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Ted Mooney, P.E. RET
Pine Beach, NJ
The authoritative public forum
for Metal Finishing since 1989
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Dichromate -passivation of zinc plated medium carbon steel
Q. Denis Moles
2006
Is there a plating specification that covers the subject plating/passivation process? What type of coloration is expected from this procedure, can the color of the parts range from a bright zinc to a golden iridescence? Other that subjecting to corrosion testing, is there a simple test that can be performed to determine if the dichromate-passivation operation was skipped?
A. ASTM B633 -98e1
"Standard Specification for Electrodeposited Coatings of Zinc on Iron and Steel" [except wire and sheet] gives 4 zinc plating thickness Classes. Each is available in 4 finish Types: as-plated, clear chromated, colored chromated or phosphated. Class, Type & color are customer specified (although available colors may be limited).
Per ASTM B201-80
"Practice for Testing Chromate Coatings on Zinc and Cadmium Surfaces," corrosion resistance of chromates increase in the order: clear, clear-blue, black, yellow, olive drab. Supplemental seals and lacquers can improve corrosion resistance. 'Jobshops' below gives several New England providers of chromated zinc finishes.
B201 gives a spot test for clear chromate coatings (unless a water-repellent supplemental coating is present) using lead acetate solution. Another spot test, for hexavalent Cr only, is given in ASTM D2092.
Zn-rich alloys with Co, Ni, or Fe can also be electroplated (ASTM B840,
ASTM B841,
ASTM B842, respectively) and similarly chromated. Likewise, mechanically plated Zn and Zn alloy coatings. The much thicker galvanized coatings are less frequently chromated.
- Goleta, California
Finishing.com honored Ken for his countless carefully researched responses. He passed away May 14, 2015.Rest in peace, Ken. Thank you for your hard work which the finishing world, and we at finishing.com, continue to benefit from.
2006
Ken
Your response is greatly appreciated - I have plenty to run with based upon your feedback.
Thanks again,
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