Pre-treatment of Aluminium Surfaces

Pre-treatment of Aluminium Surfaces
Architect Information Surface Finishes
Status 09.2012
Content
1 Schüco Overview System Finish
2 Schüco Overview Pre-treatment Choices
3 Importance of Pre-treatment
4 Requirements for the basic material Aluminium
5 Pre-treatment for Powder and Liquid Paint Coatings
6 The Pre-treatment Yellow Chromation
7 The Chromium-free Pre-treatment
8 The Pre-anodisation
9 The Seaside Pre-treatment
10 The Pre-treatment of anodisated Surfaces
11 Effects of non-optimal Pre-treatment
12 Filiform corrosion – preventable using the correct Pre-treatment
13 Additional Information regarding Schüco System-Finish & Profile Connection
14 Contact Details
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1. Schüco Overview System Finish
Schüco System Finish Surface Options (Status 09.2012)
Basis
Performance
Decor
Premium
MetallicEffekte
Fluorpolymer-Pulver
Cost
PowColor
Facade Quality
Powder Base Coat with
Topcoat
AnoLine
Pre-Anodisation with Powder
Top Coat
CosmoLine
DuraClean
MetallicLine
Anti-Graffiti
High Durability Quality
(Powder)
Fine Texture Powder
AnoLine FS
Wood & Stone Optics:
DECORAL
NATURALL
WetLine
AnColor
Anodisation
(E6/EV1, C0)
MetallicLine Plus
INOX Optic Powder
Anodisation for Structural
Glassing
(SG Anodisation)
Coloured Anodisation with /
without mechanical Pretreatment
(E1 to E6, C0, C31 to C35)
(Anodisation)
INOX Optic Classic
INOX Optic Plus
Silbver & Gold (EV2 to
EV 4)
Gloss anodisation
S315
WetColor
(Liquid Paint)
Polyurethan (PUR)
Duraflon
PUR Trend
SANDALOR
Duraflon
Duraflon Trend
AnoLine DF
PVDF
PVDF Decor
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2. Schüco Overview Pre-treatment Choices
Overview of possible Pre-treatment options within System Finish (Status 09.2012)
Comments:
1) Yellow Chromation is the most often applied method; Green Chromation is also used for
Aluminium and is most often applied via a dipping process.
2) Currently, the Chromium-free pre-treatment is required occasionally in Germany but with an
increasing tendency. In France as well as in certain industries (car and airplane production)
the Chromium-free pre-treatment is already standard.
3) For Seaside (Qualicoat quality group) also the term Qualimarine (Southern Europe) is used.
4) For pre-anodisation (abbreviation VA or MVA) the term „Aneloxal“ is used in German.
5) E0 and E6 are pre-treatment options for anodisation according to DIN 17611.
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3. Importance of Pre-treatment
Primary Protection Function for Corrosion and Adhesion on Aluminium
Industrial produced profiles and sheets have an
oxide layer as surface, which has built-in due to its
natural growth (Aluminium as reactive metal reacts
with Oxygen and moisture) during its formation
dirt, moisture, oils, fat and process additives.
To insure the prevention of corrosion and optimal
adhesion between the Aluminium base and the
finish, the contaminated initial Aluminium oxide
layer is removed down to the pure Aluminium alloy
by the controlled pre-treatment process (compare
point 4)
This oxide layer is not suitable for the prevention
of corrosion nor as an adhesive base for the Based now on the pure Aluminium alloy, the prefollowing finish due to its inhomogenity and the treatment
builds-up
an
controlled
and
incorporated non-Aluminium materials.
homogeneous conversion / adhesion layer (e.g.
the Yellow Chromate or the pre-anodisation layer).
The use of purified chemicals and additives
prevents new contaminations of this pre-treatment
layer and is regulated and controlled by quality
groups.
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4. Requirements for the basic material Aluminium
Influence of alloys and material upon the pre-treatment
The pre-treatment is an important part of the finish Quality conform pre-treatment process (photo:
process for the production of a protective and GSB):
uniform surface layer on the Aluminium surface.
Three main process steps are performed during
the pre-treatment:
1. Removal of oils, grease and und dirt
2. Removal
of
outer
inhomogeneous
oxide layer of the Aluminium
3. Formation of the pre-treatment layer
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5. Pre-treatment for Powder and Liquid Paint Coatings
Comments
The burn-in powder coating is by far the most
widely used finish method for architectural usage.
Based on the used technology, powder and liquid
paint coating processes are quite similar (but
require different production units). Therefore, both
application methods are used synonymous during
the following discussion of pre-treatment choices,
e.g. comments for powder coating are equally true
for liquid paint coating.
On request, Schüco will inform you about pretreatment possibilities because these choices must
fit to the finish process as well as to the length of
the material to be coated.
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HD Powder Surface (MetallicLine)
Down: Liquid Paint Surface (Duraflon)
Top:
6. The Pre-treatment Yellow Chromation
Yellow Chromation is currently the most often Microscope Picture (40x enlarged):
used pre-treatment method for powder and liquid
paint coatings.
This process is based upon the reaction of
Chromic acid, activating complex salts and
Fluoride in acidic solution with the purified
Aluminium surface. Depending on the reaction
time and conditions, a colourless to golden yellow
layer is formed – the colour arises from the
inclusion of Cr6+ within the formed layer.
This pre-treatment is not allowed when food stuff
is handled or prepared due to the Cr6+.
Left: Yellow Chromation layer on Alu, Right: Aluminium after
pre-treatment without Yellow Chromation layer (photo: Schüco)
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7. The Chromium-free Pre-treatment
Due to environmental concerns regarding Microscope Picture (10x enlarged):
Chromium
compounds,
Chromium-free
alternatives, based mostly upon the metals
Titanium and Zirconium, have been developed.
In this system, the dissolved Titanium / Zirconium
compounds, together with the complex agent
Fluoride and additives, react in acidic solution with
the Aluminium surface. A mixed metal oxide
containing both Aluminium and Titanium /
Zirconium is formed containing also Fluoride and
other components. The resulting conversion/
adhesion layer is colourless and requires therefore
careful adjustment and control to insure full
surface treatment.
Top layer (powder), Middle: Chromium-free Pretreatment, Right: Aluminium surface (photo: Schüco)
Left:
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8. The Pre-anodisation
By use of electric current in an acidic aqueous Microscope Picture (40x enlarged):
solution, a very stable and non-separable oxide
layer is produced onto Aluminium (principle of
anodisation).
During pre-anodisation according to Schüco / GSB
an 3 to 8 µm thick oxide layer is produced in an
aqueous sulfuric acid solution under controlled
conditions. In contrast to decorative anodisation
this layer is not sealed. The resulting quite
sensitive open cellular surface structure is an
excellent adhesion base for the powder or liquid
paint coating, which has to be finished within 8 h
(differently according to GSB 12 to 24 h).
This pre-treatment is currently the only system
providing a protection against Filiform corrosion
with a warranty by Schüco.
Enlarge View onto a pre-anodised Aluminium surface (photo:
Schüco)
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9. The Seaside Pre-treatment
This pre-treatment was developed by the quality Microscope Picture (20x enlarged):
organisation Qualicoat and is based upon the
removal of a defined surface amount under careful
controlled conditions. This process is also known
on the market as Premium Pre-treatment (quality
group GSB) and as Qualimarine pre-treatment
(southern Europe); slight technical process
variations are possible.
During this surface removal, corrosion promoting
materials are removed from the surface and the
risk of Filiform corrosion is thereby reduced.
A complete warranty against Filiform corrosion is
currently not possible – due to insufficient data,
Schüco excludes presently Filiform corrosion from
Aluminium surface after Seaside pre-treatment (photo Schüco)
its warranty.
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10. The Pre-treatment of anodisated Surfaces
The chemical pre-treatments E0 and E6
For the anodisation („Eloxal“) according to DIN Microscope Picture (10x enlarged):
17611 two chemical pre-treatments are possible
and can be obtained via Schüco:
- E0 (degreasing & desoxidation): The
Aluminium surface is only cleaned and only minor
amounts of metal is removed – surface blemishes
like marks & scratches remain visible (compare
top right picture). This pre-treatment is only
allowed for non-visible under constructions within
architectural applications.
- E6 (etching): In addition to the E0 process the Top: E0; Bottom: E6 Pre-treatment on Alu (photos: Schüco)
etching step is added to remove Aluminium and
alloy components in basic and acidic solutions.
This is done to create a defect free and
homogenous Aluminium surface for the anodising
step.
Anodised surfaces are always protected against
Filiform corrosion due to their build-up.
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11. Effects of non-optimal Pre-treatment
Functions of the pre-treatment process are to Non-optimal pre-treatment may result in
insure corrosion protection and the optimal insufficient adhesion (photo: Schüco)
adhesion to the optical important top coating layer
(based on either powder or liquid paint).
On the right, the result of a wrong or technical
incorrectly performed pre-treatment is presented in
its optical impact:
Missing pre-treatment (in this case due to over
coating) results under weathering in lost of
adhesion and delaminating of the paint layer.
The delaminating of the paint layer from the base
will increase with time and results in increasing
optical disturbance together with corrosion
problems depending on the base.
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12. Filiform corrosion
Preventable using the correct Pre-treatment
Close to the sea (up to 75 km, salt load Example for Filiform corrosion (photo: Schüco)
depending), in wet areas (specifically swimming
pools and saunas) and on main roads treated in
winter with salt for de-icing, Filiform corrosion may
occur and pre-anodisation currently the only
protection:
This kind of corrosion is a reaction between the
metallic Aluminium and Chloride. It starts on the
cutting edges, most likely in the lower area of the
part and the building. Optically, a „worm-like“
separation of the top coat (the colour) from the
metallic substrate is visible.
This corrosion does not stop but will attack
increasing areas of the building, specifically by
separation of the top coating. A permanent &
lasting repair is not possible but only the exchange
of the complete material against pre-anodisated
material.
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13. Schüco System Finish Information
Overview of available information from Schüco – Status 09.2012
Downloads via www.schueco.com:
Printed Information:
Architect info Pre-treatment
Architect info Powder Coating
Architect info Metallics
Architect info Dirt repellent Coatings
Architect info Liquid Paints
Architect info Anodisation
Colour finishes for Aluminium (Colour Booklets)
Videos Finish Methods
Edition Advance 07/2012 (Art. No. 63117)
contains also object surface options and special
finish methods used mainly for objects.
Digitale Info via SCV:
A wide series of information for current finish
products is also available as PDF.
Function and Service Information Schüco
All Information are available as PDF in German or
English.
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Edition Basic 07/2012 (Art. No. 63116) for often
requested finishes used in end customer business
such as RAL, Standard Metallics and Anodisation
according to EURAS.
14. Contact Details
For additional information and support for the presented products, please
contact the Service Centre Finish (SCV)
Schüco International KG
Karolinenstraße 1-15
33615 Bielefeld
Tel.:
Fax:
0521 / 783 - 0
0521 / 783 - 451
www.schueco.com
info@schueco.com
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