black shade after chromating

Hi friends

i have an issue of black shade on the surface ADC12 DIECAST PARTS after chromating Alodine 1200.
The diecasted parts also have black shade because of high silicon content .
The diecasted are machined and glass bead blasted before chromating.
We have been observing that black shades remain after chromating.

Can anyone post your views in this regard?

Black shade

What aluminum prep did you use before the Alodine ? I see some using Dupont 13205s , the tech sheet says " not for use on 2024 alum." ,use 13204s . I mentioned this to the local paint supplier , he say all the aircraft shops use 13205s , go figure .
I am waiting for the 13204s that I had to special order.
Tom