Anodising of
aluminium alloys is carried out in order to essentially
protect the metal against corrosion, and wear
and tear over time. The anodising process also
produces an oxide coating on the metal which insulates
it against heat and electricity, and reduces the
possibility of expansion.
The thick, porous anodic film / oxide powder
coating is deposited on the surface of the aluminium
using electrolysis in sulphuric acid. This porosity
of the oxide layer makes it able to take on various
dyes well and can help lubrication adhesion of
the component which is subjected to the anodising
process.
Strength and Adhesion
Anodising can be preferable to simply painting
or metal plating. This is because the uniform
powder layer is in itself attached to the metal
as a result of changes in the metals surface crystal
structure. The thick powder oxide layer is also
strong enough to stand up to wear and tear and
galling.
Sealing After Anodising
Since the anodic film / powder coating created
in the anodising process is porous it needs to
undergo a final sealing process to prevent corrosion
via the pores. Depending on particular job, dyes
and corrosion inhibitors can be added to the pores
before sealing. There are generally 2 main sealing
processes 'hydro-thermal sealing' or 'precipitating
sealing'.
Types of and Thickness of Anodising
This specification MIL-A-8625 outlines the quality
assurance tests for Chromic Acid, Sulphuric Acid
and Sulphuric Hardcoat Anodisation. Each of these
anodising types tend to produce different thicknesses
of anodic films with different levels of resistance.
Different amounts of time spent in processing
creates different thicknesses (measure in microns)
of the protective oxide layer. Thicker layers
may be used in external architectural structures
or on abrasion resistant surfaces whereas thinner
layers are likely to be mainly for decorative
purposes. |
