The authoritative public forum
  for Metal Finishing since 1989
-----

Yellow passivation technique and specification

Sir,

We are manufacturing deflection yoke. We are using crossarm after Zn plating& yellow passivation. We are facing decoloration problem after hotmelt application(polyamide sunmide melted at 200'C and applied between crossarm & v-coil).I am interested to know the withstanding temperature of passivation and tech details

J.S. Rajan
- Bangalore, INDIA
2002

---

Yes, you will get discoloration after exposing a zinc plating to 200 degrees. The zinc itself is probably fine, but this plating has a conversion coating that passivates the zinc (the yellow passivation). This conversion coating is a hydroscopic gel of hexavalent chromium. At temperatures over 120F, this gel loses the entrapped water molecules and the chromate layer deteriorates and becomes useless.

You'll find that if you expose parts to this high temperature and put them in a salt spray chamber, you will see early onset of white corrosion and consequently red rust. The passivation layer is required to protect the zinc plating from early onset of white corrosion. Zinc with yellow passivation will not withstand the conditions you need for your part. Your problems are not the result of poor quality plating. I suggest that you find a different plating or coating for your application that can withstand higher temperatures. Or, try plating the part after this hot melt process. Unfortunately, either of these options will likely cost more.

Tim Neveau
Rochester Hills, Michigan
2002

---

2002

Dear Mr JS Rajan,

The discoulouration faced by you may be avoided by alloy plating with either zinc cobalt or zinc iron plating. We are currently alloy plating parts with zinc yellow in alloy plating wich do not discolour after 200 degrees.

Khozem Vahaanwala
Saify Ind

Bengaluru, Karnataka, India