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Conversion coating appearance

2006

Hi, I'm process engineering for aerospace industry. We have chromate chemical conversion coating process with Alodine for various aluminum alloys. As you know, the colored conversion coating shall be distinctly colored with iridescent gold to tan.
But, in our product, parts from clad aluminum alloy 2024-O condition show noncoloring. On the other hand, other alloys which processed in same batch with above alloy definitely show beautiful color. Could you tell me what's the reason for this matter and how can I solve this appearance problem (Please see below photos).

For your reference, our general process is;
1. vapor degrease for 3 minutes
2. alkaline clean for 10 minutes
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
Best regards.

D.Y.Yoo
Aerospace - Busan, Korea

I'd say your deoxidiser isn't designed for 2000-series alloys. This is because of the very high copper content of those alloys. Does it leave any kind of grey smut on the surface after processing? If so that would prevent the chromate from adhering. Talk to your supplier about a more appropriate deox solution.

John Reid
- Brisbane, Qld, Australia
2006