Hard Anodising (Type III Anodising)

Hard anodising is a specific type of anodising process applied to aluminium and its alloys, aimed at creating a thicker, more durable oxide layer compared to regular anodising. This enhanced layer provides improved corrosion resistance, wear resistance, and electrical insulation properties.

How Hard Anodising Works:

1. Electrolytic Process:
   Hard anodising is an electrochemical process that involves immersing aluminium in an acidic electrolyte bath (often sulphuric acid) and applying an electrical current. The aluminium reacts with the electrolyte to form an oxide layer that is chemically bonded to the metal surface.

2. Thicker Oxide Coating:
   In hard anodising, the process conditions are modified to create a thicker oxide layer, typically ranging from 25 to 150 microns (compared to the usual 5 to 25 microns in regular anodising). This results in a harder, more durable surface.

3. Cooling and Sealing:
   After the anodising process, the aluminium may be cooled rapidly and the oxide layer is usually sealed to improve corrosion resistance and to enhance the appearance (often with a clear or dyed finish).

Properties of Hard Anodising:

• High Wear Resistance: The thicker oxide layer provides significant protection against abrasion and scratching, making it ideal for components exposed to harsh environments or high friction.

• Improved Corrosion Resistance: The thick oxide layer protects the aluminium from corrosion in harsh chemical or environmental conditions, making it suitable for use in marine, aerospace, and industrial applications.

• Electrical Insulation: The oxide layer is non-conductive, making hard anodised parts useful for electrical insulation purposes.

• Surface Hardness: The resulting oxide layer is very hard (typically 60-70 HRC), which enhances the material's overall performance, especially in parts subjected to wear and stress.

Applications of Hard Anodising:

• Aerospace Components – Used for parts such as aircraft landing gear, engine components, and flight control systems that require both strength and resistance to harsh environmental factors.

• Automotive Parts – Engine components, brake calipers, and suspension parts benefit from its wear resistance and corrosion protection.

• Industrial Equipment – Used for parts in pumps, valves, and machine tooling exposed to extreme conditions.

• Military and Defence Equipment – Hard anodised coatings are used on weapons, protective gear, and various military hardware for durability and corrosion protection.

Benefits of Hard Anodising:

• Longer lifespan of components due to wear and corrosion resistance.

• Reduced maintenance costs and extended service life.

• Increased strength for structural parts.

• Non-reactive surface, ideal for use in sensitive environments.

Limitations:

• Colour variation: The natural colour of the oxide layer is typically a dark grey or black, but it can be dyed in various colours, though the colour may not be as uniform as with standard anodising.

• Increased cost: The process is more energy-intensive and requires more precise control, which can increase production costs.