Commercially aluminum profiles and sheets having on its surface natural oxide layer. This oxide layer spontaneously and instantly forms on any fresh surface of products as a result of the reaction of aluminum as a reactive metal with oxygen and moisture.. In a production environment, this occurs in the presence of contaminants., moisture, grease, fats, as well as various technological materials.
Why prepare an aluminum surface for painting?
This “production” oxide layer is not homogeneous and contains impurities and inclusions.. Therefore, it does not have the ability to prevent corrosion in corrosive environments or to serve as an adhesive base for subsequent surface finishing of aluminum.
To ensure corrosion prevention and optimum adhesion between the aluminum base and the finish, for example, powder coated, this contaminated oxide layer must be removed to a pure metal (aluminum alloy) by a suitable controlled pretreatment process – aluminum chemical reactions with various chemicals.
On a chemically clean aluminum surface, a controlled and homogeneous conversion / adhesive coating is formed., for example, yellow chromation or special anode coating.
The use of chemicals and additives of high purity prevents new contamination of this prepared layer, which is then maximally preserved until the application of the specified finish coating.
Surface finish for painting
Surface preparation is an important step in the process of applying a uniform surface layer of a protective or decorative coating. The process of preliminary surface preparation includes three main stages [1, 2]:
- grease removal, fats and dirt;
- removal of the outer inhomogeneous and contaminated oxide layer;
- formation of a special adhesive coating.
Surface preparation for painting
In aluminum building structures - windows, doors, facades - powder coating is usually used to protect against corrosion and ensure an attractive appearance of products. Powder and liquid staining are largely similar processes., although they require various technological equipment. Therefore, all, what will be further described in relation to the preparation of the surface for painting, equally applies to powder, and liquid coloring of aluminum.
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Conversion chromating: yellow and green
Chromation is the most commonly used conversion process for coating aluminum products., in particular, aluminum profiles. Because these coatings are called conversion, that they do not just “stick” to the surface, and form a single whole with it.
There are two types of chromating processes:
- yellow hormation and
- green chromated.
Yellow chromation is most often used to prepare an aluminum surface before powder or liquid painting..
This process is based on the reaction of chromic acid with a cleaned aluminum surface to form complex salts and fluorides.. Depending on the duration of the process and reaction conditions, a protective layer forms with a color from colorless to golden yellow. This yellow color comes from inclusions of hexavalent chromium Cr+6 in the resulting coating.
Green chromate is also called phosphochromate.. The main components of phosphochromate baths are usually phosphoric acid., hydrofluoric and chromic acid or another source of hexavalent chromium.
The difference between yellow and green chromate
The difference between the two conversion coatings - yellow and green - chromate and phosphochromate - is not too big. Both types of chromation have similar properties.. It is believed, that yellow chromation gives better corrosion protection, than green chromating, and green chromation, in turn, provides better paint adhesion, powder and liquid.
Due to the environmental hazards of chromate compounds, various chromate-free surface preparation technologies have been developed., mainly based on titanium and zirconium compounds.
In this preparation system, dissolved titanium and zirconium compounds together with complex fluoride compounds and additives interact in an acidic solution with the surface of aluminum. Complex oxides form on the surface, containing aluminum, titanium and zirconium, fluorine and other components. The resulting conversion / adhesive layer is colorless and therefore requires careful adjustment of process parameters and strict control to ensure complete surface treatment.
Anodizing as surface preparation
Under the influence of electric current during the processing of aluminum in an acidic aqueous solution, a very strong oxide layer is formed on its surface - an anodic oxide or anodic coating.
As an aluminum surface preparation for painting, anodizing in an aqueous solution of sulfuric acid is used to obtain an anode layer with a thickness of 3 by 8 micrometers. Unlike a conventional protective and decorative anode coating, this anode layer is not subjected to compaction - pore filling. Such a porous structure provides surfaces with high adhesive properties for powder and liquid staining.. According to Qualicoat, the prepared surface must be painted no later than, than for 16 hours .
Currently, only anodic preparation of the aluminum surface provides protection against such damage to the painted surface as filiform corrosion.. Qualicoat concurrently with the license for anodic surface preparation approves powder coating applications for aluminum structures, which are used in coastal areas of the seas and oceans.
Source: Qualicoat Specification, 14 edition, 2015