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Why does zinc & cathodic e-coat lose adhesion after humidity test?

Q. Hello Sir may I know what kind of caustic soda [affil link] concentration for this purpose?

Pua Kai Chek
- Malaysia
March 7, 2023

A. Hi Pua. I don't know if Joko will be reachable and will respond after all this time, but I think I agree with Gurvin that phosphatizing is a better pretreatment after zinc plating than chromating is. In fact, I doubt that there is any real advantage to zinc plating them prior to CED rather than just zinc phosphatizing them. Weakening/removing/destroying the zinc plating by immersion in caustic doesn't strike me as a good idea :-)
Luck & Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

Q. Yes I fully agree with you, but many customers, especially automotive part drawings, mention and require plating prior to ED coating ... that's the problem now ...

PUA KAI CHEK
- Malaysia
March 8, 2023

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"Handbook of Electropainting Technology"
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Q. We alkaline zinc plate steel parts and send for cathodic e-coat, initial adhesion after e-coat parts passed, but after placing the parts in the humidity chamber 72 hours parts fail adhesion. Why is this happening?

Popatbhai B. Patel
electroplating consultant - Roseville, Michigan
August 12, 2009

? Are the components processed for Zinc Phosphate before E-coat? Or do you directly E-coat after Zinc Plating?

We have seen application of E-coat after Zinc Plating but with Zinc Phosphate, and have not seen a failure in quality.

Gurvin Singh
Mohali, Punjab, India

A. Mostly because of brightener. Usually if part is specified to be zinc plated prior to ED paint, part should have green chromate. Otherwise you can try to etch parts with caustic soda [affil link] before sending to ED paint shop.

Joko Prasetyo Yuniartanto
- Bekasi, West Java, Indonesia

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