Where aluminium components might be subjected to wear in a race engine or transmission, hard anodising is often specified as a surface treatment. This process converts the existing surface to a thick oxide structure; the main difference between decorative anodising treatments and hard anodising is the thickness of this oxide layer.
The anodised layer thickness may be specified and there is a ‘half and half’ rule that can be used to predict component growth. Half of the oxide layer is internal – that is, the existing metal is ‘replaced’ by its oxide – and half of the layer thickness manifests itself as component growth. For example, a component 10 mm thick and given a 0.04 mm thick anodising treatment will see a growth of 0.02 mm per side, 0.04 mm in total, giving a post-anodising thickness of 10.04 mm
The resulting oxide layer is very porous, and it is always ‘sealed’ to fill in the pores. There are several options for sealing, each of which has its own advantages. Hot-water sealing is commonly used, either on its own or in conjunction with one or more alternatives. Sealing of hard anodic surfaces gives treated components a greater degree of corrosion resistance and gives the coating increased toughness. It also improves dielectric (insulating) strength.
Using hot water fills the pores in the anodised surface and creates hydrated aluminium oxide, which is less dense than the as-anodised film. The anodised film is very hygroscopic, and will tend to seal itself slowly over time by drawing moisture from the air. Water sealing is done at close to the boiling point of water: the speed of the process depends very much on temperature, and the higher the temperature then the quicker an effective seal can be formed.
Another method, nickel fluoride sealing, uses a cold solution of the nickel salt but takes a long time, although the process can be accelerated by a warm water rinse after initial sealing. The nickel fluoride reacts with the aluminium oxide to produce aluminium fluoride and nickel hydroxide in the pores. Cold sealing is said to offer maximum hardness and wear resistance, and surfaces thus sealed are less prone to finger-marking.
Sealing using a hot sodium or potassium dichromate solution gives the anodising a military-style drab-olive colour, and is particularly effective in terms of corrosion prevention.
Anodic films can be sealed with PTFE to give a hard-wearing surface with relatively low friction and good water-shedding properties. It can have a slightly ‘milky’ appearance, especially if the anodised surface has been dyed black beforehand.
Sealing the anodised surface is very important if you intend to rely on its dielectric properties. If left unsealed, the dielectric strength over a fissure or pore drops to the same level as if an air gap equivalent to the anodised film were present. By sealing, we don’t necessarily improve the insulating properties of a pore-free section of the film, but we improve the film where it is at its weakest.
A note of caution: hard anodising is known to affect the fatigue strength of aluminium alloys, with the effect varying with the substrate alloy, the anodising process and the thickness of the coating. It is possible though to restore some of the loss in fatigue strength by the use of processes such as shot-peening.
Written by Wayne Ward