Unlike aluminum, bare untreated magnesium deteriorates quickly when exposed to moisture in the air, so Marchesini, Dymag and Mavic put a dichromate coating on the surface to avoid corrosion prior to final painting. This Dow #7 coating offers only fair corrosion protection (less than 24 hours 20% salt spray resistance) according to the Mil Spec [MIL-M-3171C]
Impacts from small stones and other debris will damage this coating and expose it to the weather, so an expensive magnesium wheel or swingarm should be painted or powercoated prior to track or street use. Some mags like Dymags come powdercoated with a tough clearcoat direct from the factory.
If corrosion has already started, you need to use a chromate pre-treatment compound like Alodine 1200 that prepares the surface of the magnesium. Then you can paint it like any other metal.
Dow Magnesium Processes [MIL-M-3171C]
Dow#1: A chrome pickle treatment for magnesium. Color varies from matte gray to yellow-red. Only fair corrosion resistance (less than 24 hours 20% salt spray resistance). Removes metal (Approximately .0006" for wrought material, less for castings). No dimensional change. Used mainly for protecting magnesium during shipment, storage, and machining. Can be used as a paint base.
Dow #7: A dichromate treatment for magnesium. Color varies from light brown to dark brown to gray depending on alloy. Only fair corrosion resistance (less than 24 hours 20% salt spray resistance).
Dow #9: A galvanic anodize treatment for magnesium. Produces a dark brown to black coating. Designed to give a protective film on alloys which do not react to Dow No. 7 treatment. Only fair corrosion resistance (less than 24 hours 20% salt spray resistance).
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