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Electrophoresis - copper from pickling causes problem with painting

Hi
Let me to introduce, my name is Tomas and I am responsible for supplier hot milled steel in our company. We are using stamping process -- then are our product washed in alkali solution, after that are our products painted by electrophoresis. Two years ago we were using also phosphating before painting by electrophoresis. When we cancelled this extra chemical treatment appeared problem with one supplier. After paintings are on surface small no painted stains it seems that small parts of surface are without conductivity. This problem is not with each batch of steel. We started investigation and our supplier is talking me that he found small layer copper on steel surface is seems that copper came from pickling solution on supplier line. Have somebody met with this problem.

Tomas

Tomas Kandra
Quality specialist - Slovakia
March 20, 2011

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Copper will immersion plate on steel from an acid pickling bath. Once on the surface, it will create some problems since iron will dissolve preferentially to copper in the electrophoretic paint solution. So the areas with copper are passive relative to uncoated iron, and may not get covered with paint.

It appears that you need to find the source of copper contamination in your pickling bath and take steps to reduce or to eliminate it. In most situations a copper concentration of < 50 mg/L in a pickling bath will not cause problems.

Lyle Kirman
consultant - Cleveland Heights, Ohio
March 24, 2011