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it is necessary to remove
traces of the abrasives by compressed
air and then to clean the
surface again.
1. The incrustation of iron particles on
the aluminium surface is a source of galvanic
corrosion that will lead, in the
presence of moisture, to superficial
micro-pitting.
Cleaning before painting (LAG)
142
ALUMINIUM IN COMMERCIAL VEHICULES CHAPTER X DECORATION AND FINISHING 142 | 143
4.2.2. Application of the primer
The primer should be applied
directly after the pre-treatment of
the surface to prevent the rebuilding
of the oxide film or to prevent
any dust being attracted by the
vehicle during longer periods of
waiting time. Primers (or “wash
primers”) are used as adhesive
agents to maintain the necessary
bonding forces between the substrate
(aluminium surface) and the
paint system. They are also working
as corrosion inhibitors, as they
prevent water vapour diffusion
through the paint system from
getting in contact with the aluminium
surface. Primers made of
epoxy- resins are a well suited
material for pre-treating aluminium,
but need a thoroughly treated
bare metal surface. The primer
is normally applied by spray gun
and the thickness of the wash
primer or reaction primer layer is
about 10 μm.
4.2.3. Final coating
The application of the final coating
system can be done in different
ways, but is anyhow not specific for
aluminium. In any case, it is of vital
importance to use paint systems
with coordinated properties. The
technical rules of the paint suppliers
have to be strictly obeyed.
The final coating system can be separated
into 2- or 3-layer systems
with or without the use of fillers and
basecoats. Fillers are needed to flatten
unevenness and/or to increase
thickness of the coating system.
For preparation of the surface, the
primer layer has to be ground with
a smooth grinding disk (roughness
300-400). Fillers have also to be
ground before application of the
topcoat system.
The paint is normally applied with
spray guns. Drying times and temperatures
have to be controlled. It
might be necessary to apply an
intermediate fine grinding of the
single paint layers.
A typical painting procedure of a
silo tank trailer could be2:
• Etching / degreasing inside and
outside by spraying with an inhibited
etching agent based on phosphoric
acid
• Rinsing with fresh water for
about 20 minutes
• Final assembly of the vehicle
• Grinding of the tank surface with
a manual grinding machine to
remove small surface damages
• Cleaning and degreasing with
degreasers or silicone removers
• Application of the wash primer
onto the outer tank surface. Layer
thickness 8-10 μm.
• Drying of the tank at room temperature
(20°C) or at elevated temperatures
up to 80°C
• Removal of unevenness with a
filler; grinding of the filler layer
• Removal of dirt and dust by
wiping with a moisturized cloth
• Application of the 1st paint
layer (basecoat or wet-in-wet
filler) in 2 steps with a combined
layer thickness of 60-70 μm.
Special attention should be paid
to the area of stone chipping.
• Application of the topcoat
(clear-coat) in the desired colour
after max. 2 hrs. Final coating
thickness 50 – 60 μm.
• Drying of the top layer
Extrusion profile systems used e.g.
in tipping trailers can be painted
in 2 ways: either the trailer can be
painted as a whole or the profiles
can be painted individually and
then being assembled. The general
rules for decoration mentioned
above are also valid for these
types of constructions.
In any case it is essential for a sufficient
and long lasting paint decoration
to apply a well conducted
preparation of the surface as
mentioned before. Problems with
the paint decoration often are
not related to the paint or the
aluminium itself, but more with
an insufficient pre-treatment.
2. Koewius, Gross, Angehrn Aluminium-
Konstruktionen des Nutzfahrzeugbaus,
Aluminium Verlag, Düsseldorf, 1990.
143
EUROPEAN ALUMINIUM ASSOCIATION
144
1. DEFINITION OF CORROSION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146
2. CORROSION OF ALUMINIUM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146
2.1. The natural oxide layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146
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