Pretreating Zinc-Plated Parts

Q. Please give your thoughts on pretreatment of zinc plated parts prior to cathodic e-coating. We have random defects (blistering after paint cure) and have tried to tie the defect to the zinc plating thickness, cleaning and zinc phosphate parameters with no luck. The current pretreatment system is alkaline clean (120 sec at 150°F), rinse, condition, zinc phosphate (90 sec at 110°F), e-coat and cure 20 min at 350°F. (December 2005 Clinic).

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A. I think you are looking in the right direction, but your problem may not be the zinc plating itself but a clear sealer over the zinc plate. Most platers add a clear chromate seal over their zinc plate. These clears are normally organic in nature and can be/are removed with strong, hot alkaline cleaners. You may see variations in the amount of clear on the part and removed in your cleaner stage.

In my experience as a custom coater, I had some success in getting customers (or their platers) to eliminate the clear over the zinc plate. Your zinc phosphate will work much better if the clear is eliminated as it can react with the zinc surface directly.

If the clear sealer cannot be eliminated or completely removed, your zinc phosphate will be “hit and miss” and the defects you described may result. The clears that platers use are not normally baked and if they are not removed, they could blister under the e-coat after the bake.

If the clear is not eliminated or removed, you are really e-coating over the clear chromate and are not giving your customer the quality they are expecting. The zinc phosphate over zinc plate plus the e-coat substantially increase the corrosion protection of the parts.

Try getting samples from you customer without a clear and run them with regular parts as a control. Or soak some parts in a strong solvent (like MEK) to remove the clear before processing, again running with some control parts.