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Ullrich Extruded Products Catalogue
Aluminium extrusions... virtually any shape desired by the customer can be produced

Aluminium Treatments and Surface Finishes

Anodising Aluminium

The capacity of aluminium to respond to anodising, the most familiar of finishes, make aluminium a most important metal in a quite fundamental way. The fact that aluminium can take on 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 alloys usually anodised produce a clear, hard, and extremely corrosion-resistant film capable of being coloured. The functional and decorative potential this confers on the metal is widely exploited in applications ranging from building components to domestic cookware. Varying the conventional electrolyte composition and process variables produce anodic coatings of distinctive functional properties. Thus, very hard anodic films are developed to provide 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 more usually produced integrally and permamently with the evolution of the anodic layer. Whether clear or coloured, it is important that designers understand the essential nature of anodising. Inevitable, the anodic film reproduces the physical nature of the original metal surface. Not only does this mean that any mechanical finish applied previously to the surface will be clearly evident, but even 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 a sulphuric acid or other solution, and finally sealing to reduce porosity.The finished anodised surface is in fact an inert, and therefore a 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.

The Window Association of New Zealand Anodising Standard SFA 3503-03:2005 give the following guidelines:

  1. It is recommended that in sea spray and geothermal areas at least 25 microns is required
  2. Coastal areas, but not sea spray require at least 20 microns
  3. Inland coastal and inland with plentiful rainfall both require at least 12 microns

Colour finishes are checked for accuracy against standards, and tested for leaching by immersion in a boiling 0.1% borax solution.

Powdercoating Aluminium

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 considence. 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 optimaise adhesion of the thermosetting.

 
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