Sharon

Topic: Powder coating adhesion problem on zinc plated screws

Generally zinc plated part which substrate is steel can be used for powder coating. We face the problem that the head of the screws sometimes failed the adhesion test after powder coating. The process is: heat treatment + glass bead + zinc plating + powder coating. Although we conduct the same process, some of them can pass adhesion test, some can't. It seems that the key point is related to zinc plating process. But different plating chromate chemical were tried, no stable test result so far. Would you please give some comments on what is the root cause and how to solve this problem? Thanks.

DustinGebhardt

Re: Powder coating adhesion problem on zinc plated screws

What is the plating pretreatment process?  What zinc plating process are you using?  What type of chromate? 

-Dustin Gebhardt, CEF

Advanced Manufacturing/Finishing Engineer

Moen

Sanford, NC

Sharon

Re: Powder coating adhesion problem on zinc plated screws

Pretreatment process:
1Degreasing
(Ultrasonic)
2  Degreasing I
3  Degreasing II
4  Rinse
5  Activation
6  Rinse
7  Electro-degreasing
8  Rinse
9  Plating( alkaline bath)
10  Rinse
11  Activation
12  Rinse

Chromate:
Jasco TR-175 Always fail
Jasco Tr-162 + Superseal(Pavco)    Some fail.
Jasco Tr-162 + Ultraseal(MacDermid)  Some fail
Dipsol Zt-444 + Superseal(Pavco)        Few fail.  Tend to colorful appearance
The favorable color is as clear as possible.Appearance is very important.

DustinGebhardt

Re: Powder coating adhesion problem on zinc plated screws

Where is your adhesion failure occurring?  Between the zinc and powder coating?  Between the steel and zinc?  This will greatly help troubleshoot your problem. 


Looking at your process, I see that you do not have an acid step between the last degreaser (electrocleaner) and your plating bath.  The acid will help remove any residual alkaline film and activate the part (which is typically passive after electroplating).  I would recommend that you try going into an acid bath prior to the zinc plate.  This is especially true if  your adhesion problems are between the steel and zinc layer.

Also, recall that chromates are gelatinous and contain water.  You may want to try pre-baking your parts before applying the powdercoating to dehydrate the chromate.

-Dustin Gebhardt, CEF

Advanced Manufacturing/Finishing Engineer

Moen

Sanford, NC

Sharon

Re: Powder coating adhesion problem on zinc plated screws

Thanks for your comments. Sorry for my late reply.

But the problem is adhesion between zinc plating and powder coating. We asked the powder coating supplier about this problem, the answer is that they usually do the test on the stamped piece in large size and have no idea on such a small area only the head of the screws.