Zinc plating is rusting in blind holes

Letter 113

Zinc plating is rusting in blind holes

-----

I have a plating problem that concerns a 5/16 cold rolled steel and I have spec'd a finish of zinc plating on. There are a number of blind, tapped holes (8-32) end-on into the 5/16 thickness. The problem is that after the part gets plated, rust appears days later inside the tapped holes. i have talked to my plater about the problem, and he assures me he is following this procedure: 1. ultrasonic cleaning. 2. water rinse. 3. electro cleaning. 4. water rinse. 5. descaler. 6. water rinse. 7. cyanide pretreatment. 8. water rinse. 9. zinc plating. 10. water rinse. 11. chromate 12. four water rinses 13. hot water rinse.

If this is the proper plating procedure, and every thing here looks like the right set of steps, why else might I be getting the rust inside the holes after a few days? Any help would be appreciated.

Ronny Tallman -

-----

This problem is more about the plating shop managing your expectations than their technique, Ronny.

In typical commercial zinc plating, rust inside tapped holes like this is a common thing because there is no plating inside the holes. In fact, some platers (with their customer's permission) stopper the holes during processing to minimize dragover and contamination rather than pretend there will be plating there.

If the inside of the holes MUST be plated, they can be. But this involves detailed handwork like putting a piece of wire (auxiliary anode) into the hole, and inverting the part in every tank so that neither air pockets nor cupped solution interfere with the process. But this is not the usual commercial practice. Sorry to be the messenger.

The only cost-effective encouragement I can offer is that alkaline zinc and cyanide zinc plating solutions have greater covering power than acid zinc, and you may see at least some improvement if the plating shop uses one of those processes instead of acid zinc.

Ted Mooney, P.E.
finishing.com
Brick, New Jersey

Electroplating Engineering Handbook, Lawrence Durney

Legal disclaimer boilerplate button