Why do you anodize
Exposed to the earth's atmosphere,
aluminum combines with oxygen to form a
protective surface film which inhibits further
oxidation of the aluminum. Unlike steel
or iron alloys, aluminum will not continue
to oxidize (rust) once this protective layer
is formed. This natural oxide is extremely
thin and loosely adhered to the aluminum
surface, however, and is easily removed
by handling. Anodizing is a process which
thickens the natural oxide film resulting
in a heavy aluminum oxide film of controlled
thickness having the hardness similar to
that of a ruby or sapphire.
Aluminum anodizing is the
electrochemical process by which aluminum
is converted into aluminum oxide on the
surface of a part. This coating is desirable
in specific applications due to the following
- Increased corrosion resistance
- Increased durability
/ wear resistance
- Ability to be colored
- Electrical insulation
- Excellent base or primer
for secondary coatings
During the anodizing process,
several controls are critical to assure
the specified film thickness, its abrasion
resistance and density. These controls include
a precise combination of chemical concentration,
temperature and current density. In the
production of quality anodized products,
there is no alternative to having sophisticated
monitoring equipment and highly-trained,
experienced personnel. The company you choose
for your anodizing projects must be able
to demonstrate these qualities.
When aluminum is anodized
conventionally, direct electrical current
(DC) is passed through a bath of sulfuric
acid -- the electrolyte -- while the aluminum
being treated serves as the anode. This
produces a clear film of aluminum oxide
on the aluminum's surface. Electron microscopy
indicates that this layer is mostly porous
with a very thin barrier layer at the base.
This structure lends itself very well to
electrolytic coloring or absorptive dying.
- Pre-Treatment: Cleaning
is done in a non-etching, alkaline detergent
heated to approximately 145 degrees Fahrenheit.
This process removes accumulated contaminants
and light oils.
- Rinsing: Multiple
rinses, some using strictly de-ionized
water, follow each process step.
- Etching (Chemical
Milling): Etching in caustic soda
(sodium hydroxide) prepares the aluminum
for anodizing by chemically removing a
thin layer of aluminum. This alkaline
bath gives the aluminum surface a matte
- Desmutting: Rinsing
in an acidic solution removes unwanted
surface alloy constituent particles not
removed by the etching process.
- Anodizing: Aluminum
is immersed in a tank containing an electrolyte
having a 15% sulfuric acid concentration.
Electric current is passed through the
electrolyte and the aluminum is made the
anode in this electrolytic cell; the tank
is the cathode. Voltage applied across
the anode and cathode causes negatively
charged anions to migrate to the anode
where the oxygen in the anions combines
with the aluminum to form aluminum oxide
(Al2O3). View our anodizing video.
- Coloring: Anodic
films are well suited to a variety of
coloring methods including absorptive
dyeing, both organic and inorganic dyestuffs,
and electrolytic coloring, both the Sandocolor®
and Anolok® processes.
- Sealing: In all
the anodizing process, the proper sealing
of the porous oxide coating is absolutely
essential to the satisfactory performance
of the coating. The pores must be rendered
nonabsorbent to provide maximum resistance
to corrosion and stains. This is accomplished
through a hydrothermal treatment in proprietary
chemical baths or by capping the pores
via the precipitation of metal salts in
the pore openings.