Powdercoat Blistering on Zinc plus Chromate

A. Hi John.

Powder coating offers generally better corrosion resistance than zinc plating because the powder coating is an organic topping which encapsulates the part and thus stops (or greatly slows electrochemical corrosion), whereas zinc plating is a sacrificial coating which slowly dissolves away until it's gone and the underlying steel is exposed.

On the other hand, it is much harder to scratch through and breach a metal plating than paint. And even if you do breach the plating, the surrounding zinc plating will sacrificially protect the steel like zinc anodes protect a steel hull boat (please take this as a reduced tendency to rust rather than as a guarantee of perpetual freedom from rust).

The benefits of combining the two would be that you get the corrosion protection of zinc plating and chromate plus that of powder coating.

The problems would be the extra cost of doing both zinc plating and powder coating, and some potential incompatibility problems that are addressed in this letter, letter #2922, and some others revealable by a search of the site. Good luck.

A. 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 chromate 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 to give it a thicker plate as the zinc 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 100 °C and you're going to what, 175 °C? 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 115 °C 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 100 °C rinse with boiling water to remove any residual acid.

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

Good luck!

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A. Carol,

We also powder coat over zinc with yellow chromate, and have problems with Blistering and out-gassing. The blistering in most cases seems to relate to a problem with the plating: poor preclean, or plating too thick, and can be seen easily by removing the powder coat and checking to see if the plating is still intact. The out-gassing is different, the powder coating has what appears to be tiny eruptions in the surface from the gas exiting the part and bubbling in the surface of the powdercoat while it is in it's liquid state. When the powder coat is stripped on these parts the plating still looks good.

Although I do not have anything conclusive, we have gone as far as have a nickel strike (2-3 microns) put on the base metal and then put the zinc over top of it. This is just my opinion but I think this levels the surface of the base metal, filling in any tiny voids that may trap moisture that would be released when the part is heated. Another cause of this out-gassing can be not enough time between plating and powder coating, causing the moisture in the chromate to pass through the powder as it is in the liquid state. We have had some success with running the parts through the cure oven before we powder coat, thus dehydrating the chromate.

Hope this helps.

Regards,

A. Apparently your yellow chromate is too heavy and when you cure the powder paint the heat dehydrates the chromate. Try putting on the chromate very lightly, just barely get a light yellow hue.

A. If you replace the yellow chromate by a filmogeno fosfation, that contributes great adhesion to painting. That way we will eliminate the Cr VI of its passivation. Also I change the bath of Zn Cyanided by another free of cyanided. Another suggestion is to change all the process by a covering (plate) of Zn/ni with free passivation of Cr VI; the adhesion of the paint will increase formidably. I feel my poor English.

A best regard.

Great point Miguel! Zinc electroplating usually gets a chromate conversion coating to maintain the corrosion resistance and good appearance of the plating ... but if the item is going to be powder coated rather than remain bare, phosphatization can probably be a better approach than chromate conversion coating.

Regards,

A. Hi Rob.
A. Mild alkaline cleaning is probably all you should do to avoid damaging the chromate. In turn, this means that you should do the powder coating in relatively short order (maybe 24-48 hours after plating) to minimize the handling, aging, and potential soiling.
B. Adhesion is probably better on clear chromate than yellow chromate because it is thinner and less likely to shear.
C. Salt spray resistance will be best for the yellow zinc because it contains more hexavalent chromate.
D. General questions on this subject have been perennial, and for good answers to them you may wish to review the rest of this letter, and search the site. Good luck.

Regards,

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Ed. note Dec. 2014: These days most chromating is trivalent (for RoHS compliance) and most trivalent chromates are clear. So when you see yellow chromate these days, chances are good that it is just dyed and has the same corrosion resistance as clear chromate.

A. Dear friend,

You have put two different questions.

First of all you can do a powder coat on the zinc plating. But blue passivation means you have converted the zinc into zinc chromate. Zinc chromate coating will have lesser adhesion than your plain zinc plating.

If you have done this zinc plating through a commercial zinc plater, please check with him whether he had given you a lacquer coated blue passivation. If the passivation is lacquer coated, then the situation becomes worse for adhesion of your powder coating.

You can try powder coating with only zinc plating without passivation and lacquer coating.

A. Hi. Virtually anything is possible with sufficient funds, but it may not be economically practical. For example, if you can afford the masking effort, you can plate the inside surface with zinc and blue chromate for your test, while phosphating and powder coating the outside. Or you can phosphate the whole component, powder coat the outside, and use the powder coating on the outside as the mask for plating only the inside.

But the most practical approach is probably to zinc plate and chromate the whole component, and heed VG's advice to make sure the plater doesn't use a lacquer topcoat. Many shops powder coat their zinc plated and chromated components, and it can be done with reasonable reliability. Good luck.

Regards,