Stain mark on Sn surface after dipped new make-up trisodium phosphate

In our matte Sn (MSA-based), we use 30-40 g/l trisodium phosphate ⇦ on eBay or Amazon [affil link] in post treatment for neutralisation. A stain mark is detected on the Sn surface once dipped in freshly made-up tri-sodium phosphate.
However, it goes away after a few lots are dipped in the bath.
Why it is so?
Is this migration of any metallic contamination in trisodium phosphate onto Sn surface?

Some of our readers reject anything to do with "black magic" in electroplating and may not like this answer, Ang :-)

But it is a long tradition among practitioners to observe that freshly made-up baths often do not work quite as well as broken-in baths. And, instead of writing a PhD thesis to try to determine the cause of this phenomenon, they simply counter it by retaining maybe 20 percent of the old bath and only making up 80 percent new :-)

You might try that.

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

I had tried already tried with 50% new and 50% old bath.
It had improved but not totally removed and not accepted by QA.
I thinking of replacing this neutraliser with another supplier acidic anti-tarnish.will it better for MSA Sn finishing

Yes I would certainly expect a proprietary tarnish-ban post treatment to outperform a simple trisodium phosphate dip.

But I remain puzzled how a broken-in neutralizer can work fine, but mixing the broken-in bath half & half with a newly made up one doesn't work. That result seems to say that the problem is not that some contaminant or breakdown product is missing from the fresh bath which keeps it from working, but rather that some interfering material is initially present in a fresh bath that quickly disappears from the bath as it is used. I suppose the TSP (trisodium phosphate ⇦ on eBay or Amazon [affil link] ) powder could have some kind of anti-clumping addition agent that was added by the manufacturer, or some contaminant from the packaging.

Good luck with the tarnish-ban.

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