Aluminium can be surface finished in many different ways. The most important types are
mechanical, chemical, electrolytic oxide and organic finishes.
- MECHANICAL PROCESSES are used to remove scratches, pits and other surface
imperfections. Grinding and polishing are two methods.
- GRINDING requires a rotary grinder with a bonded abrasive cup wheel or canvas
wheel faced with suitable abrasives. Low speed grinding (1 500 mm/min) using aluminium oxide is preferred to
avoid overheating the surface.
- POLISHING requires wheels or belts that have abrasives bonded to them.
Aluminium alloys are polished in the same manner as for steel except that a lower wheel-to-metal pressure and a more
gentle cutting action with finer abrasives are used. Polishing can include a buffing operation to remove
emery marks. Buffing wheels are usually made of muslin discs sewn together.
- CHEMICAL
FINISHING PROCESSES are processes in which chemical reagent react with the metal
surface to alter its form or to produce an adherent chemical compound. Etching and conversion
coatings are two types of chemical finishes.
- ETCHING requires a chemical reagent to attack and roughen the surface of the
metal. Alkaline and acid etchants can be used. The former are more common because they are cheaper and easier to
handle. A solution of caustic soda in water is most common. Since etching lowers the resistance
to corrosion by removing the natural protective oxide film formed on aluminium, another process is
required to restore corrosion resistance. Chemical dipping removes metal in a controlled manner by
selective chemical etching in either acid or alkaline solutions to produce patterns, tapers or an overall reduction
in metal thickness.
- CONVERSION COATINGS are required to thicken the natural oxide film on
aluminium to more porous and absorptive oxide coatings, which can be bonded more readily with paints, lacquers and
organic finishes.
Aluminium can be oxidised in boiling water but the natural oxide skin is only thickened
marginally. The oxide film formed in the presence of water is non-porous and will not allow water to
penetrate the film to reach the aluminium surface. The further oxidation is therefore inhibited. Chemical solutions
have a dissolving action on the coating and help the solutions to reach the aluminium surface. These solutions help
to thicken the skin and normally solutions resulting in the formation of chromates or phosphates are used.
Films produced in chemical conversion are thinner than those made by anodising and are cheaper and easier
to apply.
- ELECTROLYTIC PROCESSES used for producing oxide films on aluminium are called
anodising. These films are hard, inert durable and serve as a base for light-fast dyes of almost any colour. Oxide
coatings obtained by electrolytic means have better corrosion resistance and superior strength compared to
those made by chemical conversion treatments.
- ANODISING requires a bath containing an electrolyte capable of yielding
oxygen. The most important electrolytes used are sulphuric and chromic acids. A direct current is passed through
the electrolyte and causes oxygen ions to migrate to the aluminium surface (the anode) to form an oxide
film. Upon further electrolysis, the oxide film increases in thickness. Recommended
coating thicknesses are shown in Figure 23. The oxidation takes place at the oxide-metal interface and thickening
takes place from the inside. The penetration through the entire oxide film is possible because the film is porous
throughout. After anodising and before drying, the pores can be sealed by placing the aluminium in hot water. Since
the anodic film is porous, colours can be introduced into the film during or after anodising, but before
sealing.
Source: IAI
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