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Can yellow zinc plating lose thickness in sealed plastic bags and with no evidence of corrosion?

January 23, 2015

Q. Recently we had a rejection from our customer for an undersized thickness on the zinc plate with trivalent chromium passivate. The spec calls for a minimum of .000175", but they measured some as low as .000130" with the average of the sample size (20 parts) at .000190".

When we went back to the plater to inform them of the quality rejection they said:

"Due to the sacrificial nature of zinc plating in general, we could not authorize reprocessing any orders from 2010 without plating charges being applied."

My question is could the plating thickness be less than what they were originally plated at for parts that were plated back in 2010, stored in sealed plastic bags inside cardboard boxes showing no sign of corrosion or degradation?

Thanks for your help!

Ron Hill
Quality Engineer - Chicago, Illinois

A. Hi Ron. Thanks for a fascinating question!

Answer 1: No, I don't think it's even remotely possible for plating thickness to decline inside a sealed bag and with no evidence of corrosion; the "sacrificial nature of zinc plating" stuff sounds a bit silly to me.

Answer 2: You're going to do incoming inspection and implement a sampling protocol and rejection standards 5 years after the parts were plated and expect the shop to take them back? I'd like to hear what others say because my first impression is that you won't find a plating shop in the world who would accept back charges or an obligation to re-do the parts under the circumstances you have described, but maybe I'm wrong. Can you give us the complete specification you ordered to 5 years ago? Best of luck.

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
January 2015

January 27, 2015

Q. Thanks for the response Ted! We believed the same to be true, just using common sense, but wanted to get a second opinion as we are not experts in finishing.

We also thought the same thing in regards to our customer that just now appears to be inspecting the part, which they have had no problem with in the past - FROM THE SAME LOT...

The response we received from our plater we thought was silly (and overly defensive) as well, and by the way, we never even mentioned that they were responsible or they needed to replate the parts. There were general questions asked to help with our investigation about if they have the capability to measure parts and, if they do, what frequency, etc.

The specification is for Zinc Plate with Trivalent Chromium Passivate (Fe/Zn5) - ASTM B633
Min Thickness is 175 micro-inches.
Corrosion resistance min hrs: White - 24, Red - 72

Let me know if any other information is necessary. Thanks again!

Anyways, thanks again for your help.

Ron Hill [returning]
- Chicago, Illinois

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