The first is metal preparation. In this phase we first clean the metal surface
with a "soap" solution to get rid of any dirt, grease, grime, hand prints or the
like. Next we use different chemicals to give us the "look" we want (bright or
dull). If we want bright we use Phosphoric acid to chemically polish the
surface and if we want dull we use Caustic Soda to etch the surface. At the
end of this phase we have a clean reactive surface that has the "look" (bright
or dull) required and is ready to be Anodized.
The second phase is Anodizing. In this phase we place the extrusions in the
anodizing tank and apply electrical power to the extrusions. The bath is a 17%
solution of Sulfuric Acid and the power is 18 volts of D.C. current at 12 amps
per square foot of surface area. It takes about 15 minutes to build the required
.2 mils of aluminum oxide. After the required time has passed the parts are
withdrawn from the anodizing tank and the acid is rinsed off.
The third phase is coloring if necessary and sealing. When Aluminum is
anodized the coating is porous because as the current flows from the extrusion
surface into the electrolyte it erodes microscopic holes through the oxide. This
creates both a problem and an opportunity. The opportunity is that because of
the porosity we can absorb dye into the coating and thereby create a colored
extrusion (gold or any other color if the dye is available). The problem is
that the porosity allows any corrosive agent to reach the base aluminum and
create a corrosion problem. To stop that from happening the pores have to be
closed, that process is called sealing. Sealing is accomplished by absorbing
chemicals into the surface of the coating and causing them to swell, thereby
plugging the holes. Properly sealed anodic coatings do not corrode easily and
do not allow staining or leaching of dyes from the coating.