Anodising - How many Microns do I need?
• 5 Micron
is suitable for decorative applications or bright finish
• 10 Micron
is suitable for internal applications and outdoor applications where cleaning is very frequent, for example, kitchen trim.
• 15 Micron
is recommended for the majority of ordinary architectural requirements.
• 25 Micron
is recommended for heavy duty external permanent architectural applications where little deterioration can be tolerated.
Anodising is the name for the treatment of aluminium to produce natural oxide coatings for protection, identification, wear resistance and decoration.
On exposure to air, aluminium will immediately form a stable oxide layer. This natural oxide film is very thin however, approximately 1/100th of a micron and will provide little corrosion resistance under normal atmospheric conditions.
Anodising is the controlled formation of aluminium oxide at the surface of items made from aluminium and its alloys. The anodic coating is produced by passing an electric current through a dilute acid solution using the work as the anode. The thickness, density, hardness and colour of the coating can be varied depending on the solution and the processing conditions.
Anodised aluminium profiles are therefore anodised to increase corrosion resistance, to increase surface hardness and to allow dyeing (colouring), improved lubrication, or improved adhesion. When exposed to air at room temperature, or any another gas containing oxygen, pure aluminium self-passivates by forming a surface layer of amorphous aluminium oxide 2 to 3 nm thick which provides very effective protection against corrosion.
Aluminium extruded profiles are anodised to greatly increase the thickness of this layer for corrosion resistance. The corrosion resistance of aluminium alloys is significantly decreased by certain alloying elements or impurities: copper, iron, and silicon, so 2000, 4000, and 6000-series alloys tend to be most susceptible. Most aluminium aircraft parts, architectural materials, and consumer products are anodised.
The acid action is balanced with the oxidation rate to form a coating with microscopic pores, 10-150 mm in diameter. These pores are what allow the electrolyte solution and current to reach the aluminium substrate and continue growing the coating to greater thickness beyond what is produced by auto passivation. However, these same pores will later permit air or water to reach the substrate and initiate corrosion if not sealed. They are often filled with coloured dyes and/or corrosion inhibitors before sealing. Because the dye is only superficial, the underlying oxide may continue to provide corrosion protection even if minor wear and scratches may break through the dyed layer. Conditions such as electrolyte concentration, acidity, solution temperature, and current must be controlled to allow the formation of a consistent oxide layer. Harder, thicker films tend to be produced by more dilute solutions at lower temperatures with higher voltages and currents. The film thickness can range from under 0.5 micro meters for bright decorative work up to 25 micro meters for architectural applications.
The most widely used Anodising specification, MIL-A-8625, defines three types of aluminium anodisation. Type I is Chromic Acid Anodisation, Type II is Sulphuric Acid Anodisation and Type III is sulphuric acid hardcoat anodisation
Anodised aluminium can be found on office furniture, displays, advertising frames, computers , window frames, cladding, curtain walling, automotive roof racks and steps, shower profiles, partitioning profiles, truck bodies and floors, wall board sections, heat sinks, and on many other products both for corrosion resistance and the ability to retain dye. Although Anodising only has moderate wear resistance, the deeper pores can better retain a lubricating film than a smooth surface would.
Anodised aluminium surfaces are harder than aluminium but have low to moderate wear resistance, although this can be improved with thickness and sealing.
Before anodisation process.
Aluminium extruded profiles are cleaned in either a hot soak cleaner or in a solvent bath and may be etched in sodium hydroxide, ammonium bi-fluoride or brightened in a mix of acids.
Aluminium extruded profiles are normally best just cleaned due to the presence of intermetallic substances unless they are a high purity alloy such as LM0.
The process itself.
Anodising is an electrochemical process where the oxide film on the aluminium surface is made thicker artificially. The process involves lowering the extrusion into an electrolytic bath where a DC current is connected to the extrusion, which acts as the anode in the circuit. While the natural oxide film is only 0.02 µm, anodising increases the oxide layer to between 5 and 25 µm depending on the product characteristics required. With the anodising process, aluminium's natural colour (naturally anodised) can be retained or a colour from a wide range of alternatives can be chosen.
Aluminium Anodising is usually performed in an acid solution which slowly dissolves the aluminium oxide.