I have had a lot of success with Gateros chemicals. However, they used to sell zinc passivate chemicals that were hexavalent. However, they only sell less toxic trivalent passivate chemicals now and at the end of last year, I wasn't getting decent results with trivalent yellow passivate on some 400F carb bits so I contacted them via email. Here is the correspondence :-
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QUESTION from me to Gateros :
Hello,
I recently refreshed my zinc plating chemicals and, as always, got excellent results.
However, my experiences with the yellow chromate passivate both original formula and the new 2-part type are mixed. I did get some reasonable results but in particular with springs, I get instant smutting after the pre-dip in 5% HCl. I am thinking of trying a 1->3% dip in nitric acid instead. I have also read on the .net that if you blue passivate first and then yellow passivate you get better adhesion results. Another suggestion is that the brighteners in the zinc electrolyte affect the performance of the passivation ... but not sure how true that is.
I do have a small amount of the original, more toxic yellow passivate and also the new, 2-part yellow passivate. Given that I have both, then which do you recommend, is it best to use performance wise please? Obviously, the health and safety aspects need consideration.
I bought some olive drab passivate from you last year, as I have a registered business and results with that were superb.
Attached is a box of small parts I did with the new zinc solution / 2-part yellow chromate pass. Not too bad but the large spring took about 5 attempts (i.e. stripping completely then replating). I stretched the spring out on a former jig I made.
Best wishes. .... Ashley
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REPLY from Gateros:
Hi Ashley,
So in theory spring steel, once plated with a thick enough layer of zinc should passivate exactly the same as any other steel item.
In practice it is not quite that simple as springs are tricky to plate because as you have found you often have to use a jig to stretch them out and may also have to de-embrittle them after plating and before passivation.
I would also drop the percentage of the HCL you are using for activation and if you are using nitric then drop it to 0.5 to 1% maximum.
I have pasted a bit below on the yellow passivate.
The key with the yellow trivalent passivates is to remove as much of the brightener film from the surface of the plating before you passivate. The brightener film is on the surface of the plating and trapped in the tiny pores of the plating itself. This caused problems with both adhesion and can also lead to high iridescence in the passivat and staining.
Good rinsing is key for a start so 3 minutes under running tap water.
Then we activate in a dilute solution of HCL from 2 to 5% ( acid brick cleaner would need to be diluted ) then rinse again. You can also use the dry acid salt solution for this but HCL works faster and seems to work better. You can also use a dilute nitric acid solution so from 0.5 to 1%. If you use HCL or the dry acid salts you may see a little gassing on the part
After this activation dip and rinse it is then into the passivate. Always make sure that your passivate temp is about 25°C. Watch the time in the passivate and rinse well after passivation in clean rinse water every time.
I use a hairdryer on cool to dry the parts, you don't want it on hot.
We also run the zylite maintenance brightener level not too high, we aim for a bright finish but not like chrome, too much brightener causes problems with adhesion and gives high iridescence.
This should help you get a good yellow iridescent finish that looks good and sticks well.
No problem, I personally would use the hexavalent yellow but as a private individual you should probably use the new trivalent yellow
Kind regards
Dan
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