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topic 59247

Effect of excessive amps per square foot on cadmium plating



April 17, 2013

Q. Does someone have test documentation that when using higher ASF during cadmium plating (providing a rough or porous finish) that it tends to increase the potential of pitting of the steel substrate? In other words the smoother (lower ASF) the finish the more "dense" (less porous) the plating is therefore reducing pitting potential on the substrate?

J Roberts
- Medford, Oregon, USA


April 17, 2013

A. Hi J.

Rough, porous, burnt plating is defective plating. It can and often is caused by excessive current density. At least two bad things happen when the ASF is excessive. The electrons at the cathode too harshly pull any available cadmium ions out of solution, not allowing the metal to build a proper structure and overriding the ability of the addition agents to help modulate the placement of the adatoms. Secondly, when electrons are pumped to the cathode at a greater rate than cadmium ions can migrate through the boundary layer, the sure result is excessive hydrolysis of water and evolution of hydrogen at the cathode.

Whether this directly causes pitting of the substrate during plating, or whether the pitting is caused by the acid activators during stripping and replating, or whether the pitting results in the field, is perhaps not carefully documented but may be unimportant.

We occasionally get questions to the effect of "how many hours of salt spray can we expect if the plating is sucky instead of proper?" And your question of pitting may be along those lines: people tend to not document that kind of thing. Good luck.

Regards,

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

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