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Black Streaks in Chromic Anodize Salt Spray Panels

We regularly salt spray test our chromic anodize panels per MIL-A-8625 / MIL-PRF-8625 [on DLA], Type I, Class 1. They are made from 2024, unclad, sealed in a dilute chromate seal, and are exposed to salt for 336 hours. On occasion they turn black and streaky but without pits or any evidence of corrosion. We've X-rayed the dark lines in contrast to areas that haven't blackened however there is no difference - just a large peak where one would expect for this high copper alloy. Does anyone out there want to venture a guess as to where the black is coming from?

Milt Stevenson, Jr.
Syracuse, New York
2006

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Dear Milton,
I have discovered that the black streaks/stains are indicative of high total Chromium levels. Some specifications state that this is a failure on test panels. Our maximum limit of total Chrome is 100 g/l, but experience has shown that once you reach >72 g/l staining will occur. So as a preventative measure we dispose of the solution at 70 g/l. Also, it may be an idea to check the seal.
I hope this helpful.
Regards,

Joanne Dunford
- Gloucester, UK
2006