Where the
world gathers for metal finishing
Q&As since 1989


-----

Spots on castings with yellow zinc chromate

We have a major problem with spots showing up on cast iron pulleys we are making that have been plated with zinc with yellow chromate. These parts looked great after plating, but 4 days later we have some spots showing up on some of the parts. What do we need to do in the plating process to prevent this? Can the parts be plated after impregnation?

Jim Johnson
Customer - Waxhaw NC USA
August 12, 2009

---

Impregnated with what?
The spots very probably result because of the surface prep steps were not good enough. You may have to grit or bead blast the parts.

James Watts
- Navarre, Florida
August 13, 2009

---

Hi Jim,

You don't include anything about your process, so everything here is a guess. Most all casting have porosity to one degree or another. If you are a zinc on steel plater I'm guessing you go through a pretty strong acid as part of your normal cycle. That is most likely your problem. It can open up the surface. Try running a load through a weaker acid (like the rinse following the acid) or drastically shorten the time and see if that helps. I have had the same problem and it cured the problem right away. I also had to put at least 10-15 microns on zinc on. Acid chloride zinc. I don't like to run castings in my alkaline bath if I can avoid it.

Good Luck

Trent Kaufman
electroplater - Galva, Illinois
August 14, 2009

---

Apart from Trent Kaufman's suggestion please try risnsing the plated parts in clean water tank with strong air agitation for 2 minutes prior to chromating.
After chromating and water swill, dry the componentusing hot air blower.
If the problem still persists,try plating the component for 3 minutes,strip the plating and proceed with the usual plating and continue the abouve sequences.Do not forget Trnnt's suggestion of reduced timinig in weak acid.
Good luck

T.K. Mohan
plating process supplier - Mumbai, India
August 31, 2009