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Treatment of Magnesium Skins with Selenious Acid

For years and years the aircraft industry has been using selenious acid to treat magnesium skins prior to painting. The selenious acid crystals are mixed with water, then the solution is brushed onto the surface and washed off. This is often done 'in-situ' i.e., whilst the aircraft is in the hangar. All our reference books and aircraft manufacturer's Service Manuals call up the use of a chromic acid solution (not selenious acid) for the treatment of magnesium. Does anyone know of any technical references relating to the use of selenious acid ? I would love to know what mixing ratio to use and any specific instructions regarding prior degreasing of surfaces, time interval that the 'stuff' is left on the surface, water washing etc.

I look forward to a reply.

Bronwyn Hicks
Aeronautical Engineers Australia - Sydney, New South Wales, Australia
2003