Powdercoat Blistering after Zinc Chromate Undercoat

A. Adhesion of powder coatings onto zinc plated or galvanized surfaces was always a perennial problem, and if you search our site you will see at least a dozen threads on the subject. But adhesion onto trivalent chromated surfaces adds yet another wrinkle because these conversion coatings often also involve a topcoat of one sort or another, which could be wax, lacquer, silicates, zirconium compounds, etc.

In our earth-wide hasty quest for Cr6-free coatings, we've judged the trivalent processes to be their equal if they can simply match the salt-spray test hours of hexavalent chromate ... which unfortunately has left us with a hodgepodge of secret proprietary trivalent chromates which offer the salt-spray resistance of hexavalent chromates, but which may not be their equal in ability of paints and powder coatings to adhere, or in chemical resistance, or in electrical conductivity, etc.

You haven't told us yet whether you are the specifier or the powder coater, and whether you also do the zinc plating and chromating and have control over it, etc. So the first requirement is a detailed process description. If you are the powder coater, and the parts are plated and chromated by someone else and they won't give you the exact process sequence, it may be virtually impossible for you to solve because, as one example, the problem may be separation of the chromate layer from the plating layer.

A. Hello DJ;

You may have a strong topcoat over the Trivalent Passivation that is preventing the phosphating over zinc plating effectively.

Your Powder Coater must ensure a good darkening of the zinc plating after his phosphating.

It would also be a good idea to skip the zirconium step before the powder coat.

A. Hi. KV may not have seen your followup. I believe he was suggesting that the phosphate must react with the zinc for good adhesion, as evidenced by a gray color; and that if you are not seeing the gray color develop, it likely indicates the presence of an unacceptable zirconium topcoat on the zinc which is preventing the graying and preventing good adhesion.

Again, trying to get good adhesion of powder coating onto a secret proprietary Cr6-free chromate applied by others is shooting with a blindfold on. You must pass most of the problem back to the plating shop, who must pass it back to the vendor of the proprietary -- the powder coater probably can't solve it on his own. RoHS has created a nightmare for you :-(

Regards,

A. The fabricator is incorrect, David. Zinc plating plus powder coating is a fine specification. The zinc plating will offer sacrificial protection to deter rust in any areas where the powder coating may get scraped in service.

It is true that, if not done correctly, there can be adhesion problems in powder coating over zinc plating; so if done by separate shops there may be buck-passing if adhesion problems do develop. So I would suggest that you try to find a shop that offers both zinc plating and powder coating so that one finisher can be responsible for the finish.

A. Dear sir,
If the Zinc Plating is done not properly, then you will face problem, We suggest best is zinc phosphating done at 65°C which will give very smooth crystalline coating of zinc, then powder coat.

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Hi. Zainuddin is steering you well that zinc phosphating is a more traditional, easier, and less expensive pretreatment for powdercoating than zinc plating -- but in a real-world corrosion test my guess is that the zinc plating will win handily because of its sacrificial capability, although I'm not aware of specific comparison testing.

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

A. A further point to Ted's reply, is that zinc plating is a quite thin zinc coating, and the corrosion protection is proportional to the coating thickness (in this case).
If the application is external, where corrosion is a consideration, then hot dip galvanizing should be considered. Typically about 6-10 times the Zn thickness, and powdercoating over HD Galv is no more difficult than over Zn plate. The same advice applies, ensure competent pretreatment and preferably by one shop that does both coatings so you don't end up refereeing in a fight over who did what wrong!

A. Hi Josh. I don't think blasting is beneficial, and the thickness of the zinc plating is irrelevant.

However ... in the old days of hexavalent chromates, the trick was to keep the chromate layer as thin as possible so the chromate layer could not shear, as mentioned by Robert Probert several times on this site. But today, with proprietary trivalent chromating in "thick film", or "thin film" varieties plus proprietary topcoats, it is far trickier. Suppose you succeed the first time, then go back for more a few months later ... the powder coating shop may do exactly what they did last time, but the plating shop may have changed their chromating process and not tell you, or at very best take no responsibility for lack of adhesion of powder coating applied by others.

My own take is that these days you must get the whole process from one shop who takes responsibility for the finish.

Regards,