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 Page 2
1. Schüco Overview System Finish Schüco System Finish Surface Options (Status 09.2012) Basis Performance Decor Premium Cost PowColor Facade Quality Powder Base Coat with Topcoat (Powder) AnoLine Pre-Anodisation with Powder Top Coat MetallicEffekte Fine Texture Powder AnoLine FS Fluorpolymer-Pulver CosmoLine MetallicLine DuraClean Anti-Graffiti High Durability Quality Wood & Stone Optics: DECORAL NATURALL WetLine AnColor (Anodisation) 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) Silbver & Gold (EV2 to EV 4) INOX Optic Classic INOX Optic Plus Gloss anodisation WetColor (Liquid Paint) S315 SANDALOR Polyurethan (PUR) Duraflon PUR Trend Duraflon Duraflon Trend AnoLine DF PVDF PVDF Decor Page 3
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. Page 4
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. This oxide layer is not suitable for the prevention of corrosion nor as an adhesive base for the following finish due to its inhomogenity and the incorporated non-aluminium materials. 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) Based now on the pure Aluminium alloy, the pretreatment builds-up an controlled and 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. Page 5
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 process for the production of a protective and uniform surface layer on the Aluminium surface. Quality conform pre-treatment process (photo: GSB): 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 Page 6
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. Top: HD Powder Surface (MetallicLine) Down: Liquid Paint Surface (Duraflon) 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. Page 7
6. The Pre-treatment Yellow Chromation Yellow Chromation is currently the most often used pre-treatment method for powder and liquid paint coatings. Microscope Picture (40x enlarged): 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 Cr 6+ within the formed layer. This pre-treatment is not allowed when food stuff is handled or prepared due to the Cr 6+. Left: Yellow Chromation layer on Alu, Right: Aluminium after pre-treatment without Yellow Chromation layer (photo: Schüco) Page 8
7. The Chromium-free Pre-treatment Due to environmental concerns regarding 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. Microscope Picture (10x enlarged): Left: Top layer (powder), Middle: Chromium-free Pretreatment, Right: Aluminium surface (photo: Schüco) Page 9
8. The Pre-anodisation By use of electric current in an acidic aqueous 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. Microscope Picture (40x enlarged): Enlarge View onto a pre-anodised Aluminium surface (photo: Schüco) Page 10
9. The Seaside Pre-treatment This pre-treatment was developed by the quality 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. Microscope Picture (20x enlarged): 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 its warranty. Aluminium surface after Seaside pre-treatment (photo Schüco) Page 11
10. The Pre-treatment of anodisated Surfaces The chemical pre-treatments E0 and E6 For the anodisation ( Eloxal ) according to DIN 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 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. Microscope Picture (10x enlarged): Top: E0; Bottom: E6 Pre-treatment on Alu (photos: Schüco) Page 12
11. Effects of non-optimal Pre-treatment Functions of the pre-treatment process are to insure corrosion protection and the optimal adhesion to the optical important top coating layer (based on either powder or liquid paint). Non-optimal pre-treatment may result in insufficient adhesion (photo: Schüco) 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. Page 13
12. Filiform corrosion Preventable using the correct Pre-treatment Close to the sea (up to 75 km, salt load 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. Example for Filiform corrosion (photo: Schüco) Page 14
13. Schüco System Finish Information Overview of available information from Schüco Status 09.2012 Downloads via www.schueco.com: Architect info Pre-treatment Architect info Powder Coating Architect info Metallics Architect info Dirt repellent Coatings Architect info Liquid Paints Architect info Anodisation Videos Finish Methods Digitale Info via SCV: Printed Information: Colour finishes for Aluminium (Colour Booklets) 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. Edition Advance 07/2012 (Art. No. 63117) contains also object surface options and special finish methods used mainly for objects. 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. Page 15
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.: 0521 / 783-0 Fax: 0521 / 783-451 www.schueco.com info@schueco.com Page 16