ANODISING and CORROSION RESISTANCE
The hard anodising process coats the metal with a layer of aluminium oxide, which is chemically inert under neutral, or mild acidic conditions of exposure. It therefore offers a means of protection of the metal surface in a wide range of environments.
Because the oxide coating is derived in part from the metal itself, the constituents of the particular alloy have an effect upon the nature of the coating and upon its corrosion resistance. It should be particularly noted that anodising of alloys of poor inherent corrosion resistance (eg Al-Cu alloys) will give a useful measure of protection, but the performance of the system will not compare with that of one in which the alloy itself has good resistance to the corrosive medium.
It should be remembered that holding the work in the process will result in small unanodised areas and it is unrealistic to expect 100% coating. Jigging can usually be arranged to avoid sensitive areas.
Any anodic film improves the corrosion resistance. The thicker the film the better the resistance. All alloys have their limit on film thickness.
Sealing in hot water or hot dichromate solutions add a quantum leap to the corrosion resistance. For details see post treatments.
There are lots of these around of varying height.
There are many trade names that claim superiority for one reason or another and attempt to prove it through misleading data or statistics. There are also suppliers of chemicals or electrical plant who claim astonishing results for their products. This is all well and good, but for what it’s worth, here’s our advice;
- Choose a hard anodising specification with a good pedigree; either a national, international or military specification.
- The specification should mention testing of hardness, corrosion and especially abrasion.
- Find a supplier, such as us, who can work to this specification, and then have them prove it by providing test results on a regular basis.
There are many ways of achieving good results with hard anodising, and any method is as valid as any other, so long as the results can be proved by rigorous testing.