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Plating 4140

 

Hello,

We have a cam lobe and roller follower made from 4140. Both parts are zinc plated and converted with yellow dichromate. These parts work together in a circuit breaker as a spring charging mechanism. Under a mechanical cycling test I noticed that at about 100 activations that the plating is flaking off the parts.

What could be the cause of this?

Thanks,

Greg Schleusner
- Milwaukee, Wisconsin

There can be a number of causes but the most obvious is: 1-You are cold welding both zinc surfaces and the weakest one is failing by pulling apart from its substrate (adhesion strength exceeded) or by breaking inside itself (zinc tensile strength exceeded). You cannot do very much about the last but the adhesion might be improved following better plating practices. Other approach could be to plate different metals in cam and follower or lubricate if possible (wet or dry).

Guillermo Marrufo
Monterrey, NL, Mexico
 

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If the waste was a powder, it would be the dehydrated chromate coating, which can occur if the drying temperature is too high or drying/baking is done too soon. If you actually see metallic flakes, it's the zinc plating, and it's coming off due to improper surface preparation--i.e., the surface was dirty, or passive rather than active, or a plating bath was used that is not suited to the substrate. Do you know whether the parts are plated from a acid, cyanide, or alkaline non-cyanide bath?

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