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Anodizing Aluminum Fabrication

Anodizing aluminum is the process used in order to increase the translucent film of aluminum oxide. – or increasing the thickness of the natural film on aluminum.

This is done in order to increase the metal’s resistance to corrosion, as well as to increase the surface hardness. The anodic layer is also non-conductive, which means that it is not able to conduct heat, sound or electricity.

Aluminum alloys’ resistance to corrosion is decreased by certain alloying elements (such as copper, silicon and iron) and other impurities, meaning that the aluminum grades 2000, 4000 and 6000 are more susceptible and thus anodized.

Anodizing is also used to allow for the coloring or dying of the material, and also gives the added benefit of improved adhesion and lubrication. It adds an aesthetically pleasing finish, a stable color and a durability to it, making it popular for use in architectural applications as well.

Anodizing Aluminium

First developed in the 1930’s for commercial applications, anodizing is done through the process passing of an electrical current from the anode to the cathode.

Caused by the electrical current being passed through the bath of sulfuric acid (acting as the cathode), oxygen is released at the anode (the aluminum). This now combines with aluminum to form aluminum oxide which makes up the anodic film.

The anodic film’s depth is determined by the amount of time spent in the anodizing tank, while the quality of the film is influenced by the concentration and the temperature of the electrolyte. Time spent in the anodizing tank range between 15 and 60 minutes, creating an anodic film thickness of between 5 microns and 25 microns.

This film created is a porous, honeycomb structure with nanopores with a with a diameter of anything between ten to 150 nano-millimeters. It is very important to note that these nanopores should, at a later stage, be sealed, as it will allow water and air to penetrate to the substrate (the underlying layer), causing corrosion.

These nanopores are often filled by using a colored dye either alone or combined with a corrosion inhibitor, before being sealed. This dye used is only superficial, causing underlying oxide to provide continuous protection against corrosion, even if small scratches and minor wear and tear breaks through the layer of dye.

Sealing the pores of the anodic film is either done through hydrothermal treatment in chemical baths, or by capping the pores. Capping pores are achieved through the precipitation of metal salts in the pore itself.

In order to ensure the desired film thickness, density and resistance to corrosion, the following controls must be closely monitored:

  1. The density of the current
  2. The precise combination of chemical concentrations
  3. The temperature throughout the anodizing process

 

In order to ensure only the highest quality anodized aluminum products are delivered to our clients, ChinaSavvy ensure both the use of highly trained professionals and sophisticated, state-of-the-art monitoring equipment.

An example of the effects of the above named conditions, would be the creation of a thicker and harder film. The following is needed for the creation of this type of film: a more dilute (or thinner) solution, lower temperatures and higher currents and voltages.

Advantages of Anodized Aluminum

Appearance:
Because the anodic layer/film is integrated into the surface of the aluminum, and because it is transparent, the natural feel and appearance of the aluminum remains unspoiled. It is an aesthetically pleasing film as it will constantly interact with both natural and artificial light. Preserving the natural beauty of the aluminum, an anodic film is much better than a normal, painted surface. Paint techniques include the use of multi-layer systems and metallic pigments, often causing color variations and metameric failure.

Resistance to Corrosion:
In general, aluminium does not corrode in the same way as steel or iron does because of its natural oxide layer. However, when corrosion does occur, it permanently damages various aspects of the aluminum alloy. Anodizing aluminum causes the forming of a scientifically controlled oxide layer, which, in turn, provides an extremely high resistance to corrosion as well as preserves the natural beauty and metallic aspects of the aluminum alloy. Suited for external use, the appropriate thickness of an anodic layer can enable aluminum alloys to deliver great results in city environments, as well as marine environments (because of its high resistance to both sulfates and chlorides).

Proven Processes and Results:
With the chemical and fundamental technologies of anodizing aluminum being established for over 100 years, continuous development and research in this specific arena still takes place today, causing great advances in various aspects, including quality. A proven method with on-site results remaining fault-less after 40 years, it is a far better option than organic or chemistry coating. On-site results are also much better proven than that of organic or chemistry coating because of the various environmental and legal constraints causing ongoing modifications to organic and chemistry coatings. Recent research has also found that on-site results of organic coated aluminium (used in the past 15 years) do start to show surface failure, very much unlike anodized aluminum.

Complete Adhesion:
Unlike a paint being applied on to the surface, an anodic film is a part of the aluminum itself. Anodizing causes a transformation in the surface of the metal and isn’t simply a coating on top. This means that surface failure, like that often seen in painted aluminium, will not occur. Adhesion failure is also not a factor when it comes to anodizing, offering a stable anodic layer on both joints and cut edges. This also means that splitting, peeling, cracking, blistering and chipping will not occur

Fading:
While organic powder coatings will always be subject to fading over its entire lifespan, anodized aluminium (with finishes including natural silver, Anolok bronze and Anolok II Blue-Grey) will never fade, as it contains no organic elements.

Chalking:
Anodized aluminium does not suffer from chalking – the formation of a fine powder on the surfaces of paint films caused by weathering – and will keep its luster, color and gloss, regardless of the surrounding environment. Made to endure specific levels of wear, some anodized aluminum can withstand marine environments, as well as humid and sunny conditions.

Filiform Corrosion:
The risk of filiform corrosion – the diminishing of the internal layer between the aluminum surface itself and the coating - is not present with anodized aluminium, as the layer is a part of the aluminum itself. With a paint coating for example, filiform corrosion is a risk as that internal layer is present and susceptible to corrosion. Another added benefit of anodizing is that, if the surface is damaged (punctured for example), it will repair itself through the process of natural oxidation.

Surface Coverage:
The process of creating an anodic film delivers a much more uniform surface covering, especially when it comes to extruded sections. This is because anodizing, as an emersion (or soaking) process, is able to cover all sections, whether they are visible or not . Non-visible sections of the product is something that is often missed when powder coated or spray painted.

Surface Finish:
Anodizing is a process that can be carried out either pre- or post modification, creating a uniform and regular finish on the aluminum. Some problems associated with organic coatings (such as orange peeling and localized high coating thicknesses) are not a factor with anodizing. Creating a uniformed finish on most sizes, textures and shapes of aluminum, it can be used on various modified pieces, such as perforated sheets, mesh material or brush metal, to name but a few examples.

Resistance:
When an anodic layer is properly sealed, it becomes impermeable. Environmental fluctuations (such as weather and temperature) influencing the natural environment in which the anodized aluminum is used, will not cause it to damage the aluminium – something that does however occur in organic coatings. Organic coatings will damage the aluminium if it is exposed to temperature cycles that are either below or above its glass transition temperature.

Abrasion:
A very hard compound, aluminum oxide is, on the Mohs Scale of Mineral Hardness – characterizing the scratch resistance of minerals – second, the first being diamond. This means that the anodic layer offers a very high resistance to scratches and abrasions. Coated surfaces on the other hand, does not offer this high resistance to scratching and abrasion.

Quality Assurance:
Without damaging the anodized surface, strict quality controls and assurance can be carried out on-site.

Environmentally Friendly:
The process of anodizing aluminum is one of the more environmentally friendly metal finishing techniques used in the industry today as the by-products produced through anodizing does not contain halogens, heavy metals or volatile organic compounds (organic chemicals that are known for high vapor pressures even at normal temperatures). Some of the most common ‘waste’ (which is aluminium sulphate and aluminium hydroxide) produced through the process can be recycled and used in industrial waste-water treatment systems, cosmetics and even in baking powder.

Recyclability:
Aluminum can be repeatedly recycled simply through re-melting, which in turn only causes a marginal loss. A pure element, containing only the alloying elements, oxygen and the pure aluminium itself, it is 100% recyclable. 90% of alumunium used in construction and buildings are recycled after the building has reached the end of its life-cycle, while 30% of Europe’s use of aluminium is fulfilled with recycled aluminum.

 

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