Page 165 - Corrosion Engineering Principles and Practice

140 C h a p t e r 5 C o r r o s i o n K i n e t i c s a n d A p p l i c a t i o n s o f E l e c t r o c h e m i s t r y 141

Although the film produced is extremely thin, it has excellent
corrosion resistance and can be colored if desired. A typical bath
might contain from 5 to 10 percent chromic acid, and be run at about
35 to 40°C. There are two main processes, one using 40 V and a newer
process using 20 V.
Oxalic acid is sometimes used as an anodizing electrolyte using
similar equipment. This bath will produce films as thick as 50 µm
without the use of very low temperatures and usually gives a gold or
golden bronze color on most alloys. The typical concentration is from
3 to 10 percent oxalic acid at about 27 to 32°C, using a DC voltage of
about 50 V.
Phosphoric acid baths are used in the aircraft industry as a
pretreatment for adhesive bonding. They are also very good
treatments before plating onto aluminum. A typical bath might
contain from 3 to 20 percent of phosphoric acid at about 32°C, with
voltages as high as 60 V.
Properties of the Oxide Film
Anodized coatings are typically 2 to 25 µm thick, and consist of a
thin nonporous barrier layer next to the metal with a porous outer
layer that can be sealed by hydrothermal treatment in steam or hot
water for several minutes. The resultant oxide is also nonconductive.
This particular property of the anodic oxide is useful in the
production of electrolytic capacitors using a special bath of boric
and/or tartaric acids.
By balancing the conditions used in the anodizing process, oxides
with almost any desired properties can be produced, from the thin
oxides used in decorative applications to the extremely hard, wear-
resistant oxides used in engineering applications. The utilization of
electropolishing or chemical bright dipping in conjunction with a thin
anodic oxide produces a finish whose appeal cannot be duplicated by
other means. Matte finishes produced by etching the aluminum surface
provides a “pewter” look that is the finish of choice of many architects.
If the anodic oxide is slightly soluble in the electrolyte, then
porous oxides are formed. As the oxide grows under the influence of
the applied DC current, it also dissolves, and pores develop. It is this
property that provides the ability to color the oxide using organic
dyes, pigment impregnation, or electrolytic deposition of various
metals into the pores of the coating.
The anodizing process conditions have a great influence on the
properties of the oxide formed. The use of low temperatures and acid
concentration yield less porous, harder films. As the anodizing
temperature is increased, the oxide becomes more porous and improves
in its ability to absorb color. However, it also loses its hardness and its
luster, due to the dissolution action of the acid on the oxide surface.
As the pore size increases, sealing becomes more difficult and a greater
amount of color is leached out into the sealing bath.