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Noncoloring conversion coating on edges

2006

Hi, I'm process engineering for aerospace industry. We have chromate chemical conversion coating process for aircraft skins. As you know, the colored conversion coating shall be distinctly colored with iridescent gold to tan.
But, in our product, especially on big (200 by 70 inch) skins show noncoloring areas within 0.4 to 0.5 inch from edge since a couple of months ago. We've performed spot test and which indicates the presence of chromate film on that area. I really want to know that what kind of reasons are affecting this condition and the corrective action should be taken.

For your reference, our general process is;
1. vapor degrease for 3 minutes
2. alkaline clean for 10 minutes (Turco 4215NCLT)
3. rinse for 3 minutes
4. deoxidize for 10 minutes (Amchem 6/16 deoxidizer)
5. rinse for 3 minutes
6. conversion coat for 40 to 50 seconds (Alodine 1200S, pH 1.3 to 1.8, and 1 to 3 oz/gal concentration)
7. rinse for 3 minutes
8. dry

THANKS in advance.

Material is Aluminum 2524-T3 Clad

D.Y.Yoo
Aerospace - Busan, Korea