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Black spots on zinc plated valves after repeated washing

We currently manufacture a large Valve Body. The part is then sent to a supplier for Zinc Plate (Colorless) per ASTM B633, Type III, Service Condition 1. The thickness requirement is .0002 - .0005 .

The part is then returned to our facility for an inside bore honing operation, followed by an aqueous wash to remove the honing solution. The part is then shipped to our customer, where they wash the part again due to a means having an affect on the valve potting compound.

After the second wash the parts sometime exhibit intermittent Black Spots, which our customer believes is related to the plate quality.

The parts have passed the salt fog test requirements prior to the wash operation at our customer.

My question is, What causes the "Black Spots" and is it related to the multiple wash operations after the plating process or is it related to the plating quality?

Michael E.Rupley
machine shop - Quality Manager - Hudson, Michigan, United States
2003

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2003

Sounds to me like you are washing the chromate off. I would suggest trying to test for the presence of chromate at each stage. But it would help to have a better definition of what you mean by "black spots". Possibly some photos?

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

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Mr Mooney's reply is again endorsed by me. Clear Chromate will not stand two washes and exhibits black spots in a Salt Spray Test. If you cannot use a yellow chromate instead, take a closer look at the water quality.

Khozem Vahaanwala
Saify Ind

Bengaluru, Karnataka, India

2003