Nature forms a tenacious oxide film on Aluminium when metal is exposed to the Atmosphere, although thin this film provides increased resistance to corrosion.
A number of processes have been developed to take advantage of this natural phenomena in order to achieve a thicker film.
At Anodising Industries we use Type II class 1 & 2 sulphuric Anodising.
The maximum film thickness is controlled by the Anodising Operating conditions, for example, increasing the electrolyte temperature or concentration will increase the oxide dissolution rate. On the other hand increasing the anodising voltage or current density will increase the oxide growth.
After the Aluminium is anodised it must be sealed, this is done by immersing the anodised metal in a hot water bath, where in the aluminium oxide coating is converted to the monohydrate. Thus the microscopic pores in the coating become resistant to further staining.
In order to obtain reproducible results from batch to batch a large number of variables must be kept under close control. The type of aluminium alloy is the first variable as the alloy has a pronounced effect on shade. A general rule is the purer the alloy the clearer ht e film produced. The 2000 series aluminium is the least pure and produces the less clear film.
Hard Anodising is an electro-chemical process of converting aluminium to Aluminium Oxide.
This process allows the production of a thicker wear and corrosion resistant coating on a variety of Aluminium Alloys. These hard coatings allow the use of Aluminium in many design applications restricted to steel.
Hard Anodising requires higher electrical power and produces thicker coatings than regular anodising.
The coating penetrates into the surface and also builds up on the parts. The anodising surface is dielectric. All of the alloys show a uniform growth on the surface equal to that penetrated below the surface of the base metal.
This important factor should be considered in cases of close tolerance. The aluminium oxide film grows perpendicular to the surface of the metal so that sharp corners are left void. This is more pronounced in thicker coatings.
Therefore if possible, sharp edges should be rounded.
Other factors in Hard Anodising are the colour of the coating is dependant on the alloy and the coating thickness. Colours can range from grey to black. Threads must also be considered when Hard Anodising as thread pitch diameter will increase approximately four times that of the coating thickness.
Had Anodising in al alternative to Had Chromium Plating, the wear resistance of this aluminium coating has proven superior to hardened steel.
The coating is file hard and is also a good insulator. The Hard Anodising film will withstand higher temperatures than the base metal. It makes the part resistant to direct flame impingement and does not conduct electricity.
Its dielectric strength is 500v.
Hard Anodised Coatings will not peel since the coating is an intergral part of the base metal. It will however crack if the parts are bent. In Hard Anodising as the coating thickness increases the fine capillary pores seal themselves making the parts corrosion and erosion resistant.
Not all alloys are suitable for the process, 200 and 4000 series contain the most copper and silicon and often result in burnt parts. The best alloys for Hard Anodising are 5000, 6000 and 7000 series alloy.
There are many applications for this process, some parts currently being hard anodised are:
- Gears
- Castings
- Computer Parts
- Hydraulic Gears
- Valves
- Moulds
- Cams
- Impellers
- Sprockets
- Winches
- Pumps
- Marine Fittings
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