Aluminium anodising, also known as anodic oxidation, is an electrolytic process by which a layer of aluminium oxide is formed on the surface of the workpiece to be treated, giving it excellent surface characteristics.
The most widely used aluminium anodising process is carried out using a sulphuric acid solution as the electrolyte.
There is also an anodising process called chromic anodising, which uses a chromic acid solution as the electrolyte. This coating is not widely used in industry because chromic acid is carcinogenic and is mainly used in the military and aeronautical sectors.
The parts to be treated are immersed in a tank containing the electrolyte, an anodic current is applied to the part (hence the name “anodising”) and the cathodic current is applied to the sides of the tank. The sulphuric acid process is considered “eco-friendly” as no substances such as heavy metals, solvents or carcinogenic substances are used.
Anodising is the most used and most appreciated coating on aluminium as it gives excellent surface properties to the coated parts, with fairly low costs.
The main characteristics are:
- resistance to corrosion
- resistance to wear
- thickness uniformity
- dielectric insulation
- possibility of colouring with pigments that penetrate the oxide layer.
Most of the aluminium alloys normally used in machining can be anodised easily and with excellent results.
The characteristics of the coating may differ depending on the alloy used, as it is a transformation process from aluminium to aluminium oxide and therefore the alloy composition may change its characteristics. The greatest difficulties are encountered with alloys that contain elements other than aluminium in high quantities, as it is only aluminium that contributes to forming the oxide layer. As a result, alloys containing high amounts of copper, such as the 2000 series, cannot achieve high thicknesses and the oxide layer will be slightly less compact and less resistant to corrosion and wear. Alloys containing more than 10% silicon may show colour non-uniformity and may not achieve high thicknesses.
TYPES OF ANODISING
Two different types of sulphuric acid anodising can be distinguished, which differ both in their surface characteristics and in the process by which they are carried out:
- Natural anodising, such as anodising OX-A
- Hard anodising, such as OX-HS and OX-W anodising
The differences in the anodic layer and its surface characteristics are due to the use of different operating parameters such as the electrolyte temperature and the applied current (Volts and Amps). The equipment serving the anodizing process is also different for the two types.
Natural anodising, also called decorative anodising, is mainly used for decorative or protective purposes in non-aggressive environments.
It is light grey in colour and can be easily coloured using pigments that penetrate into the oxide layer, thus guaranteeing excellent colour rendering together with resistance to scratches and discolouration.
Hard anodising improves and increases the characteristics of natural anodising thanks to a very dense and compact oxide layer with high hardness and excellent resistance to corrosion. This treatment is mainly used in mechanical applications where excellent resistance to wear is required and in industrial or marine environments where aggressive agents are present.
The main technical norms and international standards related to anodizing are the following:
|ISO 7599||Anodizing of aluminium and its alloys|
Method for specifying decorative and protective anodic oxidation coatings on aluminium
|ISO 10074||Anodizing of aluminium and its alloys|
Specification for hard anodic oxidation coatings on aluminium and its alloys
|MIL-A-8625||ANODIC COATINGS FOR ALUMINUM & ALUMINUM ALLOYS|
Type II: Sulfuric acid anodizing, conventional coatings produced from sulfuric acid bath
Type III: Hard Anodic Coatings
|UNI 10681||Alluminio e leghe di alluminio|
Caratteristiche generali degli strati di ossido anodico per uso decorativo e protettivo
|UNI 7796||Rivestimenti per ossidazione anodica dell’alluminio e leghe di alluminio|
Ossidazione anodica a spessore - Requisiti e istruzioni generali di controllo
|MS03_25||Sulfuric Acid Anodizing of Aluminum and Aluminum Alloys|
|ASTM B580||Standard Specification for Anodic Oxide Coatings on Aluminum|
|AMS2471||Anodic Treatment of Aluminum Alloys Sulfuric Acid Process, Undyed Coating|
|ISO 10074||40 – 60 µm|
|MIL-A-8625||≃ 40 - 60µm|
(2” ± 20%)
|UNI 7796||30 – 60 µm|
In order to better understand the growth of anodizing thickness and correctly calculate dimensions, it is necessary to understand the mechanism of formation of the anodizing layer.
As we said at the beginning, anodising is a process that converts aluminium into aluminium oxide forming a layer that can have various thicknesses.
The growth of aluminium oxide inevitably leads to a decrease in the metallic aluminium that is converted into oxide. It is therefore necessary to be careful not to consider the thickness of anodising as a whole as “over-metal”, but it is necessary to know how much in percentage the coating “penetrates” and how much it “grows”.
In precision mechanical engineering, the tolerances are very tight and the designer struggles to define them when coatings are applied, as the machining tolerances are added to those of the coating. It is therefore of fundamental importance that the final dimensions and tolerances of the coating are precisely and correctly defined in order to avoid errors, resulting in scrap or rework.
In natural anodising applications, especially decorative ones, it is mistakenly assumed that anodising does not create dimensional growth.
In such applications, tolerances are not critical and it is easier to assume that there is no growth.
In truth, the oxide thickness creates a dimensional growth of about 30% of the anodising thickness, which is usually 10µm, thus creating a growth of only 3µm.
A further variable that may slightly influence the final dimensions is the cleaning treatment prior to anodising, called alkaline pickling, which slightly dissolves the surface aluminium, thus reducing the final dimension. This value cannot be defined in a standard way as it depends on the time spent in the tank and its concentration. It can range from a few microns in the case of precision mechanical components to a few microns on extruded parts for which it is desired to remove extrusion lines.
The coating grows 30% on the outside and 70% on the inside of the surface of the aluminium piece. The radial dimensional increase is therefore equal to 30% of the treatment thickness.
The treatment thickness increases by 50% on the outside and 50% on the inside of the surface of the aluminium piece. The radial dimensional increase is therefore equal to half the treatment thickness.