Anodizing is the process of increasing aluminum’s strength
and corrosion resistance by forming a layer of oxide on the surface.
Because aluminum is a widely-used metal, anodizing finds use
in many industries, such as the aerospace and automotive industries due to the
light weight of aluminum. Construction uses aluminum in places like roofs and
trim work, while furniture, food preparation, and even art make use of it to
produce objects such as tables, appliances, display cases, and jewelry.
Large-scale anodizing began in 1923, when seaplane parts
were coated to increase corrosion resistance in a process patented by Bengough
and Stuart. This process was upgraded in 1927 by Gower and O’Brien to use
sulfuric rather than chromic acid, and this technique has become the standard
one in use today. In the 1950s, the Japanese discovered a version of the
process that used oxalic acid, which had the main benefit of producing a hard
coating faster. The chromic acid anodizing process is used to give good corrosion
resistance with a relatively thin layer. This coating is softer than the
coating produced by other processes and is easily damaged. There are two
subtypes of this; one ramps the voltage up to 40 volts, while the other uses a
maximum voltage of 20 to 22 volts. The sulfuric acid process has two
subvarieties as well. The first one, type II, is used for decorative or
protective purposes, and the second is used in the aerospace industry.
Due to concerns over hexavalent chromium’s toxcitiy, chromic
acid is being phased out of the electrolytic portion of anodizing operations.
The replacement is boric sulfuric acid, which produces a coating with superior
paint adhesive ability to a coating produced by the chromic acid process. This
process is voltage controlled and ramps up to 15 volts.
Anodizing is an electric process. The aluminum acts as an
anode in an electrolytic bath. Direct current, usually between 15 and 20 volts
is run through the bath, producing hydrogen at the cathode and oxygen at the
anode, the aluminum part, converting the surface into an oxide. The acid and
oxygen actions combine to form nanopores in the oxide surface, allowing the
oxidation process to penetrate farther into the metal and thicken the coating.
These pores are later sealed, sometimes with a dye, to prevent future
corrosion. The coating thickness varies based on use, from 0.5 micrometers to
150 micrometers. Sometimes the anodizing is done combined with chromate
conversion. The anodizing increases the wear resistance of the piece, while the
chromate conversion gives it electrical conductivity.
This process takes place in a long, multi-stage tank. The
parts to be anodized are placed on a rack hung that runs along a track. They
are washed and rinsed in various baths to remove potential contaminants. Next,
they are dipped in the anodizing bath and left there for some time while the
current is applied. At this stage, it is critical to control factors such as
pH, electrolyte strength, current, and temperature for an even coat. After
that, they are rinsed, sealed, and dried before inspection.
An anodized coating has benefits over paints and powder
coats. It is thinner and more durable, lasting longer than other options. It
costs less and can be recycled, unlike paint and powder coating.
When searching for an anodizer, consider what you will be
using the part for. Chromate anodizing coatings are self-healing to an extent
but lack the durability of the sulfuric acid coatings. It works best when there
are few alloying elements in the aluminum. Sulfuric acid anodizing offers the
best possibility for dying the final product. If a thick coating is needed,
such as for aerospace applications, the Type III process is what is used. Find
an anodizer who is willing to examine your parts and listen to what your needs
are so that the most useful procedure occurs. There is a partial list above for
you to examine. Some anodizing work has been outsourced, affecting time taken
to receive parts.
Chromic Acid (Procedure I) – A chromic acid electrolyte is
used to conduct the charge; being phased out due to health and environmental
Sulfuric Acid (Procedures II and III) – Produces coatings
that are harder than those made using chromic acid
Boric-Sulfuric Acid – Replacement procedure for chromic acid
Oxalic acid – Sulfuric acid – An alternative process for
producing hardcoats, used more in Japan and Germany
Abrasion – The process of using friction to grind and
wear away at a surface.
Activation – Changing a metal surface into a chemically-active
Alloy – A substance with metallic properties composed of two or more
chemical elements. At least one element must be metal.
Alkaline Etch – Cleaning step used to give parts a matte
finish and remove any anodic coating in preparation for anodizing
Anodizing Sheet – A sheet that has suitable
metallurgical characteristics and surface quality for the development of
protective and decorative films by anodic oxidation processes.
Bath – The chemical environment in which the anodizing takes place.
Brazing – Joining metals together by flowing a layer of molten filler
metal between them.
Chemical Film – The application of chromate conversion coating on
Chromate conversion – Passivation technique that retains a
Corrosion – The gradual chemical or electrochemical
wearing of a surface. Also, this is the deterioration due to environmental
Cryolite – A white mineral used in making aluminum
Deburring – The removal of burrs, sharp edges and fins by mechanical,
chemical or electrochemical means.
Edging – Dressing metal edges by rolling, filling and drawing.
Electrolyte – Solution that carries an electrical charge
Hardener – An alloy of aluminum and at least one other
metal that is used to make additions to molten aluminum.
Impurities – Undesirable elements in aluminum compounds.
Inhibitor – A substance used to reduce the rate of a chemical or
electrochemical reaction, commonly corrosion or pickling.
Melting Point – The minimum temperature at which a metal will become a
Passivation – Changing a chemically active metal into a passive
Plating – The process of laying a thin coat of metal on another.
Pores – Microscopic openings in an anodized surface, leading to a
microscopic tube. Although porous, the anodized surface is very dense and
Rack – The electrically-conductive device used to hold the parts to be
anodized as they are lowered into the bath. Racks are made in different sizes,
types and shapes to hold the large variety of parts and products encountered in
Refined Aluminum – Aluminum that is in a very pure state.
Smelt – Fusing or melting ore in order to extract or refine the metal it
Striation – Longitudinal lines that are not uniform, caused by uneven
Ultimate Strength – The maximum stress a material can withstand.