Gray Discolorations on Chromic Anodized 2024 Panels

Greetings,

We're processing aerospace parts per MIL-A-8625 / MIL-PRF-8625 [⇦ this spec on DLA] Type I Class 1. When using 2024 Bare panels for salt spray testing we're having some of the panels come back after testing ( ASTM B117) with gray discolorations on them. The discoloration isn't present before salt spray testing although occasionally some panels are powdery. Not all of the panels processed concurrently are like this. The MIL spec. addresses this in the requirements section and amendment 1 and provides a separate inspection procedure when this occurs although not divulging on why it happens. The number of affected panels seems to be increasing as each batch of monthly panels is tested. I'm looking for an explanation for this occurrence.

The panels are Chromic anodized (low voltage 24 VDC, 45 min. 95 F), spray rinsed, immersion rinsed and sealed in dilute chromic acid (195 F for 25 minutes). Titanium racks are used, the panels on the outer edges of the rack seem to be affected first while the panels near the center are more successful. I'm suspecting the seal however we are also sealing Type II panels in the same seal and are having no problems with those panels. Any thoughts?

The problem is increasing each month because the test panels are getting older each month, Robert :-)

Old 2024 panels don't pass salt spray tests. See letter 39596.

Unfortunately, salt spray testing of 2024 doesn't have much to do with whether your actual parts are good, and it doesn't have much to do with whether your process is good; it basically tests whether your sample panels are fresh :-(

Good luck.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey