Standard anodising can be used to anodise a wide variety of aluminium parts. However, there are instances where using a harder anodizing process would be more suitable. Here are a few examples of such instances and why they are recommended.
High-Wear or Sliding Contact
Standard anodic coatings wear quickly in applications of high wear, such as parts that are subject to sliding, abrasion, etc. This is due to the nature of the anodic coating, which is a relatively soft, wear-prone, yet very wear-resistant material. The very wear-resistance that makes it so good for corrosion and abrasion-resistance in non-wearing applications, makes it wear-prone in wearing applications. In these applications, a hard anodic coating provides a much denser oxide layer, with a typical hardness of 400 to 500 HV, that can withstand the wear. Typical applications are for hydraulic pistons, valve bodies, and cam surfaces.
Tight Dimensional Tolerances
The hard anodising process builds a consistent coating, with approximately half of the coating growing into the substrate surface and the other half growing outward from the surface. This results in a predictable amount of dimensional change. Typically this is a small increase of a few mm per side, thus there is less requirement for after treatment machining of close tolerance bores and sliding parts.
A useful reference for Hard Anodising is www.poeton.co.uk/surface-treatments/anodising/hard-anodising.
A thick, hard, dense oxide is an excellent electrical insulator and is therefore particularly suitable for components that require insulation against electrical current, such as the bodies of enclosures for electronic equipment, insulators for bus bars, or components of aerospace assemblies.
Aggressive Chemical Exposure
Dense oxide surfaces are better than soft anodic surfaces at resisting wear from a wide variety of fluids, including most hydraulic fluids, some mild acids and cleaning solutions. However, some strong alkalines can attack hard anodized surfaces.
Low-Temperature Operating Environments
Some polymer-based coatings can become brittle at very low temperatures. Hard anodized aluminium does not suffer from this problem and will remain strong down to cryogenic temperatures. It is therefore an ideal material for components used in refrigeration and in very cold climates.
