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Zinc (trivalent) plating over zamak 5

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We are experiencing tiny blisters on our parts two to three days after they are plated. The parts looked fine immediately after the plating. What is causing the problem?

Jim Tou
MTI - Clarence, New York

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Blisters can come from many different things. Typical is...
1) too much brightener or organics in the solution
2) bad pretreatment

It would be helpful to get more information on this problem.

1) What type of zinc solution are you using?
2) Is the zinc layer chromated or passivated?
3) Where on the parts is the blistering occurring?

TOM KIDD
CRC Specialist - Budapest, Hungary

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January 11, 2011

Look for a pattern in the porosity, most likely it is a hot spot in the die casting process or fill related. Try to raise the die temperature and increase the pressure on the injection during solidification. Look at the die condition, you may find that there is a loss of pressure there. Try some of the die coating products... polish the die. Look at process control, often so much is based on visual criteria. Some times it can be plating solution entrapment, but that is only a expedited symptom since casting is full of pores anyway.

What it is.. there are micro pores in the casting from the solidification of the metal, these fill with solution ( even water will do this )often plate over and then bleed out, peeling and or blistering later on.

Pending on the economics you may need to impregnate the part to fill the voids. One other trick is to vibromill the parts, not to remove the surface but to peen over the surface to close the pores ( part on part, or a polish treatment.)

Paul Barnhart
- lake in the Hills, Illinois USA