We're dealing with similar issues on a number of materials. Customers want a uniform appearance. Our suppliers who do this well have a deliberate surface treatment - like mentioned above, use a conversion coating -like a "bluing" treatment to form the oxide you want. I think you'll find more...
We have a meta-lax system. We proved the effectiveness to ourselves and presented results at an ASM fall meeting a few years back.
We had an aluminum casting that was cracking when a certain bore was machined. Neutron diffraction studies of an uncracked casting before and after a meta-lax...
