Zinc Plating with Trivalent Chromate is blistering after baking

A. Mr Shrma,

You should check the following.

1) pretreatment. Clean surfaces before plating. No water break.
2) Do you have a filter on the acid zinc ?
3) if its cyanide zinc , do you use a zinc purifier ?

Removing a blister problem is a question of bath cleanliness, solution maintenance and good pretreatment and activation before Plating.

One good system is Alkaline Cyanide free Zinc, which needs good cleaning before plating, adheres well and passivates very well to a good blue.

A. Lisa,
The MAIN cause of blistering in zinc plating is poor pre-treatment. What that means is, poor removal of oils, solids and smut. Also poor activation. If your plater is doing everything he is supposed to, those parts should not be blistering.

A. Lisa, I would like to start by clarifying that the type of chromate or passivate being used has nothing to do with the blistering problem.
Blistering can come from many sources.
#1.Poor cleaning and poor activation before plating.
#2.Highly stressed zinc deposits originating from improper bath chemistry balance, using improper current density to plate, and or overload of brightening agents can open the door to many post plating problems including blistering or latent blistering. Improper bath chemistry can and will cause high organic inclusion in the plate and can cause stress. Baking at a high temperature may cause the plating to "pop" due to stress and or poor adhesion from improper part preparation.
#3. Extremely high thickness of zinc deposit. You don't mention what type of zinc plating is used here but there are definite problems that can be associated with each of the three main zinc plating baths used today. The most common baths used would be alkaline non cyanide, acid chloride, and alkaline cyanide zinc. There are potential problems with each bath. The most forgiving bath being the alkaline cyanide zinc bath. It is also perhaps the most least used in todays world. Alkaline non cyanide deposits can be problematic if the bath chemistry not kept in proper parameter.
Have you verified the thickness of zinc plate?
Lisa, If I would assume that this is a job that has shown success in the past, then I would have to point to making sure the base material is not causing the problem,(nothing has changed?), then look specifically at the part cleaning and activation prior to entering the plating bath, then look at the plating deposit itself.
If I would assume that this is a brand new job, with a new process, I would also look at the operation of applying the rubber seal to insure that the temperatures, and any chemicals being used, are under control.

A. Hi Lisa.

How long after the failure are we seeing this part? Unless I am misconstruing what I see, the component seems to be covered with rust or scale. You can't satisfactorily plate a brown part.

You say the component is made of cold rolled steel. Was it subsequently heat treated? If so, is any information available about the heat treating sequence and the pre-plate sequence? This doesn't look to me like cold rolled steel unless a lot of time elapsed between failure and photograph.

Perhaps you hadn't seen the responses to your earlier inquiry, but I concur with others that you probably have a cleaning/pretreatment problem. Good luck.

Regards,

i. Ted,

The part in the picture is right after the parts came out of post bake. The material is CR 1020 - 1030 and is not heat treated. Below is the process our customer does to apply the rubber molding after plating.

Molding Pressure: 150KG
Molding temp: 340-360 °F
Cure time: 100 Sec
Mold Cavities: 20
Post cure Temp: 400 °F for 24 hours.

I hope this gives you a better understanding.

A. Hi, Lisa. Those temperatures and pressures don't sound that threatening to me, but I don't have personal experience with that sort of operation. Hopefully someone who does will step up.

Regards,

A. Dear Lisa,

Zinc plating normally withstands a molding temperature of 150 degrees without flaking off like this. Either the plating is not adhering well enough, or the base metal is clearly not CRC and is Hot Rolled Sheet which was pickled before coming to the press shop.

Such flaking off is quite common with Hot Rolled Steel due to porosity, and gases coming out during the molding process. Even CED coated or Zinc Nickel plating over HR Sheet parts sometimes peel off.

For parts that will be molded over after any plating or coating, the only solution would be to use better grade steel in the first place.

A. Yes I agree with Saify Ind sir, that zinc plating can withstand a heat up to 150° while moulding, and beyond this zinc gets cracked and can have a similar finish too.

A. Perhaps the gasses from rubber molding are playing a role. Have you tried baking plain steel pieces without rubber molded into them?
G. Marrufo-Mexico

A. I don't know if you use a trivalent black coating over your zinc nickel but usually an hexavalent chromate loses almost all its corrosion protection after heating. The trivalent chromates have better heat resistance.

? What is the "Base" metal? Leaded steel?, Leaded brass, Tellurium copper?, Stainless steel ... What is the surface preparation?

A. Blister on Zinc Plated part- part is not cleaned right, or too high thickness on part, or check brightener -- may be unbalanced.

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Ed. note: Peeling/blistering in alkaline zinc plating is a perennial problem. For more perspectives and experiences, readers may be interested in:
Thread 50564, "Delayed Blistering in Alkaline Zinc Electroplating"