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Acorn 2012
Hard Anodising

Hard anodising produces a relatively dense, thick oxide layer. As a result, the film will tend to be darker in appearance although the final colour is largely dependent on the alloying constituents (elements) of the base material and, in its natural state, can be anything from light green/gold to dark grey/charcoal.

The use of Hard anodising has become very important where weight saving (e.g. by the use of Aluminium instead of Steel) combined with good corrosion protection and wear resistance are required. Also unsealed anodic coatings can absorb and retain oil when used on sliding surfaces for cylinders and pistons etc, providing longer lasting lubrication.

The most common solutions used to produce hard anodic layers are in the region of 15% Sulphuric acid diluted in water and operating at ~0°C. This process uses much higher electrical voltages and increased current densities than Sulphuric anodising.

The structure of the as-anodised film is in vertical columns or pores, this provides a lot of the strength of the layer and also allows the film to be dyed (Black is usually best because of the density of the film), and also sealed to increase the amount of corrosion protection provided.

Whilst sealing the hard anodise layer will significantly improve the corrosion resistance (particularly useful for offshore / sub-sea applications), there is a resultant loss of hardness & abrasion resistance of the oxide film. However, as a direct result of the sealing process the fatigue properties are improved – a clear indication that a good understanding of what is required in the end use is critical in deciding what to type of anodising to specify.

Hard anodising can be applied to a wide range of Aluminium alloys but best results are obtained on alloys with <10% silicon and < 5% copper as alloying elements.

Coating thickness is typically 25 to 75 microns and usually there surface growth in the region 50% of total measured film thickness with 50% penetration of the film into the base metal. On this basis an oxide layer of 50 microns will be approx 25 microns penetration & 25 microns growth. Therefore a film thickness of 50 µm will increase a outside diameter (or reduce an ID) by 50 µm.

Typical characteristics (N.B varies with alloy composition):
  • Excellent wear resistance and prolonged wear life. Hard anodic coatings are typically in the region of 350-500 Vickers Hardness
  • Good corrosion resistance. Sealed hard anodic coatings will typically exceed 500 hours neutral salt spray testing (2024 alloy)
  • High electrical insulative properties: A typical 50 µm coating (sealed) will withstand in excess 1000 volts d.c.
  • Good heat resistance: Hard anodic coatings can withstand temperatures in the order of 2000°C in short bursts.

Applications include:
  • Pistons
  • Cylinders
  • Hydraulic actuators
  • Braking systems
  • Marine and Offshore applications
  • Fuel systems
  • Aircraft components
  • Gears
  • Moulds
  • Engine components
  • Aluminium Cookware / Bakeware
  • Electronic components
  • Radar systems
  • Food processing equipment
  • Pneumatic equipment
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