Letter 3035

How to specify yellow zinc chromate 

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From my research, I understand that yellow zinc chromate is much recommended for protecting steel against red corrosion, and it does so by forming white corrosion. For cosmetic reasons, though, white corrosion is also not acceptable - is there a way to specify yellow zinc chromate (e.g., with a thicker chromate layer?) so that the zinc itself will be protected? Specifying ASTM B-633 [link is to spec at TechStreet] Type II, 0.0002" does not appear to say anything about how much zinc vs. chromate is present.

Fred Pan
- Dallas, TX

Fred, the zinc will corrode preferentially to the steel, so indeed white rust will precede red rust. But the purpose of the chromate is to passivate the zinc and delay the white rust you are concerned about. Your specification is a perfectly acceptable one, although it should be noted that it is possible to specify heavier coatings of zinc for longer life, and/or to specify olive drab instead of yellow chromate to further delay the onset of white rust. MFSA has an inexpensive little "Quality Metal Finishing Guide" booklet for zinc plating which I think you would find useful.

 
Ted Mooney
finishing.com - Brick, NJ

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