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Chromate discolors over time

Hello,

We use alkaline Zn-deposition on the automatic line for automobile parts, with blue passivation, the thickness of the Zn layer about 12um. The question: after the passivation we get blue colour prefect,however tow days later ,on the edge area appears yellow colour. By the way we use the activation of HNO3 with PH about 1, so please give me your suggestion!

Thanks best Regards,

Colin Tong
- HANGZHOU, ZHEJIANG, CHINA
2002

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The old saying is "better late then never" which I hope is what you think of this reply! Yellow on the edge upon aging may be a sign that: 1. You over chromated on the edge and should use a chromate wetter or better agitate the parts on removal from chromate bath 2. Rinsing after zinc plating needs to be improved 3. Rinsing and drying after chromating needs improvement (do not heat part to over 130 deg F.)or ? Hard to guess without seeing parts unfortunately. If you are using a trivalent blue bright material your pH may be high (generally above 2.2) If so, lower with chromate not acid.

Gene Packman
process supplier - Great Neck, New York
2002