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topic 3180

Blisters/adhesion Problems when Powder Coating over Zinc Plating and Chromating




An ongoing discussion from 1999 through 2014 . . .

(1999)

Q. I wanted to know the pros and cons of powder coating over parts that have been zinc plated and chromated. Benefits? Problems?

john petro
- Reading, Pennsylvania


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.

Ted Mooney, finishing.com Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey



Blistering of Powder Coating on a Zinc Plated surface

(2002)

Q. We have experienced an issue with powder coating over a zinc plate with yellow chromate. The defect is blistering which occurs immediately and also after 48 hours room temperature. I have been advised that this is not unusual and the reason is the hydration condition of the chromate. Is this correct? If yes, what is the solution to this problem? Thanks for your responses.

Carol Shibles
- North Andover, Massachusetts


Zinc Plating
by Geduld

(2002)

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,

Kevin Davis
- Versailles, Kentucky, USA


(2004)

Q. How long does it take Clear Zinc Plating on steel to complete its out-gassing if it is not Baked?

Allan Wright
Sheet Metal Fab - Austin, Texas



Powder Coating Systems

(2003)

Q. I need advise on the best process for powder coating over clear zinc (zinc chromate). We will build a product out of Hot Rolled Pickle and Oil (HRPO), send it out for clear zinc plating, and then powder coat over the clear zinc.

Questions...

A. What is the best wash method/pre-treatment of the clear zinc part before powder?
B. Is adhesion best on clear or yellow zinc?
C. Is salt spray best on clear or yellow zinc?
D. What questions or instructions should I discuss with the plater?

Rob Bonner
- Monticello, Indiana, USA


(2003)

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,

Ted Mooney, finishing.com
Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey

----
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.



To minimize searching and offer multiple viewpoints, we've combined multiple threads into the dialog you're viewing. Please forgive any resultant repetition or failures of chronological order.



Adhesion problem in powder coating on zinc plated product

(2005)

Q. Dear sir

My problem is powder coat adhesion problem over zinc plate blue passivation.
I want zinc plating with blue passivation before powder coating because I am coating only outer surface of component ... inner surface I can't coat due to CE test getting failure; for same reason I can't go for phosphating.

regards

P.Prabhakara
ELECTRONICS - BANGALORE, KARNATAKA, INDIA


(2005)

Q. Dear sir
Before powder coating on mild steel (CRCA) shall we can go for zinc plating (blue passivation) YES or NO?
YES Means what type of Zinc plating
NO Means why ?

P.Prabhakara
ELECTRONICS - BANGALORE, KARNATAKA, INDIA


(2005)

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.

v g rajendran
V. G. Rajendran
- Trichy, Tamilnadu, India


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,

Ted Mooney, finishing.com
Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey



(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. RET
finishing.com
Pine Beach, New Jersey


(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. RET
finishing.com
Pine Beach, New Jersey



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. RET
finishing.com
Pine Beach, New Jersey


(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. RET
finishing.com
Pine Beach, New Jersey


(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



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