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Dichromate -passivation of zinc plated medium carbon steel
Q. Denis Moles
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
"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 [affil link] "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 [affil link] solution. Another spot test, for hexavalent Cr only, is given in ASTM D2092 [affil link].
Zn-rich alloys with Co, Ni, or Fe can also be electroplated (ASTM B840 [affil link], ASTM B841 [affil link], ASTM B842 [affil link], respectively) and similarly chromated. Likewise, mechanically plated Zn and Zn alloy coatings. The much thicker galvanized coatings are less frequently chromated.
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