Aluminum Surface Treatment - Anodising.

Why Need Aluminum Surface Treatment?

Aluminum alloy surface is easy to produce a thin layer of alumina (0.01µm ~ 0.02µm), with a certain degree of corrosion resistance. However because it is amorphous, the original aluminum surface would be untarnished. In addition, this oxide film is loose and porous with poor corrosion resistance also easy to stain.Thus, thanks to great corrosion resistance, surface treatment is necessary to provide corrosion protection, decoration, and functionality of materials. According to the nature of the protective layer and process characteristics, aluminum surface treatment technology can be divided into anodising, electrophoresis, organic coating treatment. Organic coating treatment includes powder coating, fluorocarbon coating, and grain processing.The purpose of this articles is to introduce some normal methods of aluminum surface treatment: anodizing, electrophoresis and powder coating, and their strengths and weaknesses.
Examples of attributes that could be achieved by surface treatment include:
  • Colour
  • Hardness
  • Electrical Insulation
  • Wear Resistance

Anodising

What is Anodising

Anodising is an electrolytic passivation process used to increase the thickness of the natural oxide layer on the surface of metal parts.Anodizing increases resistance to corrosion and wear, and provides better adhesion for paint primers and glues than bare metal does. It is possible to use anodising for decorative purpose as well since the surface could be dyed or with thin transparent coatings that add interference effects to reflected light.(Wikipedia)

Layer Thickness Application Area
5 µm Protective anodising before matching, a short period of etching
10 µm Normal stress indoors
15 µm Indoors and outdoors in dry and clean atmospheres.
20 µm Normal to hard stress outdoors e.g. construction and transport. Great stress under chemical influence indoors, e.g. in the food industry
25 µm Severe stress in the form of wear or corrosive environment

Anodising Process

There are normally four steps in the process: pre-treatment, anodising, coloring (where required) and sealing.As you can see in the accompanying illustration, the aluminum surface forms an oxide film, which, instead of being on the surface of the metal, is incorporated into the aluminum.Aluminum oxide is a hard, durable and weather resistant substance that protects the base metal.

Merits and Demerits of Anodising:

Merits
  • Good weather resistance and fire resistance
  • Electrically insulating coating
  • Decorative surface with durable color and glossy
  • Strong stain resistance
  • Coating would not peel off which is the part of the metal
Demerits
Though it has strong corrosion resistance and longer lifespan, it would be damaged in the acidic environment for its poor chemical resistance. Like other metal structures, it should be protected from the acidic environment. Thus, the lifespan of anodizing coating depends on its layer thickness and building’s environment.
Anodizing coatings might accumulate dirt and stains which could be removed with neutral detergent and washing technology. A thin anodic coating would be removed leaving a renewed one which could be used for another 20 years. But once the finish is damaged, the only choice is to re-coat the surface with another paint.