In 1946, Southern Aluminum Finishing (SAF) began work on its first
anodizing line. Four years later, the family-owned company ventured
into the architectural field selling aluminum sheet and extrusions
with and without anodized finishes. In the 53 years since, architectural
aluminum and the finishes applied to it are probably SAF’s
strongest markets.
SAF’s long history within the architectural and anodizing
markets has allowed it to grow and develop along with these markets.
It has developed standards for finishing architectural aluminum
that can be used in most anodizing shops and are often specified
by builders and architects. The SAFINISH Designation System is what
SAF uses for specifying exterior finishes used on architectural
aluminum building products. “A SAFINISH has the longest expected
lifetime of any available finish for architectural aluminum,”
noted Penn McClatchey, vice president. “These finishes are
only available on aluminum, because aluminum is the only metal with
the longevity and corrosion resistance equal to the quality implied
by the SAFINISH designation.
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| The
Clark County Athens Regional Library storefront has a 2-step
anodized coating that was subsequently dyed with a Lightfast
Red dye to produce this burgundy color.
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“It doesn’t make sense to apply an inferior aluminum
finish to cut costs. An aluminum finish is typically less than 1%
of a project’s cost, but it can be 100% of its appearance,”
he continued. SAF is so confident in its finishes that it offers
5- and 20-year limited warranties on them.
So, how do you specify the right finish for architectural aluminum?
Not only is there anodizing, but you can also choose from paint
and powder coating. SAF supplies all three finishes, but offers
consumers the benefits and shortcomings of each coating, depending
on the final architectural application.
Basically,
What Is Anodizing?
Anodizing is a simple electrochemical process developed more than
50 years ago. It forms a protective coating of aluminum oxide on
the surface of the aluminum. The lifetime of the finish is proportional
to the thickness of the anodic coating.
The coatings have been historically developed from either a chromic
acid or sulfuric acid bath. Sulfuric acid anodizing is probably
the most commercially important, since it can produce coatings in
a range of thicknesses. These coatings can also be dyed. Chromic
acid anodized surfaces are also porous and, therefore, able to be
dyed. There are advantages to sulfuric acid anodizing over chromic
acid anodizing. These include:
- Less expensive with respect to chemicals used, heating, power
consumption and length of time needed to obtain the desired thickness
- More alloys can be treated
- Coatings are harder
- It is a clearer coating that permits dyeing with a greater variety
of colors
- Waste treatment is simpler
There are, however, limitations to sulfuric acid anodizing. These
include:
- Reduction of the fatigue characteristics of the alloys
- Difficult to control when small dimensional changes are required,
since the coating grows more quickly than a chromic acid anodize
- Solution may remain in recesses and other areas, causing corrosion
A chromic acid anodized coating is harder than the base metal; however,
it provides limited abrasion resistance because of the low thickness
and structure. These coatings do have higher resistance to salt
spray corrosion, and very thick coatings of this type are electrically
insulating.
There are several advantages to chromic acid anodizing. These include:
- Not necessary to remove all traces of residual solution
- Excellent base for organic coatings
- Coatings are thinner but meet the same corrosion resistance
requirements as conventional sulfuric acid anodized parts
- Coatings are excellent as maskants for selective hard anodizing
applications
Chromic acid anodizing also has its drawbacks compared to sulfuric
acid anodizing. These include:
- Lower abrasion resistance
- Limited to alloys containing less than 5% copper or 7% silicon
- Higher voltage and more time required
- Cannot anodize alloys in the annealed condition
- Burning may occur
- Waste treatment for hexavalent chromium is required.
Anodizing is controlled using current density. When beginning an
anodizing job, the initial voltage should be low and slowly increased.
Aluminum oxide is a hard, durable, weather-resistant substance that
protects the base metal. The coating may be colored by dyeing or
may exhibit bronze tones through diffraction phenomena produced
by the coating. The coating grows from the base aluminum metal through
this electrochemical process. The coating is integral to the metal
and cannot peel or flake. The structure of the coating is many small
hexagonal pores that are filled with a seal that hydrolyzes them
to fill them with inert aluminum oxide.
Hard anodizing or Type III is different from Type I or Type II
anodizing with typical chromic or sulfuric acid based solutions.
Hard anodizing uses sulfuric acid, additives, a lower temperature,
higher current density, vigorous agitation and produces heavier
coatings. The coatings exhibit excellent corrosion resistance; however,
chemical resistance is not good. Cracking or crazing may result
as the temperature increases to ambient temperature or higher sealing
temperatures. Since sealing reduces hardness by as much as 60%,
hard anodized coatings should not be sealed if maximum corrosion
resistance is required. Flaking or chipping can occur at temperatures
above 200-300C. There is very little of this type of coating used
in architectural applications.
In general, anodizing is less expensive than painting, with the
exception of coil-coated products. Anodizing is harder than PVDF
and better for aluminum in high traffic areas where the coating
is subject to physical abuse and abrasive cleaners. Anodizing gives
aluminum a deeper, richer metallic appearance than is possible with
organic coatings, because an anodized coating is translucent, allowing
the base metal to show through the coating. This translucence contributes
to color variation problems, but SAF uses computerized color matching
with quantitative, objective color data to overcome this problem.
Anodizing is unaffected by sunlight, making it perfect for outdoor
architectural applications. Unlike organic coatings, it will not
fade due to ultraviolet light.
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| The
studios of WCNC in Charlotte, NC are clad with fabricated
aluminum sheet that has been 215 clear anodized.
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Anodizing
and the Environment
Anodizing is compatible with today’s environmental
concerns. However, more research needs to be done to determine the
total environmental impact of different aluminum finishes. The chemicals
from anodizing can be used by municipal wastewater treatment facilities.
The aluminum sulfate from anodizing plants’ effluent actually
improves the solids settling efficiency of some wastewater treatment
plants. There are no heavy metals involved in the process.
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| Front
of an office building with gold anodized features.
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Curtain
Walls and Metal Roofing
When a live, translucent building exterior is desired,
anodizing is a good finish for a curtain wall. Architects should
require color range panels from the anodizer. The installer should
be aware of the variability expected from anodized panels. An installer
may be able to sort some of the panels by elevation for a desired
effect. If a panel is outside the approved color range, the finisher
should not ship it. However, SAF feels it is also the installer’s
responsibility to avoid installing metal that is not within range.
“This is why we recommend the SAFINISH (Aluminum Association
Class I). This makes the anodic coating as durable as possible.
Coating thickness is one of the most significant indicators of durability
for anodized coatings,” noted Mr. McClatchey. “Coating
thickness for architectural use can be specified as either Class
1 (0.7 mil) or Class 2 (0.4 mil) per the Aluminum Association DAF
45, but often there is no specification. The ‘cheapest priced’
piece is then installed, and in a few years the finish is pitted,
stained and eroded. Inexpensive anodized sheets are often sold with
a coating thickness of 0.1 mil.”
| SAF’s
Guidelines for Anodizing |
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1. Always check the alloy to be sure it is anodizing
quality. Mixed alloys or tempers will not produce uniform
results. Die castings are difficult to anodize because
of their porous nature. For anodizing, 5005 aluminum
sheet and 6063 aluminum extrusions are recommended.
2. Assemblies must have drain holes to prevent solution
entrapment. Top holes are required for admission of
air, and bottom holes are required for drainage. Anodizing
chemicals can leak through even the tightest weld joints.
3. Two flat surfaces fastened to one another can trap
anodizing solutions and leak out later causing damage
to the finish.
4. Assemblies to be anodized should never include non-aluminum
materials.
5. Be sure to use the correct alloy when welding is
involved. 4043 aluminum is the worst choice because
it will turn a smutty black. The preferred alloy is
5356.
6. Anodizing surface quality depends on the metal.
Anodizing will not cover up scratches or water stains.
7. Aluminum must be kept dry and handled with gloves
to prevent stains. It is critical that aluminum be kept
dry in transit.
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8. Because aluminum must carry a certain amount of
current per sq ft of anodized area, full size and formed
sheets should be at least 0.032 inch thick.
9. Specifications should always be provided upfront.
The finishers should also know about the end use of
the aluminum.
10. The anodizer should be told where to rack the material.
At the racking point, the aluminum will carry current
needed for the anodizing process, but no anodizing will
occur. For architectural parts, a 3/8-inch bolt is often
used for racking that has a halo of perhaps a square
inch. Racking instructions are critical for the anodizer.
11. Fabrication should be done prior to anodizing.
An anodized surface will crack or craze when formed.
12. Make sure material is packaged well before shipping
it to the anodizer. When possible, SAF reuses its customers
packaging to reduce waste and ensure that the metal
arrives dry and free of scratches and dents.
Reprinted from the SAF
website.
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A finish with a thickness of 0.1 mil is known as a 200, A21, A22
or A24 finish. This coating thickness is suitable for many applications;
however, the integrity of this finish will not last more than a
few years in exterior architectural applications, and even less
in coastal regions. Also, because it is a thin coating, there is
less color agent, which will cause it to fade more quickly.
Cleaning can renew anodizing; however, little can be done if the
coating has failed. Both anodized and painted surfaces require regular
maintenance, which is something building owners need to consider.
Often, masonry contractors use acid to pressure wash the bricks
or concrete. Any of this that washes over onto an anodized surface
could destroy it.
Storefront
Kickplates, door handles, door stiles and push/pull bars
should be anodized, since the abrasion resistance will outlast any
painted surface.
Whatever type of anodized finish is chosen depends on the end use.
Anodizing is best suited to storefronts and anywhere else a rich metallic
appearance is desired. Anodized and polyester coatings are best for
storefront and handrails. Anodized, PVDF and polyester coatings could
all be used on curtain wall, roofing and storefront applications.
