What is anodising?
Anodising is an electrolytic process for producing thick oxide coatings, usually on aluminium and its alloys. The oxide layer is typically 5 to 30µm in thickness and is used to give improved surface resistance to wear and corrosion, or as a decorative layer.
The oxide layer is increased by means of electrical current, which makes the aluminium surface more durable. It is possible to use anodising for decorative purposes as well since the anodised surface can be dyed. From an aesthetic perspective, anodising preserves the metallic surface of aluminium and makes it appear more uniform.
In the electrolytic process, the components to be treated are made an anode in a dilute acid solution. Oxidation occurs at the component surface, resulting in the formation of a coherent oxide film that is very adherent to the underlying metal substrate. The majority of anodising is done on aluminium and its alloys. Other materials which can be anodised include magnesium and titanium alloys.
“From an aesthetic perspective, anodising preserves the metallic surface of aluminium and makes it appear more uniform.”
The oxide layer thickness determines the application
The different electrolytic bath factors, e.g. temperature, amperage and time, all affect how thick the oxide layer will be. Oxide layer thickness influences the areas of application of the different anodised aluminium pieces:
| Normal Anodising | |||||||||
| 10 µm | Normal stress indoors. | ||||||||
| 15 µm | Hard wear indoors and outdoors in dry and pure air. | ||||||||
| 20 µm | Normal to hard stress outdoors, e.g. in transport and construction. Great stress under chemical influence indoors, e.g. in the food industry. | ||||||||
| 25 µm | The surface is exposed to great stress in the form of a corrosive environment or wear | ||||||||
The anodising process
As you can see in the accompanying illustration, an aluminium piece is lowered into an electrolytic bath and current is passed through the electrolytic solution. Aluminium here functions as the anode electrode of an electrical circuit, hence the name anodising. The electrolytic bath forms an oxide film, which, instead of being on the surface of the metal, is incorporated into the aluminium.
“The anodic oxide structure originates from the aluminum substrate and is composed entirely of aluminum oxide. This aluminum oxide is not applied to the surface like paint or plating, but is fully integrated with the underlying aluminum substrate, so it cannot chip or peel. It has a highly ordered, porous structure that allows for secondary processes such as coloring and sealing.”
– Richard Mahn –
Before anodising can be undertaken, the aluminium alloy surface needs to be given a pre-treatment. This pre-treatment will influence the final appearance and properties of the anodised coating. The types of pre-treatment can range from mechanical processes such as abrasive polishing, to chemical treatments such as chemical brightening or electrolytic polishing. In addition, any machining, drilling or welding of the component should be done before anodising.
Anodizing is accomplished by immersing the aluminium into an acid electrolyte bath and passing an electric current through the medium. A cathode is mounted to the inside of the anodizing tank; the aluminium acts as an anode, so that oxygen ions are released from the electrolyte to combine with the aluminium atoms at the surface of the part being anodized. Anodizing is, therefore, a matter of highly controlled oxidation the enhancement of a naturally occurring phenomenon.
Three types of electrolyte solution are commonly used with the anodising process. The first is a 10-15% solution of sulphuric acid at 25°C. This electrolyte gives a coating formation rate of about 25µm/hr. The second electrolyte solution is a mixture of sulphuric acid and oxalic acid at 30°C. This gives a higher coating formation rate of about 30µm/hr. The third electrolyte is 10% chromic acid at 38-42°C, which gives a film formation rate of about 15µm/hr. These conventional anodised coatings are porous and clear, and are normally used with dyes for decorative coating.
