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topic 3414

Spotty Conversion Coating of Cold-worked Aluminum Clad


(1999)

We have recently conversion coated (Alodine 1201 [linked by editor to product info on Amazon] ) 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 ?

Anil Patel
- merritt island, florida


(1999)

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?

jim conner
Jim Conner
Anoplex - Dallas, Texas USA


(1999)

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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