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Anodizing Aluminum
By J.C. HECKER, JR.
Aluminum Consultants
Madison, Wisconsin
Anodizing, unlike electroplating and
organic coatings, is commercially unique
to aluminum. Developed in the early
1930’s, it has greatly extended the applications of aluminum in products and uses
where the metal might otherwise not be
utilized. The finish is readily available
from finishing job shops throughout the
world and is relatively inexpensive.
Anodizing converts the surface of aluminum to an oxide. While aluminum naturally forms aluminum oxide on its surface, this is a very thin film. Anodizing
provides a much thicker oxide coatingseveral mils thick if required.
Coating
Properties
The hardness of this aluminum oxide
coating rivals that of diamond. Thus
anodizing improves abrasion resistance.
Anodizing (with suitable prefinishing)
also can appreciably alter and improve the
appearance of aluminum. By using dyes
and special anodizing procedures, the
finisher can make aluminum look like
pewter, stainless steel, copper, or brushed
bronze.
Anodizing improves corrosion resistance, especially when the metal surface
is exposed to industrial, humid and marine atmospheres.
The electrical insulating properties of
the anodic finish find application when
dielectric properties are important for
electrical components.
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Anodized aluminum is easy to clean
and resists heat to the temperature at
which the aluminum itself melts.
Coating Formation
Anodizing is an electrochemical conversion process, not an applied coating.
The surface of the aluminum metal is converted to aluminum oxide as a result of
reactions occurring at the anode in an
acidic solution. The thickness and properties of the anodic coating will vary with
alloy, anodizing process employed and
cycle time (ampere-hours).
Oxide formation proceeds inward, toward the source of fresh metal. The first
formed oxide remains in contact with the
anodizing solution throughout the process cycle; the last formed oxide is at the
metal interface.
The coating is 30-50 pct thicker than
the metal it replaces, since the volume of
oxide produced is greater than that of the
metal replaced.
Structure of most common anodic coatings is predominantly cellular/porous.
There is a very thin non-porous barrier
layer at the interface. Sealing is normally
required to “set”’ dyed colors, prevent
staining and improve corrosion resistance
of the anodized surface.
Existing Processes
Anodic coatings can be formed in a variety of chemica1 solutions, although only
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a handful have been commercialized and
are in industrial use. Some of the existing processes and their characteristics are
as follows:
Chromic Acid. The fairly thin (0.1-0.3
mil), normally grayish-colored coating
formed in a chromic acid electrolyte is
used primarily to improve corrosion resistance and as a base for paint in aircraft
and marine applications. Typically produced in a 3-10 pct chromic acid solution
at 40 volts for 30-45 min, chromic acid
anodizing produces coatings that are thinner and less abrasion resistant than those
produced in sulfuric acid electrolytes.
Oxalic Acid. Primarily used in Japan.
Characteristic yellowish color. Typical
process conditions: three pct solution, 1020 asf, 75-95F, for 30-40 min. Electrical
power may be AC only or DC superimposed on AC. The coating is somewhat
harder and more abrasion resistant than
that of a conventional sulfuric acid anodic
coating of comparable thickness.
Phosphoric Acid. Very limited use as
a base for electroplating (large pore size)
and as surface preparation for adhesive
bonding. Coatings produced in 3-20 pct
by volume electrolyte at 85-95F, 50-60
volts for 15-30 min.
Boric Acid. A thin, non-porous, barrierlayer-type anodic coating is produced in
hot boric acid solution. Used as a dielectric for capacitors. Coatings are applied
to continuous aluminum strip at voltages
up to 600.
S u l f u r i c A c i d . The predominant
anodizing process. Coatings 0.l-1.0 mil
thick formed (typically) in a 15 pct solution, 12 asf, 18-24 volts, 70F for 10-60
min. The basic anodic coatings may exhibit yellowish, tan or gray colors, depending upon aluminum alloy and coating thickness. This coating is commonly
dyed or otherwise colored with mineral
pigments or precipitated metals (electrolytic two-step process).
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Sulfuric/Oxalic. A mixed-acid variation used at high current density (24-36
asf), low temperature (30-50F) and necessitating high voltage (75-100 volts) to produce coatings one to three mils thick.
Very dense “hard coatings” produced.
Used where extreme wear and/or corrosion resistance required.
Organic Acids. The most widely used
“integral” processes employ 90-100 g/liter
solutions of organic acids, containing a
small amount of sulfuric acid (for increased conductivity). Operating conditions: 70-80F, 24 asf, voltages up to 75.
Produce amber, bronze and black coatings, primarily for architectural applications. Coating thickness varies from
about 0.4 mil to slightly over 1.0 mil.
Color is achieved by aluminum alloy/
process variations, without need of secondary coloring agents. The oxide coating
is hard and dense, similar to hard coatings. It is light fast and weather resistant.
This finish is losing popularity to the
electrolytic “two-step” coloring process
because of energy saving, lower equipment costs, color flexibility and claims for
better color uniformity and greater tolerance for variations in material.
Producing Good Anodic Coatings
Process control on the anodizing line
starts with metal quality, precleaning and
racking, and ends with unracking, possible “clean-up” and final inspection. The
job shop finisher or captive anodizing
department usually has the least control
over a very important necessity for good
anodizing: metal quality. Type of mill
product, quality of that product, alloy,
temper, gauge and so on have significant
effects on the appearance, oxide coating
properties and functional properties
(abrasion and corrosion resistance) of the
end’ product.
Specific information on the effect of
metal vs. anodizing (and other finishes)
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may be found in the Conference Proceedings for Aluminum Finishing ‘87, a conference held October 28-30, 1986 by
PRODUCTS FINISHING.
Suffice it to say, it behooves the
finisher to know the metal that is to be
treated. Certain process changes may
have to be made, and specified quality
standards may be difficult to achieve if
other than the proper alloy is being
anodized.
Finally, the point often arises - whose
fault is it-the metal supplier or the
finisher-if work is rejected by the buyer
or architect? In investigating such conflicts it pays to know the characteristics
of the aluminum being anodized.
Pretreatment. Pretreating may be
necessary prior to racking of aluminum
parts for the anodizing line. Mechanical
finishing may be desirable to remove extrusion die lines or casting parting lines.
Protective paper and adhesive on sheet
panels will have to be removed manually
and by solvent wiping. Excessive machining oil may have to be removed in a vapor degreaser to avoid gross contamination of a soak cleaner solution in the
anodizing line. And the list goes on.
Racking. Racking is the first step
necessary to produce good anodic coatings. The parts must make good electrical contact with the rack, which carries
current to the part during anodizing. Poor
contact because of insufficient rackcontact area or because of loose contacts
can cause iridescent appearance, powdery
coatings, poor dyed-color match, burning and other problems.
The continuing build-up of insulating
oxide tends to break flow of current to
the part. The thicker the coating the more
pressure is exerted. Inadequate racking
may also cause parts to loosen due to
mechanical, rather than electrical, action.
Considerable force is exerted by entry and
exit from solutions and by air agitation.
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Rack design and part placement on the
rack are important. A good rack design
will hold parts securely, conduct current
adequately and carry a full load without
shielding (“robbing” current = nonuniform coating thickness).
Part position must allow for good
drainage and avoidance of air pockets.
Small, lightweight pieces are racked on
“finger clip” racks or on coiled springs.
Large, heavy sections, on the other hand,
will require strong work bars of sufficient
current-carrying capacity, and possibly
bolted contacts.
Aluminum, titanium and combination
racks are used. Aluminum racks must be
stripped after each cycle. Titanium racks
last longer but are more expensive and require greater contact area because of their
lower electrical conductivity.
Racks may be plastisol coated, but solution entrapment (and subsequent contamination) can be a problem, as the
plastisol is undercut with use.
Adequate cleaning is the first required
tank process operation. Many organic
compounds will act as a resist to later
etching and anodizing steps. They must
be removed.
The most common type cleaner in large
lines is probably the non-silicated, inhibited alkaline “soak” cleaner, which is
available from a large number of chemical companies. It will remove most surface soils such as plant dust, oils and light
buffing compound. A typical operating
temperature might be 140-160F. Too high
a temperature may produce a dried-on
foam pattern. Overly vigorous agitation
can also produce excessive foaming and
carry-over on the rack and parts.
Process control will include control of
cleaner concentration and temperature.
Avoid any oil accumulation on the tank
surface. Cleaned and rinsed parts should
present a water-break-free surface.
Thorough rinsing must follow each
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chemical step in the sequence. These may
be single- or multiple-tank rinses and/or
spray or immersion. Water should be
clean, flowing and equipped with an overflow lip at the end or along the side. Bottom inlet and top outlet are usually
recommended.
Conductivity meters also have been utilized in order to control water purity and
conserve water. Tanks should be free of
galvanic currents (insulating pads and dielectric pipe connections) and be, equipped
with insulated work-bar pads on the tank
ends.
Deoxidizing would normally follow
cleaning and rinsing. Using an acid solution at somewhat elevated temperature
(120-160F), its purpose is to remove nonuniform oxide films present on the surface of many aluminum products and
other contaminants not dissolved by the
soak cleaner. Work not properly prepared
in this manner may not etch uniformly in
the following step.
Deoxidizers are typically mixtures of
chromic, sulfuric, nitric or phosphoric
acids. They require chemically resistant
tank linings. Analytical procedures and
test kits are often provided by suppliers
of proprietary solutions. Use of these plus
temperature control and visual observation of cleaned metal surfaces would constitute usual process control.
The work is now ready for etching, a
treatment step designed to remove the
natural shiny aluminum look and provide
a soft, matte, textured appearance. (Bright
dip, on the other hand, is used to enhance
the pre-polished specular surface).
Etching is carried out for periods of 35 min at 90-120F in nominally five pct sodium hydroxide solutions. Excessive solution temperature can produce “caustic
burning,” a non-uniform etch pattern
that will usually make it necessary to reject the work.
Both generic and proprietary etch so288
lutions are used. Small amounts of sodium gluconate are often added at low
concentration to sequester sodium
aluminate built up in the etch solution.
This will prevent precipitation of hard,
alumina hydrate scale on tank walls and
burner tubes.
It is normal practice to dump a portion
of the etch tank when dissolved aluminum
content gets too high (about 40 g/liter).
At this point the solution is quite viscous
and difficult to rinse.
A fairly recent development is the
“never dump” etch. In these proprietary
solutions, aluminum is allowed to build
to a point where the rate of dissolution
is balanced by the rate of aluminum removed by dragout. The tank may be operated for a long period of time under these
conditions and etching is said to be more
uniform. In either case, analysis by titration is used to monitor and maintain alkali
content and aluminum concentration.
Etched parts are usually desmutted in
the acid deoxidizer, although they may be
carried directly to the sulfuric acid
anodizing tank. Acid desmutting removes
most of the aluminum alloy metallic constituents not dissolved by the caustic etch
and clinging to the surface as “smut.”
These copper, iron, manganese, silicon
and other elements tend to contaminate
the anodizing solution. And they may be
carried through the entire process and
show on the finished parts as a darkish
film if not previously removed by a desmutting operation.
Anodizing is usually carried out in a
nominal 15 pct sulfuric acid solution at
70-80F, depending upon whether the
coating is to be clear or dyed. Temperature should be held within a few degrees
to produce consistent coating properties.
Control of current flow (12- 16 asf) is normally recommended, but many plants
operate at fixed voltages.
The amount of oxide produced will be
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Anodizing Aluminum. . .
ANODIZING line at left, with colored parts emerging from electrolytic
coloring station.
a function of amperes x time, as with all
electrochemical reactions.
Agitation is necessary to prevent localized overheating on parts and to provide
uniform solution temperature throughout
the tank.
Cathode location is important, particularly with large sections. Even though
the oxide buildup is self-limiting and
throwing power is good, those surfaces
closest to the cathode will receive thicker
anodic coatings. With a metal tank connected as cathode, it may be necessary to
selectively mask certain areas with a
nonconductor.
Acid concentration and aluminum content are determined by titration. You
should avoid solution contamination by
chlorides, fluorides, iron, copper, mercury and so on. Aluminum content is normally allowed to rise until it reaches approximately 20 g/liter. Then a portion of
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the solution is discarded and replaced
with fresh acid. Some companies offer
equipment for removal of aluminum to
eliminate the need for this procedure.
Production sheets should record all
anodizing parameters as well as concentrations, temperatures and chemical additions made in cleaning, etching, dyeing
and sealing solutions. Inspection at the
anodizing tank would usually include
general appearance and coating thickness
on selected parts.
Where desired, the dyeing operation is
next carried out in tanks of various watersoluble, organic-dye solutions. Dye concentration, pH and temperature are recommended by the dyestuff manufacturer.
Air (if recommended) or mechanical
agitation should be employed to maintain
uniform concentration and temperature.
Very short (less than about three min) dye
times should be avoided. Longer immerPF DIRECTORY
Anodizing Aluminum. . .
sion times will allow for deeper dye
penetration into pores of the oxide coating and this is desirable.
It is necessary to avoid galvanic currents and chemical contamination in dye
solutions. Impurities such as aluminum,
sulfates, phosphates, silicates and iron
can affect absorption characteristics and
dyestuff service life. Tanks also should be
covered to prevent introduction of dirt,
rust and oil and unnecessary exposure to
light.
Dye concentration and pH should be
controlled. Concentration may be
checked by spectrophotometer and/or
standardized dyeings using test samples.
Dye performance will vary with the
specific dye, amount of dye absorbed,
sealing treatment a n d exposure
conditions.
A very important final process step is
sealing. Unsealed, the oxide pores are
subject to staining and lowered corrosion
resistance. For clear coatings, sealing in
boiling deionized water converts the
amorphous form of aluminum oxide to
a more stable crystalline hydrate form.
This reaction tends to “plug” and “cap”
the oxide pores. Dyed anodized aluminum requires specialized sealing in nickel
acetate, to prevent bleeding and to improve light fastness.
Sealing times may be 3-5 min for nickel
acetate and up to 20-30 min for water.
Double seals are also employed. A
dichromate seal may be used for improved corrosion resistance. It imparts a
light greenish color to the anodic coating.
The pH must be controlled closely to
insure efficient sealing. Again, it is also
necessary to prevent chemical contamination by phosphates, silicates and some
metallic elements. Deionized water is
recommended for tank make-up and
additions.
Slight agitation is normally used to insure uniform temperature distribution.
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Cover the tank to protect the seal solution from overhead contaminants. Solution filtration helps to remove suspended
particles.
Surface “smut” after the sealing treatment is fairly common. Degree of smut
may vary with alloy, pH, water purity,
sealing time and other factors. It can be
removed by wiping. Chemical companies
also market additives that tend to decrease smut formation.
Final inspection includes checking for
appearance and coating thickness and
performance of one or more of the seal
tests. Coating weight and salt-spray resistance also may be required for some
specifications.
Newer Developments
Electrolytic coloring processes were introduced into the United States during the
late 1970’s, largely because of the need
to reduce energy consumption. They were
in use in Europe and Asia for some time
before that.
The “two-step” name indicates a dual
process: anodizing in a conventional sulfuric acid solution and coloring in a subsequent operation. It differs from organic
dyeing or mineral pigmentation in that
coloring is produced by electrochemical
action. In a proprietary solution, applied
AC power deposits metallic particles of
tin, nickel or cobalt in the pores of the
previously formed anodic coating. This
causes colors to be developed as a result
of optical effects produced by light scattering that occurs when these other metals are deposited in the pores.
Color range is similar to that described
for the integral colors. Light fastness is
said to be very good. The processes do not
require the amount of energy needed for
integral anodizing because lower current,
voltage and time are used. Each of the
five to ten electrolytic processes being actively promoted in this country has its
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Anodizing Aluminum. . .
own characteristic process parameters and
stated advantages (or disadvantages).
Typical coloring conditions are 60-80F,
one to five asf and five-25 volts AC.
Coloring time, largely independent of
oxide-coating thickness, may be as little
as 10 sec and up to 10-15 min.
Detailed information on these processes should be obtained from the chemical supplier who markets and/or licenses
a specific treatment.
Two variations of the electrolytic coloring process are over-dyeing and the possible application of a wide range of colors
through interference effects. In the first
method, bronze background colors
produced electrolytically are dyed with
conventional organic dyestuffs before
sealing. This “three-step” process is said
to be in use at several plants in Europe.
Light-fast dyes are employed for architectural applications. Integral bronze colors
also may be over-dyed.
The second coloring procedure, not yet
in production, is considerably more complicated. It involves modification of the
pore structure produced in sulfuric acid
by a subsequent anodic treatment in phosphoric acid. Pore enlargement occurs at
the base of the pore. Metal deposition at
this location produces colors ranging
from blue, green and yellow to red. The
colors are apparently caused by opticalinterference effects, rather than by light
scattering as with the basic electrolytic
coloring process. The feasibility of this
approach will have to await further
development.
Spray dyeing is a relatively new technique said to offer several advantages
over conventional immersion in dyes.
As the name implies, anodized and
rinsed parts are introduced into a closed
chamber and sprayed with dye emerging
from a series of spray nozzles. Time, temperature and concentration of the dye solutions are similar to those for immersion
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coloring. Rinsing also may be carried out
in the spray chamber.
This process reduces space requirements considerably. Instead of individual
dye tanks in the line for each color, one
spray chamber is used and various colors
are fed as needed from auxiliary storage
tanks off-line.
Capital investment in dye solutions is
reduced, since the quantities used in spray
dyeing are much less (recirculation to
storage). Solution life reportedly is extended, since the dye baths, in closed containers, are protected from light, oil and
dirt.
Coil anodizing should be mentioned,
not because it is new-which it is notbut because the reader should be aware
that anodized material is available in wide
(60 inches) sheet and foil form. Finishes
available from at least five production facilities include mechanical, etched and
bright dip pretreatments followed by
clear, dyed and, within the last seven
years, electrolytic coloring.
Advantages of this system include prefinished stock for fabrication; lower cost;
product uniformity; and less handling (vs.
many individual parts).
Possible disadvantages, on the other
hand, include gauge limitation (.080-inch
max depending on vendor), raw edges
(where slit to width) and crazing. Some
anodizing treatments such as hard coatings and integral colors are not available
in coil form.
As in many other industries, automation, particularly of process-cycle control,
power supplies and cranes is becoming
more common with new lines. Once
manually racked, the parts may be
cleaned, etched, /‘anodized, dyed and
sealed automatically. Cycle times in each
tank are pre-programmed. Hoists operate at prescribed speeds (horizontal and
vertical) and tilt for drainage. Rectifiers
may “ramp” and then maintain constant
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Anodizing Aluminum...
current, constant voltage and/or amp-hr
control for set cycles. Conveyorized automatic processing offers better control
of process parameters (temperature, concentration, agitation and so on). This type
of equipment should find greater application in the future.
In the hard-coating field, improved
power supplies have decreased the tendency for “burning” and made it easier
to process difficult-to-run high-copperbearing alloys. Two modifications are
pulsed power and AC superimposed on
DC. The first system applies 5-30 millisecond pulses of direct current superimposed on a base current. Pulsing apparently prevents overheating of the metal
surface and resultant burning. One rectifier manufacturer states that (pulsed) current densities as high as 200 asf may be
applied without causing any burning.
The second method employs alternating current of variable potential superimposed on DC to accomplish similar goals.
Being able to run thin gauge, critical
machined parts and/or high-copper alloys
without excessive process time or burning is a great benefit to this industry.
Energy conservation, and reduced heating cost, has prompted the development
and promotion of low-temperuture sealing solutions during the past few years.
Several proprietary chemicals are being
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offered, allowing sealing of anodic coatings at temperatures of 150- 160F down to
ambient temperature (80-9OF). The cold
seal is said to involve chemical impregnation of the pores, followed by reaction
to form a resistant barrier. Vendor
claims, in addition to energy savings, include reduced sealing times, less smut and
greater tolerance for seal bath contaminants. Although these sealing treatments will pass all of the standard seal
tests, there is a reluctance to use them for
exterior architectural applications because
of the lack of long-term exposure data.
A final, fairly new development that
currently is of interest is the use of organic
additives to sulfuric acid for higher speed
anodizing. Organic additives have been
promoted through the years for hard coating. More recently, however, several companies have expanded the use of organic
additives to conventional sulfuric acid
anodizing. Claims are made that these additives will improve productivity and reduce operating costs by allowing the use
of higher current densities (at higher temperatures). The anodic coatings are said
to be comparable to the normal 70-72F
coating in hardness, and burning has not
been encountered at the higher current
densities. Chemical cost is higher than
that of straight sulfuric acid and the adPFD
ditive must be monitored.
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