Why Need Aluminum Surface Finish?
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, Aluminum Surface Finish is necessary to provide corrosion protection, decoration, and functionality of materials. According to the nature of the protective layer and process characteristics, aluminum surface finish technology can be divided into anodizing, 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:
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)
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:
- 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
- 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.
|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|
Powder Coating is a type of coating that is applied as a free-flowing, dry powder. The main difference between a conventional liquid paint and a powder coating is that the powder coating does not require a solvent to keep the binder and filler parts in a liquid suspension form. The coating is typically applied electrostatically and is then cured under heat to allow it to flow and form a “skin”.Powder coating is mainly used for coating of metals, such as household appliances, aluminum extrusions, drum hardware, and automobile and bicycle parts.( Wikipedia)
Advantages and Disadvantages of Powder coating:
- Energy Saving: Because of one-time film, it could increase 30-40% productivity and save about 30% energy.
- Environmental Friendly: No organic solvent evaporation(without toluene, xylene, and other harmful gases) and release little or no amount of Volatile Organic Compounds (VOC) into the atmosphere.
- Excellent corrosion resistance, chemical resistance than anodizing than oxidative coloring profiles
- No risk of running or blistering coating
- More Colors Option
- Poor weather resistance
- Easy to fall off powder
- Susceptible to UV and strong sunshine and easy to discolor
Electrophoresis coating can also be referred to e-coating, electrodeposited coating, electrocoating, electrophoretic coating and cataphoretic painting etc. In any case, the aluminum casting parts are immersed into solution in an electrified tank containing the required paint. The paint is drawn to the aluminum casting parts, where the addition or subtraction of electrons changes the paint from ions in solution to solid particles adhering to the aluminum casting parts.
- Compared to powder coating, high coating film adhesion, not easy to fall off and aging
- Compared to anodizing, higher hardness coating film and impact resistance
- Colorful and metallic surface
- Protective layer with higher corrosion resistance than anodizing and powder coating
- Resistant to cement, mortar acid rain invasion
- Better wear resistance, weather and chemical resistance than anodizing
- Short processing time: as it works in water-based electrophoresis tank, it could finish the painting in several minutes which is easy to achieve the automated assembly line work.
- High utilization of painting: Because the painting has low viscosity and the electrophoresis workpiece could be washed with water, the utilization of electrophoretic coating could be up to 95% or more by recovery equipment.
Electrophoresis finish is based on the anodizing finish, thus the quality would be better than anodizing finish. So the price of electrophoresis is also higher than that anodizing coating.
- The layer of e-coating will affect the surface conductivity of heat and electricity.Thus we usually use silicon glue to protect the nearby area where it should be free from e-coating for aluminum casting products.
- Fewer Colors Choice: Because the electrophoresis coating can only use water-soluble paint, it can not arbitrarily change the color in the coating progress.[Dark, Silver White, Bronze, Champagne and Gold]
- Higher requirements of aluminum surface: Because the electrophoretic layer is transparent and light, any slight defect and stain will be completely unmasked in transparent lacquer. Thus, it requires a strict management of oxidation and coloring.