine, nitric acid,
etc.).16–18
Salt spray tests are also widely used, par-
ticularly the “neutral salt spray test,” which
simulates the high salt content found in
seawater or that used on highways during
cold winter periods.19 In general, the choice
of test was dictated by the application and
the expected corrosion severity.

Figure 5: Process sequence for the plating of

copper-based samples. NITRIC ACID VAPOR (NAV) TEST
The ASTM B-735 standard uses the “nitric
acid vapor test” to evaluate porosity, and
the extent of corrosion is microscopically
measured by a quantitative pore count per
surface area. Test requirements include:

• Temperature: 20–25°C
• Air humidity of approximately
55% (never exceeding 60%)
• Acid vapor provided from 70%
w/w nitric acid reagent.
Figure 6: Corrosion inhibitor comparison (as
corroded area after NAV testing) for Betatec
and conventional products. The procedure involves placing nitric
acid into a dry chamber or desiccator, fol-
lowed by a delay period of approximately 30 minutes prior to introduction of
the corrosion test samples. The ASTM standard specifies a duration of 60 min
for gold thickness to 2 μm.

Required test time as a function of gold deposit thickness20:

60 ± 5 min for Au = 0.6–2.0 μm
75 ± 5 min for Au = 2.0–2.5 μm

The actual duration used for this study was extended to 120 minutes to pro-
vide an increased severity of two times the ASTM standard.
The gold thickness used throughout this study was fixed at 0.3 μm (well below
that used in previous studies). Samples removed from the chamber were oven-

235
 Sample Ni-Sulphamate HS Aurocor SC Post-treatment
[10 A/dm ] 2
[μm]
0 1.5 μm 0.3 None

1 1.5 μm 0.3 Benchmark 1

2 1.5 μm 0.3 Benchmark 2
3 1.5 μm 0.3 Benchmark 3

4 1.5 μm 0.3 Benchmark 4

5 1.5 μm 0.3 Betatec
Table 2: Part of the Sample Matrix Highlighting the Benchmark Products

dried at 80°C prior to pore examination,

which was undertaken using a microscope
together with Aquinto a4i docu/analysis
software. Calculations involved pore count
per area and individual pore diameter, pro-
viding an overall % corrosion area in accor-
Figure 7: Stable contact resistance values dance with the ASTM specification. Pores
before (red) and after (orange) an 8-hour were size-categorized as follows:
pressure-cooker test.

• <0.05 mm
• 0.05–0.12 mm
• 0.12–0.4 mm
• >0.4 mm

For each test specimen, a target area of 36 mm2 was used for each set of
measurements. The process sequence used to prepare the samples was as per
Figure 5.
A total of 200 samples with a target gold thickness 0.3 μm were prepared
and examined alongside samples treated with commercially available corro-
sion inhibitors (Benchmarks 1–4). Table 2 highlights a small fragment of the
experimental matrix used.
After being subjected to 2 hours of NAV testing, all samples were categorized
by calculating and evaluating the pore count per corroded surface area. Figure
6 compares the calculated corrosion area of the samples.

CONTACT RESISTANCE MEASUREMENT

Extensive measurements establish that treatment with Betatec post-dip does
not cause an increase in contact resistance. The parameters: I = 10 mA, U = 20
mV, F = 5 cN were used to satisfy the EN IEC 512 standard. The mean value of
30 measurements was used for each test sample.

236
 ZCT Fmax
Reference As plated 0.34 sec 1.91 mN
Pressure cooker 4 h 0.36 sec 1.49 mN
Pressure cooker 8 h 0.81 sec 0.71 mN
Betatec As plated 0.33 sec 2.07 mN
Pressure cooker 4 h 0.35 sec 1.54 mN
Pressure cooker 8 h 0.80 sec 0.56 mN

Table 3: Results Demonstrating no Loss to Solderability After Post-Dip Treatment

Steam-aging: The “pressure cooker test” is used to promote corrosion, diffu-

sion, and deposit cracking. This test was conducted for a duration of 8 hours,
after which contact resistance measurements were carried out. The pressure
cooker test parameters included:

➢ high temperature 105 °C

➢ high humidity 100% RH
➢ high pressure 1.192 atm

Results inferred no increase in contact resistance after 8 hours of exposure.

Figure 7 depicts little change in contact resistance values for Betatec 1 and 2 test
samples, before and after the pressure cooker test, using the very low tracking
force of 5cN.

SOLDERABILITY STUDY
This study was carried out to confirm that Betatec post-dip would not effect
solderability properties. The gold thickness was reduced to 0.1 μm for these
solderability tests.
The lead-free alloy SAC (SnAgCu) was used as solder with the following
conditions:

Solder SnAgCu
Temperature 245°C
Density 7.5 mg/mm3
Immersion time 10 s
Sensitivity 2.5
Submergence 3 mm
Velocity 21 mm/s

All tests were conducted in accordance with the IEC-68-2-69 standard using
a Litton Kester 950 E3.5 flux. Both zero crossing time (ZCT) and wetting force
(Fmax) were used to assess solderability, highlighted in Table 3.

237
 BONDING PROPERTIES
(WIRE PULL TESTING)
Gold-wire loops (25-μm
diameter) (Au HD2) were
used to determine any
loss in bonding proper-
ties after Betatec post-dip
treatment. A pull tes-
Figure 8: Crack classification rating 1 to 5, with 1 and 5 showing ter (DAGE 4000) and
insufficient surface bonding. Ratings 2, 3, and 4 are acceptable.
TS-bonder (Delvotek
5410) were used for this
study.
Wire pull testing deter-
mines the quality of the
gold wire bond to the sur-
face. It consists of apply-
ing a specified upward
force under the gold wire
attached to the surface.
The crack mechanism is
studied to evaluate the
bond quality.
The crack ratings in
Figure 8 highlight the
Figure 9: Thin gold wire crack rating distribution (as % of popu-
lation) for Betatec-treated nickel/gold-plated samples. crack at the wire/surface
All samples passed the test. interface. Ratings 1 and
5 indicate an insufficient
bond with the surface,
whereas ratings 2, 3, and
4 are acceptable.
Sample processing
involved nickel sulpha-
mate (Ni-Sulphamate
HS) and pure-bond gold
(Aurocor K 24 HF) plating,
followed by Betatec post-
Figure 10: Strong discoloration of untreated samples compared to
Betatec treatment (above) showing no discoloration after an 8-hour dip treatment. To confirm
pressure cooker test. measurement reproduc-
ibility, 4× samples were
measured and compared to 2× untreated references. All samples passed the test
with no crack ratings of 1 or 5 evident. The majority of samples provided a rating
of 3 (central crack), considered the best bonding quality (see Fig. 9).

TIN SURFACE PROTECTION STUDY

To maximize cost-effectiveness (mentioned in the introduction) many connec-
tors are plated with gold only at the functional contact region with the “solder
flank” receiving a tin coating. However, the processing sequence does not allow
for selective treatment of each region with different post-dips. Hence, the ideal
gold post-dip candidate must ensure no detrimental effect on the properties of

238
 Zero Crossing Fmax
Time (ZCT)
Reference As plated 0.35 sec 2.89 mN

Pressure cooker 8 h No wetting –0.55 mN

Betatec As plated 0.35 sec 2.92 mN

Pressure cooker 8 h 0.41 sec 1.49 mN

Table 4: Solderability Results for Untreated and Betatec-treated Samples on Tin Deposits

the tin but also provide preferable increased corrosion resistance.

Oxide formation of tin (evident as surface discoloration) greatly impairs
solderability properties. Therefore, Betatec-treated samples were subjected to
8-hour pressure cooker tests. The results shown in Figure 10 highlight the ben-
eficial corrosion resistance of the Betatec treatment.

SOLDERABILITY STUDY OF TIN DEPOSITS

The solderability study of tin deposits was undertaken using the same param-
eters and conditions used for gold. All samples were plated with 1.2 μm nickel
and 8 μm tin. It is clearly apparent that without Betatec post-dip treatment, no
wetting occurs within the 10-second test period. The solderability results are
summarized in Table 4.

CONCLUSIONS
Comprehensive test results have been presented for a new and patented gold
post-dip treatment that significantly improves the corrosion resistance of nickel/
gold deposits and contributes to the reduction of gold costs. Extended NAV
testing (to two times the ASTM B-735 standard) shows a significant increase
in corrosion resistance for a 0.3 μm gold thickness.
It has been demonstrated that the Betatec post-dip can impart beneficial
hydrophobic properties to the gold surface with subsequent blocking of pores.
This hydrophobic and protective mechanism has been evaluated by water con-
tact angle measurements on pure copper, nickel, and gold deposits together
with nickel/gold plating on copper (before and after treatment with the gold
post-dip).
Results show that this beneficial post-treatment has no adverse effects on
electrical, solderability, or bonding properties of the gold electrodeposits. It has
also been demonstrated that this treatment can provide a dramatic increase in
protection of tin deposits, allowing for easy incorporation into most selective
gold plating lines.

REFERENCES
1. Kaiser H. Edelmetallschichten. Bad Saulgau: Leuze Verlag, 2002.
2. Braunovic M, Konchits VV, Myshkin NK. Electrical Contacts. CRC Press, 2007.
3. KITCO Precious Metals. Historical Data and Charts. Available at: www.kitco.com/charts/.

239
4. Reid FH, Goldie W. Gold Plating Technology. 3rd rep. ed. Amer Electroplaters Soc, 1987.
5. Reid FH, Goldie W. Gold als Oberfläche. Bad Saulgau: Leuze Verlag, 1982.
6. Kurtz O, Lam P, Barthelmes J. New approaches to palladium-nickel and palladium plating for the
semiconductor & connector industry. Presented at: SF China 2006.
7. Kurtz O, Barthelmes J, Rüther R. Die abscheidung von palladium-nickel-legierungen aus chlorid-
freien elektrolyten. Galvanotechnik 2008;99(3):552–7.
8. Kurtz O, Lagorce-Broc F, Danker M, Rüther R, Barthelmes J. Hoch-korrosionsbestündige nickel-
gold oberflächen. Galvanotechnik 2008;99(9):2136–42.
9. Schramm B, Ott F, Kurtz O, Barthelmes J. Hochkorrosions-
beständige nickel-gold oberflächen. Presented at: ZVO Oberflächentage 2008, Würzburg.
10. Halley JW, ed. Solid-Liquid Interface Theory. American Chemical Society: ACS Symposium Series
789, 2001.
11. Sibilia JP, ed. A Guide to Materials Characterization and Chemical Analysis. 2nd ed. Wiley–VCH, 1996.
12. Thomas T. An essay on the cohesion of fluids. Philosophical Transactions of the Royal Society of
London, The Royal Society, London 1805;95:65—87.
13. de Boer FR, Boom R, Mattens WCM, Miedema AR, Niessen AK. Cohesion in Metals. Amsterdam:
North-Holland, 1988.
14. Boettger JC. Phys. Rev. B 1994;49:16798.
15. Corrosion–Understanding The Basics. ASM International, ISBN 0-87170-641-5, 2000.
16. DIN EN ISO 7384, Korrosionsprüfung in künstlicher Atmosphäre–Allgemeine Anforderungen.
17. ASTM G 87–02, Standard Practice for Conducting Moist SO2 Tests.
18. ASTM & 60-01, Standard Practice for Conducting Cyclic Humidity Exposures.
19. ASTM-B-117, Standard Practice for Operating Salt Spray (Fog) Apparatus.
20. ASTM-B-735-06, Standard Test Method for Porosity in Gold Coatings on Metal Substrates by
Nitric Acid Vapor.

240
plating processes, procedures & solutions
ZINCATE- OR STANNATE–FREE PLATING
OF ALUMINUM AND ITS ALLOYS
BY JOHN W. BIBBER, THE LABORATORY AND RESEARCH DIRECTOR AT
SANCHEM, INC. CHICAGO, IL

For more than 80 years now the most generally accepted process for the prepara-
tion of aluminum and it alloys for plating has involved the use of a “zincate” or
“stannate” processing solution. Environmental issues relative to the use of these
processes are becoming more and more of a problem. The process itself is long
and involved and will differ from one alloy to another. This article presents a
much more environmentally acceptable alternative that is considerably easier to
work with, gives far more consistent results and is more cost effective.
Aluminum is light, strong, environmentally acceptable and relatively inexpensive.
As such it enjoys wide use in the electronics industry. It is, however, easily corroded,
relatively soft and not easily welded or soldered. As a result, it is quite often plated.
The process being presented in this article has now become part of “ASTM
specification B253”– “Standard Guide for the Preparation of Aluminum Alloys
for Electroplating” which also covers the electroless deposition of metals on
aluminum and its alloys by the use of “Zincates”. Although other methods of
preparation are available[i], “zincate” and/or “stannate” have been the most
widely used method for over 80 years now[ii].
“Zincates” basically consist of an alkaline solution of zinc as zinc hydroxide
and a number of variations to this basic composition are commercially available.
“Stannates” consist basically of an alkaline solution of tin as the hydroxide, and
a number of variations to this basic composition are commercially available.
After an extensive series of cleaning steps, which may or may not require more
than one application of a given “zincate” or “stannate” composition, the sur-
face of the aluminum or aluminum alloy ends up being coated with a very thin
film of zinc or tin metal which will be dispersed over the surface of the metal
in an irregular pattern depending upon the nature of the alloy being processed
and / or the characteristics of the particular “zincate” and / or “stannate” solu-
tion being used. This then removes or displaces a large portion of the aluminum
oxides on the metal and sets up electrochemical cells whereby the aluminum or
aluminum alloy will more easily accept the metal being plated on it while at the
same time displacing the zinc and / or tin out into the plating solution along
with any other metals that the given “zincate” or “stannate” solution may have
contained and were deposited on the aluminum or aluminum alloy.
In many cases, this will act to shorten the bath life of your plating solution.
In particular, “electroless” deposition solutions such as “electroless” nickel. As
is the case of metals other, than, aluminum, copper is frequently plated directly
onto a given “zincate” or “stannate” prepared surface to facilitate the plating of
nickel or chromium as the type of “zincate” or “stannate” composition used will
influence the ability of the aluminum or aluminum alloy to accept subsequent
metal deposits[iii],[iv] In addition the use of “zincates” and/or “stannates” will
require you to use an alkaline cyanide copper plating bath rather than the more
environmentally acceptable acid copper plating bath.
The “zincate”and/or “stannate” based processes are totally dependent upon
generating and continually maintaining very reactive surface conditions. This
may be possible under laboratory conditions, but very difficult (if not impos-
sible) to maintain in any given plating shop depending upon the aluminum alloy

241
being processed. Thus the need for very clean rinse solutions and numerous
processing tanks that need constant attention to avoid the expense of having
to reprocess parts.
Once the parts have been processed and rinsed, they need to be
immediately placed in the plating bath and quickly plated while the surface is
still active and ready to accept the metal being deposited on them. The larger
the parts, the greater the degree of difficulty involved. Table 1 illustrates one
of the more commonly used methods for processing wrought aluminum with
“zincates” or “stannates”[v].
Table two illustrates one of the more commonly used methods in the process-
ing of aluminum castings[vi]. Castings are much more difficult to clean in that
the silicon and other elements in a given casting are generally present in much
higher percentages. Silicon, for example, will range from about 6 percent to as
high as 25 percent and it is rather difficult to remove.

1. Clean in a sulfuric acid or phosphoric acid-based cleaner at about 120 degrees F.

for two to three minutes
2. Mineral-free water rinse.
3. Strong potassium hydroxide etch for one to three minutes at ambient tempera-
tures.
4. Rinse in mineral-free water.
5. Acid etch in 15% - 25% sulfuric acid containing 1% - 2% fluoride at ambient tempera-
tures.
6. Double rinse in mineral-free water.
7. Process in “Zincate” or “Stannate” solution.
8. Double rinse in mineral-free water.
9. Immediately place the parts in the plating bath.
Table 1. Commonly used procedure for processing wrought aluminum alloy parts.

1. Strong alkaline (pH of 12 or higher) cleaner at about 150 – 170 degrees F.for two
or three minutes.
2. Double rinse in mineral-free water.
3. 1% – 3% fluoride salt added to 20% - 25% sulfuric acid with the balance being
nitric acid at ambient temperatures for one to three minutes.
4. Double rinse in mineral-free water.
5. “Zincate” or “Stannate” processing of the casting.
6. Double rinse in mineral-free water.
7. Go directly to plating bath.
Table 2. Commonly used procedure for the processing of cast aluminum alloy parts.

Table 3 is similar to the process outlined in the “ASTM B253 specification” -

“Standard Guide for the Preparation of Aluminum Alloys for Electroplating”
and now in use as an alternative to the standard “zincate”and/or “stannate”
process for the preparation of wrought alloys to be plated.
Once the organic-based “Plating Catalyst” is applied, the surface of the metal
being processed is more or less sealed from the outside elements and may, if so
desired, be dried and stored in a clean/dry area to be processed at a later date by
first of all reactivating the surface by soaking the parts in warm (100 – 120 degrees

242
F.) mineral-free water for about 15 minutes, dipped in a 1% aqueous ammonia
solution for about 10 seconds and then rinsing again in mineral-free water before
going into your plating bath. Parts may be processed in a rotating basket if so
desired.
Table 4 outlines the currently used process for processing cast aluminum alloys
with the organic-based “Plating Catalyst” and once again the casting may, if so
desired, be dried and stored in a clean dry area to be processed at a later date by
once again soaking them in warm (100 -120 degrees F.) mineral-free water for
about 15 minutes, dipped in 1% aqueous ammonia for about 10 seconds and then
rinsing again before going into your plating bath. The parts may be processed in a
rotating basket if so desired. Regardless of what aluminum alloy is being processed,
the same “Plating Catalyst” used and your major source of loss is “drag out” from
the bath. The very thin catalytic film (about 100 – 200 nm) is displaced out into
your plating bath and will have no adverse effect upon the bath. The electrochemi-
cal deposition of the catalytic film is not directional and once the deposition pro-
cess starts, the film will deposit itself in any and all recessed areas or small holes.

1. Clean in a Phosphoric acid or Sulfuric acid based cleaner at about 110 – 120
degrees F. for two to three minutes.
2. Rinse in mineral-free water.
3. Activate by immersion in a 5% Potassium Hydroxide solution at ambient tempe-
ratures until you have uniform and even gassing of the part, generally not more
then 10 – 15 seconds.
4. Rinse in Mineral-free Water.
5. Acid etch in 20% sulfuric acid containing about 1.5% ammonium bifluroide for
two minutes.
6. Rinse in mineral-free water.
7. Rinse in 1% ammonia water solution to remove all traces of acid or acid Salts for
10 – 15 seconds.
8. Rinse in mineral-free water and go immediately directly into “Plating Catalyst”
making the part the anode at about 15 - 16 amps per square foot for about two
to three seconds.
9. Rinse the part off in a 1% ammonia water solution followed by a mineral-free
water rinse and take directly to the plating bath.
Table 3. Procedure for the processing of wrought aluminum alloy parts by new process.

1. Strong alkaline (pH of 12 or higher) cleaner at about 150 – 170 degrees F.for two
or three minutes.
2. Double rinse in mineral-free water.
3. 1% – 3% fluoride salt added to 20% - 25% sulfuric acid with the balance being
nitric acid at ambient temperatures for one to three minutes.
4. Rinse in mineral-free water followed by a rinse in a 1% ammonia water solution
to remove all traces of acid or acid salts.
5. Rinse in mineral-free water again and go directly into “Plating Catalyze” making
the casting the anode at 15 – 16 amps per square foot for two to three seconds.
6. Rinse casting in 1% ammonia in water and mineral-free water and go directly
into the plating bath.
Table 4. Procedure for processing of cast aluminum parts by new process.

243
CONCLUSIONS
As outlined above the electrolytic deposition of a thin and environmentally safe
catalytic organic film on wrought or cast aluminum or its alloys will serve as a
alternative to the conventional “zincate”and/or “stannate” processing to prepare
aluminum and its alloys for plating. The new process is accomplished with far
less difficulty.

ABOUT THE AUTHOR

John W. Bibber is the laboratory and research director at Sanchem, Inc. He has a B.S. degree
in chemistry from Shippensburg University, Shippensburg, PA. and a Ph.D. in Inorganic
Chemistry from the University of Georgia, Athens, GA. He has U.S. and International
Patents on surface treatment processes for Aluminum, Magnesium and Titanium. He is
an officer of ASTM, a member of AESF and a Member of ACS.

REFERENCES
1. Arrowsmith, D. J. and Clifford, A.W., “Int. J. Adhesion and Adhesives”, volume
5, page 40, 1985.
2. Hweitson, E.H. (Eastman Kodak, Co.)U.S.Patent1,627,900 (1927).
3. Mallory, G.O., “Plating and Surface Finishing”, 72, No. 6, 86, 1985.
4. Leloup, R. (W. Canning and Co., Ltd.). British Patent 1,007,252 (1955).
5. Wernick, S., Pinner, R., Sheasby, P.G., “The Surface Treatment and Finishing
of Aluminum and its Alloys”, Metals Park, OH., ASM International, pp. 1043
– 1045, 1987.
6. Wernick, S., Pinner, R., Sheasby, P.G., “The Surface Treatment and Finishing
of Aluminum and its Alloys”, Metals Park, OH., ASM International, pp. 1043
– 1045, 1987.

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