The authoritative public forum
  for Metal Finishing since 1989
  

  -----

Zinc plating / Clear Chromate Powdery Residue

We have specified zinc/clear chromate on an 18 GA sheet metal chassis used in an electronic assembly. We have blind hole 6-32 X 1" tall PEM standoffs in the part. There is a white powdery residue in the threads and on the tops of these standoffs when we receive the parts from the plater. We also have this residue and poor plating coverage around other areas of the chassis where a divider partition is spot welded to the main chassis. We have allowed for the escape of trapped air during racking. I have two questions: 1. What is the white powdery substance? please be specific (chemical name) 2. What process can help prevent the generation of this powder, and improve coverage? Using thru hole standoffs is not an option. Cosmetics is not an issue, the plating is for the prevention of bare metal rust and electrical conductivity. Thank You for your help.

Chris Saulnier
Electronics Manufacturer - Gray, Maine, United States of America
1999

Chris, the white powdery substance could be from contaminated cold and hot water rinses following the chromate treatment. As the parts may be racked vertically, the threaded holes are probably retaining a build-up of "salts" in this area. The tops of these standoffs is probably the last place water accumulates in the drying process. Any time a spot welded area is encountered you will potentially see an area not plated. The temperature of the spot weld actually will change the temper of the steel. If your plater won't work with you, find another zinc plater. Explore how the weldment procedure is utilized as well.

Bill Hemp
tech svc. w/ chemical supplier - Grand Rapids, Michigan
1999

---

I agree with Bill an all points especially the fact that a good zinc plater worth his salt should be able to use good rinsing and a better plating process. You haven't mentioned the plating process in use here. I assume that you are using an alkaline process since your complaint sounds all too familiar ! Try a Potassium chloride based bath which has better throw and will also cover all spot welded areas beautifully irrespective of surface stresses. Good luck ....

regards

Khozem Vahaanwala
Saify Ind

Bengaluru, Karnataka, India

1999