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  for Metal Finishing since 1989
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Too Much Chromate

2007

I work for a manufacturer of hydraulic components. We have a product/process that we have struggled with to some degree over several years. Recently, things have gone from bad to worse. The part is 12L14 steel with a zinc chromate finish. The first step in our production process involves resistance welding a SS mesh to the part. We have measured a wide range of contact resistance on the part which creates a moving target for our weld parameters. Our most recent batch of parts has so much resistance the welder cannot burn through. Here is our finishing spec for the part.

1. Zinc plate (colorless) per ASTM B633, type III, service condition 1.
2. Optional: Apply water soluble lacquer (Aqua-lac 8 or equivalent) after zinc plating.
3. After plate, parts must only be processed with amine free product with less than 100 ppm active sulfur and 5 ppm silicon.

I would be remiss if I did not mention the parts go through a final machining step and are washed after plating.

I am looking for a spec equivalent to MIL-C-5541 for the part in question.

Thank you in advance for your input.

Kurt Skov
Consumer - Portage, Michigan

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I'm not familiar with that particular water soluble lacquer, Kurt, but if you have an optional lacquering step you are going to have varying surface resistance. Another factor is that some of your plating shops have probably shifted from hexavalent chromates to trivalent chromates and you are seeing extra resistance due to that.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2007