Fisheyes

Question:

We’re having a problem with fisheyes and need some advice. We have a seven stage pretreatment, epoxy electrocoat and TGIC polyester powder topcoat. The fisheyes are obvious after electrocoat. We can see where the fisheyes will occur after pretreatment—there are bright silver spots, sometimes with a “comet” tail. EDS of the center of the fisheye shows aluminum and silicon, sometimes calcium. The defect does not happen on hot-rolled steel. The defect does not occur on vacuum degassed interstitial free hydrogen annealed cold-rolled steel. The defect does occur on commercial quality cold-rolled, draw quality aluminum killed cold-rolled, and INTEK nickel flash cold-rolled steel.

We have an alkaline cleaner (steel only), chlorate accelerated iron phosphate, chrome seal and DI rinse.

The problem went away without our purposely changing anything. All the pretreatment parameters were within the recommended range. The problem came all at once and went away all at once. I think that one of our pretreatment chemicals may be reacting with a condition that exists in certain cold-rolled steels. I’m not sure which stage to concentrate on. Any ideas? K.H

Answer:

I suggest you concentrate on the cleaning and its rinse stages. The aluminum killed cold-rolled steel and the nickel flash cold-rolled steel should also go through a cleaning stage. If you can’t use the alkaline cleaner, use a detergent cleaner.

I would also try to determine the source of the contamination. Did it come from an operation in your plant or was the metal contaminated when it was received? If it came from your plant, did you do something different during the problem period?