Graph Engineering

Metal Finishing Specialists

Hard Anodising

Hard anodising is a process which employs the use of specialist acidic solutions at low temperature and high voltage and current density electrical conditions to produce particularly hard anodic oxide coating with excellent wear and corrosion-resistant properties. These coatings have particular advantages on aluminium used for specialist engineering components where the properties of lightweight plus a hard surface are of particular benefit.

The process imparts a film of up to 100 microns on certain alloys, however maximum hardness is usually achieved at approximately 40 – 50 microns and this will have a rating of around 1100 Vickers.

Typical Applicatons

* Gear box components and brake calipers.
* Valve housings and shuttles
* Slideways and other moving machinery components.
* Injection and blow mould tooling.
* Housings which are used in particularly aggressive environments.

Alloy Selection

The colour and appearance of hard anodized components varies significantly depending on the alloy employed. Hard anodized components manufactured from 2000 series alloys will have a light straw colour, 6000 series alloy components will have a dark stone grey colour. For this reason it is important to familiarize yourself and discuss the alloy with us prior to processing so as not to be expecting a ‘clear hard coating’ which is not achievable.


When considering hard anodising there are a few restrictions to be aware of, should any of these restrictions apply, it may well be more prudent to consider a semi hard coating.

* Hard anodising may have an effect on the surface condition of the component, the surface should therefore be prepared to a higher level than that required on the finished product.
* Hard anodising is unsuitable for alloys which contain greater than 4% copper in the alloy or alloys which contain more than 7% silicon.
* It is advisable that the edges of hard anodized components be radiused.
* Colour matching of hard anodized components is extremely difficult and colours tend to be darker. Colours between alloys vary widely.
* Blind holes will tend to receive significantly lower coating thickness, the deeper the hole the thinner the coating expected