Aluminium Treatments and Surface Finishes - Anodising
The capacity of aluminium to respond to anodising, the most familiar of finishes, make aluminium a most important metal in a fundamental way. The fact that aluminium can accept this attractive, durable and tough-wearing finish makes it possible to exploit its strength and lightness in a large number of applications, particularly in building construction.
Anodising essentially is an induced thickening of the natural protective oxide film on the metal's surface. It converts the surface of the parent metal, not a 'coating' in the usual sense. Unless severely deformed or stressed by excessive thermal movement, the anodic film will not chip, peel, or crack. With conventional sulphuric acid anodising, the anodised alloys produce a clear, hard, and extremely corrosion-resistant film capable of being coloured. The functional and decorative potential this offers is widely exploited in applications ranging from building components to domestic cookware. Varying the conventional electrolyte composition and process variables produces anodic finishes with distinctive functional properties. Thus, very hard anodic films are developed for abrasion-resistant surfaces on gears, pistons, bearings and similar components.
Anodic films can be coloured in many ways. Conventional sulphuric acid films are microscopically porous, and organic or inorganic dyes and pigments may be incorporated and sealed in the film. Very durable coloured films used for exposed environments are usually produced integrally and permamently with the evolution of the anodic layer.
Whether anodising is clear or coloured, it is important that designers understand the essential nature. Inevitably, the anodic film reproduces the physical nature of the original metal surface. This means that any mechanical finish applied previously to the surface will be clearly evident, and the characteristics of different metal forms will persist. Thus, extruded element and sheet metal, if colour anodised to the same specification and placed together, will show an apparent colour difference due solely to minor but characteristic differences in surface profile peculiar to the individual mill processes.
The basic anodising process consists of a suitable chemical pre-cleaning dip, followed by etching in a caustic soda base solution, anodising electrolytically in sulphuric acid or other solution, and finally sealing to reduce porosity. The finished anodised surface is in fact an inert, and therefore protective, film of aluminium oxide.The thickness of the aluminium oxide anodising is varied by processing time to suit its application. The following guide quotes minimum figures, film thickness being checked on a batch by batch basis by electronic means.
12 micron Common Standard for internal and outdoor applications where cleaning is frquently required.
15 micron General architectural requirement.
25 micron Heavy duty external architectural or marine applications where little deterioration can be tolerated.
Colour finishes are checked for accuracy againnst standards, and tested for leaching by immersion in boiling 0.1% borax solution.
The application of thermoset powdercoating on aluminium has increased dramatically. The wide variety of powder types now available in the process allow the engineer or architect to specify powdercoating for a wide variety of pplications with confidence. Polyester thermosetting powder is the most commonly specified product becuse of its excellent resistance to natural weathering, a high degree of chalk resistance and colour fastness, high levels of mechanical properties and good chemical and corrosion resistance.
Standard colour ranges provide ample choice but colour-matching to specific colours can be acheived on request. After a cleaning process, the surface is chromate-converted to enhance corrosion resistance and optimise adhesion of the thermosetting.
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