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topic 3180p2

Powdercoat Blistering after Zinc Chromate Undercoat



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A discussion started in 1999 but continuing through 2020

2005

Q. Hello,

I have some problems to get good adhesion of a powder coating on a zinc treated surface. This problem concerns especially the zinc with Cr6-free passivation.
What is the difference between the Cr6 and Cr6-free passivation concerning the adhesion of an organic layer?
How can I solve my problem?
Thanks in advance.

DORLY Jérôme
- Lyon, France


2005

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.

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


2005

Q. My company sub-contracts this operation of zinc plated + powder coating. So we have 2 sub-contractors : one for the zinc, the other for the powder coating. The distance between them is around 600 kms.

The process steps for zinc deposition is :
- standard surface preparation
- nitric or sulfuric activation
- passivation CrIII
- drying
- packaging

The process steps for powder coating is :
- phosphate degreasing
- water drying
- Zirconium passivation
- air drying (170-180 °C for 5 minutes)
- painting (with Epoxy wax)
- oven (155 °C during 12 minutes)

If you need other information, I can send it to you.
Thanks in advance.

DORLY Jérôme [returning]
- Lyon, France


2005

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.

Khozem Vahaanwala
Khozem Vahaanwala
Saify Ind supporting advertiser
Bangalore, Karnataka, India

saify logo


2005

Q. Hello KV !

Thanks for your answer.

Nevertheless, what do you mean by "must ensure a good darkening of the zinc plating after his phosphating". I do not understand darkening...

Moreover, my powder coater apply a phosphate degreasing and not a phosphating. Is it a problem for you. Perhaps a phosphate degreasing is not hard to prepare the surface?

Regards

DORLY Jérôme [returning]
- Lyon, France



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,

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



Zinc Plating before Powder Coating for Architectural Projects?

2006

Q. I am an architect in Michigan. My firm designs many projects with decorative metal fabrications on the exterior.

I had recently specified the decorative railings on a project to be zinc-plated and powder coated.

The fabricator indicated that it is not possible to powder coat over zinc plating?

Is this true? Is there another substrate finish that we should specify for powder coating?

David C. VanderKlok
architect - Lansing, Michigan


2006

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.

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


2006

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.

Zainuddin
- Karachi, Pakistan


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


2006

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!

Geoff Crowley
Geoff Crowley
galvanizing & powder coating shop
Glasgow, Scotland




How to enhance adhesion of powder on zinc-plated mild steel surfaces

August 4, 2015

Q. Hello, guys. I'm new to this site and I'm a sales person from a Chinese factory producing shopping trolleys made of normal mild steel. I'm asking simply because I'm not getting much explanation from my colleagues for my question and I expect some professional answers here.
Our factory is making trolleys with zinc-plating plus coloured powder coating. But it turns out that the adhesion of powder on zinc-plated surfaces is not as good as on phosphated surfaces because the powder peels off where the trolley nests (It's better after we use better quality powder).
One of my customers show me test results that e-coating on zinc-plated surfaces performs better in adhesion but worse in corrosion resistance by NSS test than powder coating on zinc plated surfaces. Is this the usual case?
I'd also like to know that if adhesion of powder coating onto phosphated surface is usually not as good as on zinc-plating surface? Is there any way to improve it? Will it be better if no chrome passivation is applied after zinc-plating? Thanks!

William Shen
Sales for shopping trolley - Shanghai,China


August 2015

A. Hi William. We appended your inquiry to one of many long threads on similar subjects. To summarize the subject from my perspective, although others might see it differently --

Phosphatization is the traditional pretreatment for powder coating on steel and is less expensive, easier and more trouble-free than zinc plating. But if you achieve full adhesion of the powder, zinc plating under the powder coating almost surely offers greater real-world corrosion resistance because of the significant additional amount of cathodic protection afforded by the sacrificial zinc plating. But in these days of RoHS-compatible trivalent chromating of zinc, adhesion of powder coating onto the surface seems to be a perpetual development project because of the many different secret proprietary chromate formulations and topcoats.

You situation is further complicated by the mistake of trying to equate salt-spray life with real-world life, which becomes ridiculous when trying to compare zinc plating to powder coating. Zinc plating is intended to deliberately corrode away in a salt spray test; powder coating is designed to remain inert to the salt, but allows the underlying steel to fail catastrophically through a scratch or pinhole because it offers no sacrificial protection.

My personal take is that powder coating on top of zinc plating is not a very good spec these days and should only be used if a single shop does both steps -- in which case I'd think about applying no chromate, and then phosphatizing the zinc plating before powder coating.

I think the best process of all would be to skip the zinc plating ... and zinc phosphatize the plain steel, followed by e-coating it, followed by powder coating. I think this will lead to excellent corrosion resistance in both salt spray testing and the real world, and no adhesion issues. Good luck.

Regards,

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


August 5, 2015

A. There are several options for pretreatment between zinc plating and powdercoating.
PC on top of Zn without pretreatment is a sure recipe for adhesion failure.
Chromate is certainly the best way, but various countries restrictions on its use are making this almost impossible.
We PC on galvanized steel. Options for pretreatment that work include Chromate (as mentioned), Zinc phosphate, and manganese phosphate.
ZnPhos has disadvantage that it needs heating and produces a sludge. Mn Phos does neither. Both give good reliable adhesion.

Agree totally with Ted on salt spray. It does not replicate real life, only gives some comparison between different organic coatings.

Geoff Crowley
- Glasgow Scotland



January 9, 2020 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. Hi,

I am looking to get a couple parts zinc plated and was hoping to get some advise on pretreatment processes and what chromate to use for the zinc plating. Ideally I'd like to powdercoat the lids after they are plated to have them match the rest of the system. However, from looking around I have seen that there are a lot of common issues associated with doing this.

I was curious if it's beneficial to blast the steel after plating to help with the adhesion of the powdercoat or would this be counterproductive and take off the zinc as well? Additionally, does the thickness of plating impact how well the powder can bond with the surface? Are there any specific types of chromates and pretreatment techniques that typically do better when powdercoating on top of a zinc plated surface?

Please let me know what you think, any advise would be appreciated!

Josh Daniels
- Denver, Colorado, USA


January 2020

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,

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

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