Metal finishing Q&As since 1989
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Inconsistent yellow with ASTM B633 Type V SC3 zinc plating and chromating
Q. To Whom it may concern,
I'm having an inconsistency issue with my plater using ASTM B633 Type V SC3.
He is telling me that there no way to control the amount of yellowing due to the thickness of the plating, because there's too many variables involved.
Is it possible to achieve the same corrosion protection as Type V SC3 in Zinc without or at least minimum amount of yellowing?
Any help you can provide is greatly appreciated.
Product Designer - Canada
September 20, 2010
A. This finish is colorless ROHS-compliant passivate (usually trivalent) over 0.5 mil thickness. There is no apparent reason why this should yellow under normal conditions.
Some questions that might lead to a resolution: What type of zinc electrolyte is used? Is a seal or lacquer used over the passivate? Is the passivate a high-corrosion-resistance chemistry?
- Chicago, Illinois, USA
October 9, 2010
A. The yellowing has nothing to do with the zinc thickness. It has to do with the passivation thickness.
The thicker the passivation, the more it tends to yellow.
You should operate the passivation at lower pH, lower concentration and lower temperature.
If the passivating solution is contaminated with iron, it tends to yellow.
The rinses between the plating solution and the passivation solution must be good and clean.
Most important -- the nitric acid after the zinc plating should not be less than 1% and must be recharged every day.
Sara Michaeli
Tel-Aviv-Yafo, Israel
December 30, 2010
A. Plater is wrong, it should not be yellow and the thickness can be maintained. Find a new plater. Also Specification is now ASTM B633-11.
Gilbert Loy- USA
December 11, 2012
Ed. update Jan 2024: Thanks Gilbert! Today it's ASTM B633-23. For convenience and reduced confusion, except when otherwise requested & noted, we try to automatically keep all links updated to the most recent version of the specs.
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