Anodizing does not increase strength
croswell1 said:
Seriously, I doubt the black anodise is any heavier than the silver, although it could be. The CD or "hard anodising" treatment is heavier, but reportedly increases the rim's strength by about 40%.
Hard anodizing is an entirely inappropriate surface treatment for aluminum rims. It's negative qualities far outweigh its positives for this application.
Anodization is basically a method to create a layer of aluminum oxide on the surface via electrolysis. Aluminum oxide, being a ceramic, is harder than aluminum, and can add wear resistance to an aluminum surface. In addition, aluminum oxide is relatively inert, and so adds environmental protection against acids, corrosion, etc. However, because the anodize coating is brittle (i.e. it cracks instead of flexing), it imparts no additional strength to the base aluminum. In fact, the oxide layer cracks easily far below the yield point of aluminum.
The anodizing layer can be generated to varying thicknesses. A thin layer of oxide is relatively transparent. Different color coatings can be created by adding a dye to the oxidizing tank. The dye basically gets "trapped" in the oxide coating, coloring the surface of the final product. If a thick oxide layer is created, the dye can not be seen through the oxide, and instead remains the natural color of aluminum oxide (a slightly brownish grey).
As mentioned, the oxide layer is relatively brittle, as compared to aluminum, and cracks easily. If a thicker the the layer of oxide, the deeper the surface cracks will be. The deeper the cracks in the surface are, the more likely they are to propogate through the base metal. Hard anoded rims are therefore less durable than non-anodized rims in terms of fatigue cracks.
Aluminum oxide also has a lower coefficient of friction than bare metal. Hard anodizing on braking surfaces therefore provide less braking force. Typically, braking on hard anodized rims actually improves once the anodize layer on the brake track has been worn through.
