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topic 10837

To chromate or dichromate that's the question


 

This one has me stumped. I am a large volume zinc plater. I thought we have seen plating specs. written in almost every way. This one has me a little confused, it came from a large electronics company.

.0002 zinc chromate-clear .
.0002 zinc dichromate-clear

I presume the chromate-clear is "blue bright" or silver in color. What the heck is dichromate-clear?

Don P [last name deleted for privacy by Editor]
- Elkhart, Indiana


 

The Cronak process was one of the first conversion processes. Mohler "Electroplating and Related Processes" [link is to info about the book at Amazon], Chemical Publishing Company, New York, 1969) says that this was a sodium dichromate solution acidified with sulfuric acid. My guess is that some 'specs' may still call for a dichromate when they want a chromate conversion coating, although I don't remember seeing any in my travels. Maybe most of them were recalled by the late 70's in the companies where I worked. I would offer the newer conversion coatings as offering a better system than the dichromate, probably for the same cost as a dichromate dip.

tom pullizzi monitor
Tom Pullizzi
Falls Township,
   Pennsylvania 


 

Tom may be right about the origin of the term "zinc dichromate-clear". Or it may simply be a typo; perhaps they want "zinc dichromate-yellow" and just mistyped it as the typist skipped a line on the source document?

I would agree with Tom that you should definitely consider one of the new environmentally friendly conversion coats in lieu of the dichromate. But to call it a "better system" and for "probably the same cost" strikes me a large leap of faith followed by an over optimistic expectation. The chrome-free coatings are not only more expensive but in several ways are not as good.

Ted Mooney, finishing.com Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey


 

You may be making the assumption that the large electronics company knows there is a difference. They are probably just asking for the same thing, zinc plating with a clear hexavalent chrome conversion coating. Ask to be sure, but I'll wager they have no idea what the difference would be, and would accept the same plating for both requests. Often a person ordering plating will read a phrase off a print or spec, not understand it, but put it in their request for quote.

tim neveau
Tim Neveau
Rochester Hills, Michigan


 

I recently ran into a problem with chromate discoloring 260 brass. The chromate was on a steel part that was in an assembly that also had brass and copper parts. This is an electrical relay assembly. During the electrical testing the brass turned a blueish color almost like it had overheated. My research uncovered brass has barium in it which is a alkaline-earth metal used to deoxidize copper. Chromate can have a reaction with barium but does not have a reaction with dichromate. This is the only difference between the two that I have found.

Mike O [last name deleted for privacy by Editor]
Metal Stampings - Anoka, MN, USA



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