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Trivalent zinc and galvanic corrosion

My company produces 12L14 threaded screws which are then zinc trivalent plated. The problem is that these parts are showing signs of galvanic corrosion. I have been put on this project after a few failures and don't want this to continue with me. I have read that a zinc phosphate coating will help or adding a yellow dichromate layer will help. I meet with the platers later this week to discuss current and future procedures and would appreciate any help I can get.

Randy Zielke
screw products - Crestwood, Illinois, USA
2006

Trivalent chromates have traditionally been vastly inferior to hexavalent chromates in corrosion resistance. However, great strides have been made in the last few years in response to RoHS pressure, and your plating shop should be using a newer formulation and should be able to provide corrosion resistance equivalent to hexavalent chromating. Zinc phosphate is usually a pretreatment for paint, although it can also be used to hold a bit of oil to deter corrosion.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2006

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