Powdercoat Blistering after Zinc Chromate Undercoat

letter 5542

Powdercoat Blistering after Zinc Chromate Undercoat

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Hello, We apply a yellow zinc chromate finish to cold rolled steel parts before powdercoating with a TGIC-Polyester powdercoat. This is done for corrosion resistance, in our tests this combination was better by far than using a zinc-rich powder undercoat or a zinc-phosphate undercoat.

The problem we are having is the powdercoat is bubbling on some parts. From stripping the parts, it appears that the problem is related to the zinc finish. Has anyone had a similar problem?

My plater says there is no problem and if there is it must be the powdercoating. My powdercoater believes that it is the plating, and I agree with him, because the problem appears to be batch dependant (coming from the platers).

Thanks for your help.

Craig Christensen
Campbell Scientific Inc - Logan, UT, USA

1 of 2 simultaneous responses .

Craig...it would appear that you have moisture/solvent on your substrate and/or in your powder. Trying one or both of the following might put your finger on the culprit; 1)recoat the same part after stripping and drying,hopefully the heat history from the previous cure schedule will have eliminated the problem liquid if it was in the substrate and the next caoting will be trouble-free. 2)coat another substrate(without plating) of similar size or weight with the same powder and same cure schedule. If the problem is in the powder, then the problem should occur on the new substrate as well. Good luck.

Manuel Tyler
- Augusta, Georgia

2 of 2 simultaneous responses .

We have found that the problem you are describing is a shared problem between the paint and the plating. We successfully (without fail) powdercoated pre-plated (electro-galvanized) metal for years and then switched to zinc with yellow chromate due to customer design changes. Ever since we changed to the zinc/chromate process (using the same paint), we have had a miserable throughput due to paint outgasing/blistering.

We believe there is a chemical reaction occurring between the plating and certain types of paint. Our worst failures are occurring with zinc/gold chromate & semi-gloss polyester powdercoat. We have not had any failures with clear chromate using the same paint nor have we had any failures with zinc/gold & textured paint.

The platers claim that there is a higher water content in the gold. Chromate begins to evaporate (dehydrate) when elevated above 150 deg. F. Knowing this condition, we are confident that the chromate is outgasing through the paint.

There are 2 solutions: Test the clear chromate. If successful, get your customer to change the color. Ask your powder supplier to formulate an anti-gasing additive to your powder. We are using this with Cardinal and the throughput has greatly increased.

Kriss Stein
Accurate Metal Fabricators - Seal Beach, CA

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I read the previous three threads and wanted to add a few tidbits.

I zinc-chromate plate parts frequently and the process may give you a clue as to why it is behaving as you see. If you are going to just xhromate plate a part you can plate as thick as you choose to suit your needs. If you are going to subsequently zinc chromate color it, you need tp give it a thicker plate as the xinc coloring is actually an acid etch. This tends to remove a bit of the chromate layer. The deeper the color desired the more is etched away.

To stop the etch the part is rinsed in distilled water and the process is complete. Any water from the chromate process MAY be trapped under the zinc finish. I say may, because I only suspect this from your problems. By heating the part in curing the powder coat, the moisture may be released. I suspect something else may also be happening. The zinc color as I said is an acid etch with a water rinse. If there is any residual acid on the surface it is also feasible that it might also be released during the heat cure. I would be more inclined to think this is the culprit as opposed to water. If water were outgassing it would occur at 100C and you're going to what, 175C? I would imagine the water would be outgassed well before the polymers even began to set-up. Water in minute quantities shouldn't blister the coating other than possibly causing a blush. Acid however could conceivably damage the polymer coating and wouldn't completely go away till a much higher temperature.

So what to do? One thing to try might be to bring the part to about 115C after the initial chromate step and hold it for awhile to dry the part out. Secondly after the zinc etching and rinse with distilled water bring the part up to the cure temperature you are using for your powder coat. Hold it there for a time and then as it cools to around 100C rinse with boiling water to remove any residual acid.

This would obvioulsy add a cost to your overall process and maybe you might want to conside other substrate options. But if it is critical to use a zinc chromate then this may help with your problem.

Good luck!

Mark Wetzel
- Buffalo, New York

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