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surface.
To keep the mirror-like surface
throughout a long period, it
makes sense to apply a clear coat
system, as exposure to normal
atmosphere would lead to a
bleaching of the polished surface.
Aluminium tipper (Benalu)
Aluminium tank (LAG)
140
ALUMINIUM IN COMMERCIAL VEHICULES CHAPTER X DECORATION AND FINISHING 140 | 141
4.1. Anodizing
Anodizing is an electrochemical process
to reinforce the natural oxide
film on the aluminium surface.
Anodising is done in a sulphuric
solution at a certain electric current.
The natural oxide film is
thereby newly built and the
process can be controlled to reach
4. Chemical decoration
STRUCTURE OF THE ANODISED LAYER
FIGURE X.1
Notch
Sealed
cells
Aluminium
Pore
Cell wall
Barrier layer
(base of the
cell)
certain thicknesses of the oxide
layer (in the range of 1000 times
of the natural oxide film).
Anodising not only produces
most frequently a silver- matt surface,
but at the same time
enables increasing hardness, corrosion
resistance and resistance to
abrasion. The process is applied
discontinuously on components
like castings, extrusions and plates
or continuously on coils.
The structure of the anodic film is
determined by the process
parameters (type of bath, applied
current etc.) and consists of
hexagonal cells. The center of the
cells includes a micro-pore with a
diameter of micrometers. These
pores have to be sealed to close
them and to guarantee an excellent
corrosion resistance. This is
done in boiling water under the
use of sealants. (Figure X.1)
4.2. Painting
4.2.1. Introduction
Painting is the most usual way of
decoration for commercial vehicles.
Due to the natural oxide
film on the aluminium surface, it
is of vital importance for a well
adherent and durable organic
coating to apply an efficient surface
preparation.
It is therefore not sufficient to
clean the bare aluminium surface
and to degrease it prior to paint
application. It is essential to
remove also the natural oxide
layer, because it disturbs adhesion
of the paint system.
This can be done in 2 ways:
Chemical pre-treatment
by etching (after degreasing
or by a combine
degreasing/etching process)
Degreasing of aluminium surfaces
can be done with fluid
degreasing solvents, supplied
e.g. by paint producers. The
objective of cleaning and
degreasing are:
• to remove any kind of fatty or
oily residues, or traces of dirt and
dust from the surface
• to prevent electrostatic charging.
141
EUROPEAN ALUMINIUM ASSOCIATION
To apply a degreasing solvent
properly, it is necessary to wipe
the surface with a fresh moistened
cloth and then clean it
with a new, fresh and dry cloth.
Aluminium has amphoteric properties,
which means that it can
be dissolved either in an acidic or
alkaline environment. Etching of
commercial vehicles is normally
done by applying the etching
agent by spraying. Alkaline etching
agents are based on caustic
soda, silicates, phosphates, carbonates
and sodium hydroxide.
The concentration of sodium
hydroxide and the temperature
of the etching agent have a large
influence on the speed and rate
of the etching process. Etching
can also be done on the base of
acidic solutions with phosphoric
acid or nitric acid. Etching leaves
a rough and very moisture-sensitive
surface behind. It is therefore
essential to rinse carefully with
fresh water after etching (about
20 minutes).
Mechanical treatment
sby grinding or blasting
Grinding is to be done on a clean
and degreased surface to prevent
oil being trapped into the aluminium,
which could lead to adhesion
problems of the paint. The graining
of the grinding disk should
have a grain size of 120-180.
Blasting allows a more uniform
treatment of the vehicle and
reaches areas which cannot be
reached by a manual grinding
machine. It is essential to use
iron-free blasting abrasives like
non-recycled corundum, as iron
can lead to corrosion problems1.
The rate of abrasion during blasting
is very low and well below
0.1 mm and therefore in the
same range as etching.
After grinding (which is also used
to flatten the welding seams and
to plane out scratches) or blasting
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