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No color anodize after beadblasting

2004

Q. I am restoring a couple of small turbine engines from the 1950's. Problem is re-anodizing some aluminum castings. These were originally clear anodized but needs new anodize due to salt water corrosion. -Parts were beadblasted at high pressure using glass beads. -Then parts were blast polished at 2bar(30PSI) using new spherical glass beads. This resulted in a bright and shiny surface. -Then half an hour later, the usual anodize procedure: quick dip in sodium hydroxide, sulfuric anodize for 30min, dye, acetate seal. However the parts quickly turned dark gray in the anodize tank, and would not absorb dye. Continuity test showed that the parts had a thick and strong oxide.

But here is the interesting part; after the blast polishing I filed down some burrs...these areas did not turn dark gray in the tank...and they took the color nicely!

See some pictures of these parts here:

     

What is the cause of this? Is it the blasting or the blasting media that does this? How can I prevent the parts from getting dark gray, and have these dyed in the shown yellow color? Due to the complex geometry of most of these parts buffing/brushing is not an option.

Any help greatly appreciated.

Eirik Karlsen
hobbyist - Svolvar, Norway