Corrosion of Metallic Coatings 295
monoxide is then admitted to the retort, reducing the red oxide to magnetic
oxide, which is resistant to corrosion. Each operation takes approximately
20 min.
Iron and steel may also be coated by electrolytic means. The article to
be coated is made the anode in an alkaline solution (anodic oxidation).
These coatings are primarily for appearance, such as for cast iron stove
parts.
8.5.4 anodized Coatings
The electrochemical treatment of a metal serving as an anode in an electro-
lyte is known as anodizing. Because aluminum’s electrode potential is nega-
tive and its oxide lm is stable in natural environments, surface treatments
have been developed for the purpose of producing more stable oxide lms.
The anodic lms formed can be either porous or nonporous, depending on
which electrolyte is used.
Porous lms result when electrolytes such as sulfuric acid, oxalic acid,
chromic acid, and phosphoric acid are used. These lms have the advantage
of being able to be dyed.
Sulfuric acid is the most widely used electrolyte. A large range of operating
conditions can be utilized to produce a coating to meet specic requirements.
Hard protective coatings are formed that serve as a good base for dying. To
obtain the maximum corrosion resistance, the porous coating must be sealed
after dying. The anodic coating formed, using sulfuric acid as the electrolyte,
is clear and transparent on pure aluminum. Aluminum alloys containing
silicon or manganese and the heterogeneous aluminum-magnesium alloy
yield coatings that range from gray to brown and may be patchy in some
cases. The absorptive power of these coatings makes them excellent bases for
dyes, especially if they are sealed in nickel or cobalt acetate solution.
It is not recommended to use sulfuric acid as the electrolyte for anodizing
work containing joints that can retain the sulfuric acid after removal from
the bath. The retained electrolyte will provide sites for corrosion.
When chromic acid is used as the electrolyte, the coatings produced are
generally opaque, gray, and iridescent, with the quality being dependent
on the concentration and purity of the electrolyte. These are unattractive
as compared to those formed using sulfuric acid as the electrolyte. When a
0.03% sulfate is added to the electrolyte, colorless and transparent coatings
are formed. These coatings are generally thin, of low porosity, and hence
difcult to dye. Black coatings can be obtained in concentrated solutions at
elevated temperatures. Attractive opaque surfaces can be obtained by add-
ing titanium, zirconium, and thallium compounds to the electrolyte.
The chromic acid oxidizing process is the only one that can be used on
structures containing blind holes, crevices, or difcult-to-rinse areas.
Chromic acid anodizing generally increases fatigue strength, while sulfuric
acid anodizing may produce decreases in fatigue strength.