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"Cratering" after cataphoretic painting on galvannealed steel

 

Hello everyone,

Could anyone explain me why when we paint galvannealed steel (steel with a zinc-iron alloy) in a cataphoretic line, appears sometimes "cratering". Can the quantity of iron in the alloy affects, or there are other reasons, like impurities from the manufacturing process of the steel, produce this surface defect?

A lot of thanks,

Isabel Ruiz
- Barcelona, Spain

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It sounds to me that you have "fish-eye" which is a common complaint with paints. It can be caused by numerous things, including a dirty or poorly cleaned substrate, de-wetted substrate or the cataphoretic paint is out of spec. It can also be caused by contaminating the bath with impure water; it is recommended that all make-ups should be done with de-ionisied or demineralised water I have found that cataphoretic paints and similar products have a relatively short operating shelf life. Apparently it adsorbs carbon dioxide from the air and this results in co-deposition of the gas. It is sometimes possible to dummy the gas out by depositing the paint on some scrap material. I would check your cleaners and water source. I would also examine at the metal substrate and look for any islands of either corroded metal or unusually looking metal that could cause de-wetting.

Trevor Crichton
R&D practical scientist
Chesham, Bucks, UK