Hard Anodising Process
Hard anodising is very similar to sulphuric acid anodising. Using a higher acid concentration, it is processed at a solution temperature of 0°C with a significantly higher voltage being applied.
A thicker oxide layer and therefore increased corrosion resistance, electrical insulation and increased wear resistance compared to standard sulphuric anodising.
As aluminium “oxidises” (the natural reaction between air and aluminium, it produces a loose dry white powder, aluminium oxide. Sulphuric acid anodising is an electrolytic process using higher voltage (DC current) and a higher concentration sulphuric acid electrolytic solution causing oxidation of the aluminium surface to form thicker “harder” layers of aluminium oxide under controlled conditions.
The passing of the current liberates high volumes of Oxygen at the surface of the aluminium thus aiding controlled formation of the oxide layer. These processes do not produce a coating but are a conversion of the surface aluminium to aluminium oxide and are therefore metallurgically bonded.
The techniques produce a tight but porous layer which because it is porous requires a sealing process to close the pores and prevent further natural attack on the aluminium.
Sealing is carried out by immersing the component in near boiling de-mineralised water, which if required can have small quantities of other chemicals, such as Nickel acetate or sodium dichromate to further enhance corrosion resistance.
The process is the most common protective treatment for aluminium. The anodised layer form both within the surface substrate (penetration) as well as on top of the surface, growing the size of the component.
Before sealing, immersion in specially formulated dyes can be carried out. The open pores soak up the dye which will then be locked in by the sealing process. This can produce very attractive finishes but will depend on the thickness of the anodised film as the thinner the film, the less dye will be absorbed.
Why Choose Hard Anodising?
- Excellent wear resistance - increased durability
- Close tolerance control
- Resistant to high temperatures
- Increased corrosion resistance
- Provides electrical insulation
- Aesthetically pleasing finish - from clear to light grey through to black in colour
Similar or obsolete designations and standards
Most Common Specifications:
- ISO 10074
- MIL-A-8625 Type III
- Def Stan 21-5 2-3
- ASTM B580 Type A
Components, Applications & Industries
Hard anodising is often used in aerospace and automotive components where the hard wearing properties are most beneficial. Hard anodised aluminium can also be found in architectural and construction applications due to resistance to weathering. General engineering applications also benefit from hard anodising.
- Swivel joints
- Sliding parts
Penta Precision is able to offer Hard Anodising as part of our one stop machining service in the UK. As well as our own machining expertise, we are able to access our network of approved quality suppliers for the finishing of your parts, saving you the time and effort and helping to streamline your supply chain.