plating, anodizing, & finishing Q&As since 1989
Spotty Conversion Coating of Cold-worked Aluminum Clad
We have recently conversion coated
Amazon affil link]) a frustum (12 feet diameter structure) that is composed of cladded aluminum (Al 7072 over Al 7075). After Alodine coating we are noticing (in 2 out of 6 vertical sections) of the structure spots that look like large rain drops, two more section have slightly elongated droplet look and the other two sections that look normal continuous film.
Our process has always been same - - we strip the paint using plastic and glass bead blasting, clean structure using alkaline detergent (Turco #23), confirm water-break-free and then Alodine. The surface of the frustum is extremely rough due to repeated plastic and glass bead blasting. Could the differences in cold-working of aluminum 7072 clad be the cause of spotty conversion coating ? Or would it be likely some contamination and then what kind ?
- merritt island, florida
Sounds more like a bleeding problem than a metal problem. Either in the rinsing of the alkaline cleaner or the chromate solution. Any residual alkaline will cause the chromate to spot out. Is there a possibility of entrapment of alkaline residue in the structure of the part?
Mabank, Texas USA
Yes three is a possibility of alkaline residue left over. Though we had checked pH of rinse water to be close to neutral, I can not rule out the possibility of surfactant left over as a film on the "roughened" Al-Clad.Anil patel
- merrit island, Florida
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