PACKRMAN

Topic: DIFFUSE NICKEL CAD PLATING

WE HAVE RECENTLY DEVELOPED A PROBLEM WITH OUR NI-CAD PLATING. THE CHROMATE (OR COLOR) IS RUBBING OFF OF THE PART AFTER THE BAKING PROCESS. WE HAVE REPLACED THE CHROMATE WITH A NEW BATH AND IT SEEMS TO HAVE HELPED, THE COLOR WILL LAST THRU THE 630 DEG. FOR 30 MIN. BAKE BUT OUR CUSTOMER PERFORMS A SECONDARY BAKE AT 500 DEG. FOR 2 HRS. AFTER THIS BAKE THE COLOR WILL RUB OFF WITH VERY LITTLE FRICTION TO A SILVER LAYER AND LOOKS LIKE ASH.

DOES ANYONE HAVE ANY THOUGHTS?

FOR THAT MATTER DOES ANYONE KNOW THE FUNCTION OF THE CHROMATE FILM?

THANKS FOR THE HELP.

DustinGebhardt

Re: DIFFUSE NICKEL CAD PLATING

I don't have much experience with Cd plating (let alone NiCd), but I'll try to throw out some ideas anyway.

The chromate layer forms a protective coating over the plated deposit to resist corrosion.  Typically chromates are used with zinc or cadmium plating because the chromate actually converts some of the base metal into a gelatenous film, which is why it is called a "conversion coating". 


Typically, the chromate will come off for one of several reasons: the layer is too thick (formed too fast or allowed too much time in the bath or the pH is too high or temperature too high or bath is new), the base metal is not clean (free from residues from the plating bath, not oxidized, etc).  Since you are NiCd plating, and nickel doesn't form the conversion coating with the chromate, you can also include the alloy ratio as something to consider.  Getting too much Ni in the deposit can cause a loose adhesion of the chromate.  So check your alloy (if possible) and/or make sure your plating bath is in the correct parameters.

-Dustin Gebhardt, CEF

Advanced Manufacturing/Finishing Engineer

Moen

Sanford, NC