Mag Alloy refinishing

Hi to all:
I am a new user of this site; my business is classic car restoration, specifically, old Porsches. I am interested in all aspects of plating / refinishing that relates generally to restoring these cars.
At the moment, I am trying to find out what is involved in restoring the original yellow chromate finish that the magnesium alloy crankcases had, back in the late sixties,when they were new.
I have been told that it is potassium dichromate, and the cases were simply immersed in it for a few minutes. I tried doing this on a test piece, using yellow passivate from my zinc plating setup, but it has no effect at all.
Can anyone enlighten me on this subject?? I am no plating expert!
Also, any suggestions for removal of the oil and grease that gets absorbed into the surface of the metal over the years??
Any help would be greatly appreciated!!