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Strange Zinc Plating Corrosion

We just received photos from our telecommunications base station in Florida that shows black and areas of corrosion on our CRS shelves. These shelves were zinc plated type III, SCE clear per ASTM B633. What are the most probable causes of this corrosion? Also, how can plating thickness be verified? Could any cleaning solution, used prior to packing for shipment, be a cause? What process controls should be verified at the plating facility?

Thank you for your help.

Steven Beers
- Tustin, California, USA
2002

While zinc plate may look great, corrosion wise it may not hold up under damp weather / outdoor conditions. In a salt spray cabinet for example, you may be lucky to get 3 days before red rust. Black and white corrosion is also visible, usually before red, because the coating is sacrificing itself before the steel does. The black you see is "normal".

Steve Sarmiento
- Newbury, Ohio
2003