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"Alkaline non-cyanide zinc problem"
Q. We currently have a problem where we can't achieve a blue finish after passivating. Have tried 3 different passivates (2 hexavalent and 1 trivalent) in last 5 days with no progress whatsoever.The zinc deposit looks OK until it starts to dry when it takes on a hazy, bloomish type of appearance over all current density ranges. Prior to this problem appearing we had plated satisfactorily with our existing passivate for 10 years.
Any ideas ?Dougie Lightfoot
- Fife, Scotland
A. It is much more difficult to passivate an alkaline non cyanide zinc deposit than a cyanide one. Most suppliers have an additive named conditioner or purifier for the zinc bath. Try to add it to your solution.
Also check your rinses, they have to be much more clean than for a cyanide deposit.
How is your iridescent chromate? Usually the iridescent is less sensitive then the clear chromate.
(I notice that you are from Fife in Scotland and it reminded me of the Macbeth....)
chemical process supplier - Tel-Aviv, Israel
A. Dear Dougie ,
Sorry for the long wait for a reply , but you problem could most probably be overcome with a predip before passivating in either
1 ) 5 % Hydrochloric Acid
2 ) 2 % Nitric Acid
remember the Caustic Film of Alkaline non - cyanide Zinc plating solutions is very tenacious , usually effective rinsing after plating can only be achieved in rinses that are violently agitated with air or solution movement then acid dipping prior to chromating
John Tenison - Woods
- Victoria Australia
A. Estimado colega:
Nosotros hemos padecido este problema con el pasivado azul durante varias semanas, pero en los ultimos dias hemos probado el pasivado azul IRIDITE (no recuerdo el no.), distribuido por Macdermid (por lo menos en Espana) y los resultados son estupendos muy azules y sin iridiscencias ni amarilleos, pruebalo.
Miguel Angel Martín
surface finishing shop
Ed. note: It is fine to suggest clear vs. yellow, trivalent vs. hexavalent, and operational suggestions -- but please no brand or sourcing suggestions ( huh? why?)
Thanks for the suggestions, the problem was eventually traced to a deterioration in the incoming water supply. Calcium and other minerals in the water prevented the chromate in the passivate working. We added Ammonium Bifluoride to the water prior to adding the passivate concentrate passivate and this cured the problem immediately.
We had considered the possibility of poor quality water but had discounted it out of hand as historically the water in this area had always been of very high quality.
A lesson learnt, albeit an expensive one.Dougie Lightfoot
- Fife, Scotland
A. Dear Dougie
We have seen this problem many times, particularly in the south of England due to water quality. We have a trivalent blue pasivate which overcomes these problems without pretreating the water.
Yours sincerelyALAN GARDNER