Nature has provided aluminum with a highly protective “skin” in
the form of a clear barrier oxide on its surface that forms quickly and
is tough enough to hinder the deeper intrusion of oxygen and
other gases and liquids to the subsurface aluminum atoms. This
oxide is tightly chemically bound to the underlying surface, and if
damaged, reforms immediately in most environments. On a freshly
abraded surface, the barrier oxide film is only 1 nm thick, but is highly effective in protecting the aluminum.
The oxide film develops slowly in normal atmospheres to greater
thicknesses, and when corrosive environments are present, the
oxide may both thicken and darken. However, it generally
retains its protective character. Thus, in normal environmental
exposure, aluminum does not corrode (rust) away as does steel.
Aluminum surfaces do oxidize when exposed to air, but this
differs from the oxidation of steel in two important ways:
Aluminum oxide is effectively transparent and invisible to the
unaided eye. Aluminum oxide clings tightly to the surface of aluminum and forms a protective film that blocks progressive deterioration.
It does not flake off, thereby exposing fresh surfaces to further oxidation. When damaged, it quickly reforms again, providing continuing
With this natural corrosion resistance, the aluminum bodies
of many commercial motor vehicles, rail cars and aircraft are
unpainted; aluminum has proven durability in such applications.
Anodic coatings are among the most useful for many applications
Increase corrosion resistance.
Increase paint adhesion.
Increase adhesive bond durability.
Improve decorative appearance.
Increase abrasion resistance.
The basic approach in anodizing is to increase the thickness of the
natural oxide coating on aluminum by converting more of the
underlying aluminum surface to aluminum oxide while the part
being anodized is the anode in an electrolytic cell.
Chemical conversion coatings are adherent surface layers of lowsolubility metal oxide, phosphate, or chromate compounds produced
by the reaction of suitable reagents with the metal surface. They differ
from anodic coatings in that conversion coatings are formed by
a chemical oxidation-reduction reaction at the aluminum surface,
whereas anodic coatings are formed by an electrolytic reaction.
Same as steel.
I had to take an SAE course on corrosion during a job where corrosion was causing major failures. So I just copied this out of the instruction manual.