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Inconsistent yellow with ASTM B633 Type V SC3 zinc plating and chromating

September 20, 2010

Q. To Whom it may concern,

I'm having an inconsistency issue with my plater using ASTM B633 [affil. link] 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 ASTM B633 [affil. link] Type V SC3 in Zinc without or at least minimum amount of yellowing?

Any help you can provide is greatly appreciated.

Billy Hui
Product Designer - Canada

October 9, 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?

Mike Burnson
- Chicago, Illinois, USA

December 30, 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
sara michaeli signature
Sara Michaeli
Tel-Aviv-Yafo, Israel

December 11, 2012

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 [affil. link] -11.

Gilbert Loy

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