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Abrasion Resistance of Metallic Coatings

What causes darkening on aluminum-coated platinum surfaces after abrasive treatment?

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Q. Our powder-coated platinum surface darkens significantly after a short exposure time if abraded with a CS10 Taber wheel or pencil eraser. Our supplier blamed unbonded leafing aluminum, so we tried a bonded leafing aluminum and it did the same thing. It seems that only certain types of friction create this darkening effect. Can you explain what's going on and why only certain abrasives cause it? What is the solution? What properties do we need to prevent this with platinum short of a clear topcoat?

A. I am not aware of any particular name for the discoloration of the metal flake when exposed to abrasion. Abrasion can remove the resin that is formed over the aluminum flake and abrade the surface of the aluminum so that it does not reflect light. Some abrasives will polish the surface and not cause the darkened look. A vulcanized rubber eraser often contains some high-silica pumice or similar abrasive that scores the soft aluminum surface and creates a smudge. The Taber abrader cuts the aluminum surface. A soft substance like leather will polish the aluminum.

The solution lies in having enough powder film over the aluminum flake to resist mild abrasives. This is not possible in the bonding process and it will not work at all if the material is dry-blended. I know it's probably not what you want to hear, but the only sure way to provide the necessary abrasion resistance is to add a clear coat over the metallic coating. The clear coat will protect the metallic flake from abrasion and also add some resistance to sunlight. You should definitely consider adding a clear coat or consider a non-metallic coating. 

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