Surface Treatment

SURFACE TREATMENTS ON ALUMINIUM

Surface treatments serves to enhance the aesthetics of the product, or to reinforce it against corrosion and wear. Furthermore, surface treatments may also be used in altering the electrical characteristics or thermal absorption or reflection characteristics of aluminium profiles.

WHAT IS ANODIZING?

The Turkish term for anodic oxidation (eloksal) is borrowed from German, and refers to electrical oxidation of aluminium. The international lingo for the process often uses the terms “anodic oxidation” or “anodizing” to describe it. Anodic oxidation is a very specific surface treatment applied on aluminium, via an electrochemical process. The electrolyte used in this context is usually an acidic solution. The aluminium to be coated, in turn, is the anode in the electrolysis. A specific and adjustable current (usually direct current) is applied between the aluminium (the part to be processed) and a suitable cathode, for a limited time. The time frame is set in accordance with the specifications and thickness of the anodic oxidation layer to be applied. The process produces heat, and hence the electrolyte requires cooling to keep its temperature fixed. Anodizing is among the most important surface treatments applied on aluminium products. For seventy years anodizing has been in use for both decoration-related and industrial applications. Extensive efforts were made to produce color in the anodizing, for architectural purposes, and processes were developed to achieve success to this end. Most of these processes are based on introducing color pigments into the porous structure of the anodic oxidation layer. Prior to many of these surface treatments, pre-treatment (surface preparation) is applied to prepare the surface for the coating process itself. Pre-treatment can not only change the view of the surface for aesthetic purposes, but also improve the adhesion capabilities of the surface, to enhance the durability of the anodic oxidation layer.

Treatments to alter the view of the surface may be mechanical or chemical in nature. Among the mechanical treatments, polishing, satin finish, deburring or sanding are the most common. Deburring and sanding are most often used for small parts. Polishing and satin finish, on the other hand, are usually applied on profiles. The satin finish process is used to develop light strips in accordance with the thickness of the brush wires, with a view to hiding extrusion lines and/or making chemical etching easier. Polishing, on the other hand, produces a shiny surface.

The chemical pre-treatment methods applied are chosen according to the desired level of surface quality. Matte surfaces are produced by etching (causticisation) with sodium hydroxide solutions, while shiny surfaces are produced by chemical dipping in acid solutions, or by electrochemical polishing. The solutions used for electrochemical polishing may contain various combinations of phosphoric, sulphuric, nitric, or chromic acids. Chemical pre-treatment entails a few stages. Prior to causticisation for matte surfaces or electrochemical polishing, degreasing is applied in the form of a non-abrasive cleaning material. Causticisation or electrochemical polishing, on the other hand, are followed by cleaning with an acid, in order to remove the remnants the reaction may produce on the surface.

WHAT IS ELECTROSTATIC POWDER COATING?

Electrostatic powder coating is a surface coating method which entails coating the part with a powder containing the paint pigments, and thereafter application of oven-drying. In order to ensure that the powder is fixed on the surface, washing and chromatizing processes are applied in the beginning. Then, the dried and well-grounded part is pushed through the powder cabin. In this stage, paint guns in the cabin spray electrostatically load paint particles on the part, whereupon the particles stick on the grounded part. The powder-coated part is then placed in an oven preheated to 180°C to produce a homogenous and smooth layer on the part through polymerisation.
Even though the layer produced by the electrostatic powder coating is not as strong as the one produced by anodic oxidation, the color varieties it offers, as well as its brightness, texture, and ease of application makes it a common choice.

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