Letter 42077

Black spots on zinc plating with trivalent (clear) chromate [Ohio] 

August 30, 2006

Has anyone seen black spots on zinc plating with trivalent (clear)chromates? We recently converted from hexavalent to trivalent and began seeing what appeared to be a white haze that turns into very small black spot. We use a sealant (silicate) over the chromate.

Gary Clyburn
Captive shope plating fittings - Dayton, Ohio, USA

Is your zinc bath Alkaline or Acid Chloride? Rack or Barrel? Giving more details would narrow things down a bit.

JEFF W LENTZ
Plating Co. - Lancaster, PA

Gary

There is a wide set of probabilities , which may or may not help.

However..

If perhaps you are a cyanide zinc fan, the black spots are entrapped cyanide in porosities , bleeding out post passivate and drying...
.
If you are an alk Zinc fan, Try beefing up your Post Plate Nitric acid dip to ensure no caustic carry over into the Trivalent passivate.

And if you happen to be an acid zinc fan , using a barrel here are 2 probabilities.

One: Your Current is too High and causes imprints of the barrel holes on the parts which is burning due to inability of solution to carry current. Check Zinc and Total chloride to data sheet levels. Reduce current.

Two. If you run rack, it might be loose zinc particles co depositing and then corroding faster than the regular surface.

In alk zinc Rack too, such a possibility exists.

Now Tell us what you do..?

Rack or barrel, Cyanide acid or alk ??

Regards,

Asif Nurie Atlanta Global Limited New Delhi, India

Gary,

One more possibility existe..

Are you applying the Topcoat while the part is wet, or do you dry first , then Topcoat ?

If the second possibility is being followed, the parts will behave as described by you..
Check.

Asif Nurie Atlanta Global Limited New Delhi, India

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