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Salt spray test: Organic thread coating makes plated fasteners rust

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Q. Hi Finishing Folks,

I'm working in fastener manufacturing.
The parts often need to be plated, and there are many parts which are required to have thread coating (adhesive/teflon).

Recently a customer complained that the part with an adhesive coat failed the salt spray test (SST), and requested an explanation as to why our reports were OK.

The adhesive coat, is applied after plating, and involves a curing period at an elevated temperature. The failed parts were thoroughly rusted.

Our QC performed the SST before the adhesive coat was applied, and the customer performed the SST on the finished part.

It is logical that a subsequent process will reduce the performance (due to high temp, handling, shipping, sorting, etc). But at what stage is it supposed to be tested? Is the plating supposed to account for the reduction from subsequent process?

The coating in question is an FCA standard PS.50031 - 8 micron Zn-Ni unpassivated. SST is 500 hrs without red rust.
Within the standard, the SAE USCAR1-3 [affil link] is being referred for the procedure.

Daniel Philbert
- Taipei Taiwan
November 15, 2023

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