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Chromating Problem

I have a problem of conversion coating on Aluminium 6063. After finishing the process of pretreatment, some of specimen appear GREY some like GREENISH. Who has any idea about this ?

CHUTHA SRITA
QA MANAGER // PVDF COATING - KRATUMBAN, SAMUT SAKORN, THAILAND
2003

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Please be as specific as possible about your chromate conversion bath, etc.

I'm currently working on a similar problem and I have had some improvements in the results achieved.

Santiago José Siccardi
- Santa Fe, Argentina
2003

We had similar issues with a MIC-6 Aluminum plate and Chromate conversion. From my non-expert experience, I learned that there are many factors that can effect the physical appearance and properties of Chromate. The coloration variation is likely due to slight variations in thickness in the coating. I understand that Chromate on Aluminum is typically 10-30 millionths in thickness. The thicker coatings tend to appear more greyish green while the thinner is more bright. The thickness can be driven many many factors.

Obvious variables include the etching, bath, and cleaning concentration and duration. In addition, the process is extremely sensitive in this range which can even be driven by the purity of your stock aluminum.

These are my observation. Can an expert confirm or correct me in this area?

Bryan Clarke
Printers - Vernon Hills, Illinois, USA
2004