Coating of aluminium

Aluminium or aluminium aloys often have various types of finishes applied to their surfaces to enhance appearance or improve functional properties [1]. Here is discussed the methods employed in the coating of aluminum.

Aluminum in construction

Aluminum has a unique combination of properties, which makes it suitable for many different types of products:

• pressed,
• rolling,
• pressed,
• kovanoy and
• cast.

Aluminum products can be complex systems for use in various spheres of life, including, construction, engineering, manufacture of products for leisure and sports. Tens of aluminum alloys make it possible to maximize the advantages of aluminum in order to achieve maximum performance of aluminum products, such as:

• required dimensional accuracy;
• high level of strength;
• a given level of plasticity and viscosity;
• corrosion resistance under specified conditions;
• varied and attractive appearance;
• given life.

Therefore, aluminum is widely used as a material for building constructions, such as:

• curtain wall systems;
• window, doors and shop windows;
• roofing of industrial and civil buildings;
• elements of external facing of buildings;
• furniture and furniture accessories.

Aluminum as a basis for painting and anodizing

Aluminum and its alloys have an advantage over other metallic materials in the, that spontaneously on the surface protective layer is formed from a natural and solid inert oxidic layer. This oxide coating is formed on the air or air-saturated water and instantaneously recovered, if metal scratches or abrasions occur.

Therefore, the aluminum surface even without special treatment has a highly satisfactory appearance and sufficiently high corrosion resistance. For many building components in temperate conditions of the oxide layer is sufficient both in terms of appearance, and from the point of view of resistance to corrosion. Not so long ago, in 1960-1970-ies, aluminum in the state of delivery - in the compact or rolling – widely used in construction, especially in industrial and agricultural.

Modern construction is characterized by high demands on aesthetics, design and corrosion protection of building construction elements. Designers and architects strive for maximum color diversity of structural elements of buildings, external and internal. In such cases apply, such methods of surface treatment of aluminum articles and semifinished, as coloring and anodizing. Aluminum products without special decorative and protective treatment is applied only when, wherein the corrosion rate of aluminum is very low, for example, in rural areas, and special requirements for the appearance are absent.

Figure 1 – The use of colored profiles
in building facades stekloprozrachnyh

Основными промышленными методами для создания защитно-декоративных покрытий являются:

• anodizing aluminum;
• aluminum powder staining и
• aluminum painting with liquid paints (mainly, aluminum sheets in rolls).

Защитные и декоративные покрытия алюминия

If the appearance of the surface of an aluminum profile or plate in the condition, that they arrive directly after compacting or rolling, not satisfied with the architect or designer, or, If you need additional protection against corrosion, the aluminum surface is subjected to processing for creating various types of special coatings or textures.

The following methods of finishing aluminum are most widely used for building aluminum:

• Anodizing (anodnoe oksidirovanie) [2]
• Powder coating [3]
• Liquid coloring sheets in rolls.

Figure 2 – Methods for applying organic coatings [3]

Figure 3 – Line bath for anodizing aluminum profiles

Moreover, for the modification and preparation of the aluminum surface before painting and anodizing, the following aluminum surface treatments are used:

• Mechanical surface treatment, including, applying a texture
• Himicheskoe lighting
• alkaline etching (sometimes acid)
• Formation of conversion coatings (chromate and non-chromate) [4]

Figure 4 – vertical line
for surface conditioning aluminum profiles and
their powder coating

quality factors

The quality level of protective and decorative coatings - after anodization, powder coating and liquid painting - is directly dependent on the quality of pre-treatment. Sami anodizing processes, powder coating and liquid painting are complex technologies, which require constant attention and monitoring.

Listed below are the main factors, that have the greatest influence on the quality of the final protective and decorative coating.

The chemical composition of the aluminum alloy for anodising

Various aluminum alloys have different combinations of alloying elements. The differences in the content of some chemical elements, for example, copper and iron, can cause differences in color and texture of anodizing, even at exactly the same anodizing technology. Therefore, the alloys generally used for anodizing 6063 и 6060. Alloy 6060 with narrowed compared with a standard alloy chemical composition give maximum color uniformity and brightness products in the Party.

Mechanical surface treatment

If machining is defined in order, it is performed before the chemical treatment or anodizing. Machining the aluminum surface is carried out in order, to give it some texture or remove defects and inhomogeneities. As a result of mechanical surface treatment, the following is obtained:

• uniform matt anodised surface;
• anodized surface with directional or chaotic "scratches";
• shiny (mirror) anodized surface.

Colorless anodic coating is transparent or translucent. Since the anodic oxide surface texture is repeated, then virtually all traces of surface machining to be seen through the anode coating.

Examples of machining aluminum surfaces are:

• polishing;
• sandblasting;
• grinding;
• shot blasting;
• metal brushes treatment.

Chemical surface preparation for anodising

Chemical preparation of the aluminum surface – this application aluminum chemical reactions with a variety of suitable chemical reagents. Alkali etching aluminum gives a greater or lesser degree of matte surface. Electrical or chemical polishing forms brilliant, mirror surface. Chemical preparation of the aluminum surface, which is produced before anodizing, It has a decisive influence on the appearance of the finished anode coating.

conversion coatings

Prior to application in a powder or liquid paint its aluminum surface is subjected to chemical preparation. After alkali degreasing and acid aluminum surface lightening products act on the chromate treatment or beskhromatnuyu, which resulted in the conversion coating formed.

Figure 4 – Model forming a chromate conversion coating
containing fluorides [4]

This coating is called conversion, because, that it is formed from the material of the aluminum surface itself as a result of its reaction with the chemical substances of the solution (chromate or chromate-free). Conversion coatings provide maximum paint adhesion, powder or liquid.

Sources:

1. Cleaning, Finishing, and Coating / Aluminum and Aluminum Alloys – ASM Speciality Handbook / Ed. J.R. Davis, 1993
2. Anodizing of Aluminium – TALAT Lecture 5203 / José L.Gazapo and J. Gea, INESPAL Lamination, Alicante – 1994
3. Organic Coating of Aluminium – TALAT Lecture 5204 / J. Vereecken, Free University of Brussels – 1994
4. Conversion Coatings – TALAT Lecture 5202 / G.E.Thompson, AWESOME, Manchester – 1994