Powder Coating on Galvanized Steel

"Handbook of Hot Dip Galvanization"
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A. Powder coating of galvanized steel is commonplace in Australia. It is important to note the following.

Any welding or "hot work" of galvanized product defeats the purpose of galvanizing in the first place. It is best to complete all fabrication prior to galvanizing so you have a complete envelope of zinc on your product.

It is wise to allow your product to air cool and avoid any quench process at the end of the galvanizing process. This is because it causes contamination of the surface prior to powder coating. Often the pre treatment for the powder coating will not be sufficient to remove all trace of quench.

Oven time for baking the powder is no problem.

Remember that any surface imperfections on the galvanized surface will be magnified through the finish coating, so it is common practice to sand the product before powder coating.

A. As for the wash line, an alkaline cleaner can be your best option if your production is suited to it. The cleaner will have to be mild enough so that it doesn't over etch the galvanized surface, but still alkaline to provide excellent cleaning. Your production rate and quality requirements will determine your best route, the suggestion of galvanizing the parts after assembly is a good one if it can be done efficiently.

It is very doubtful that a solvent dip will turn out to be your best option, air-quality laws being what they are it's hard to go back to that older technology.

A. Hi Arul.

I believe that you can successfully powder coat on pre-galvanized material, but you may be missing the fact that pretreatment is necessary. You must phosphatize before powder coating. Can you tell us anything about what you are doing in that regard? Thanks.

Regards,

A. It is quite possible to powdercoat on galvanized steel, but as ted has said pretreatment is essential.
Without any pretreatment the powder is almost guaranteed to fall off.
With appropriate pretreatment it can be very difficult to get off even if you wanted to.

There are several pretreatment routes:
Chromate system
Zinc Phosphate system
Dry in place systems
Silane systems

Each has advantages and disadvantages. Some are better sprayed on, some by tank based immersion. Any good "system" will involve at least 4-8 stages.

An example of one is:
Alkaline degreaser
Rinse 1
Rinse 2
Phosphoric acid etch
rinse 3
rinse 4
titanium dioxide suspension
Zinc phosphate
Demineralised water rinse 5
Drying

Be suspicious of anything with fewer than 4 stages.

We use a manganese based pretreatment of 8 stages, and coat hundreds of tonnes of galvanized steel a month.

A. Hi Suresh. Degreasing is not adequate pretreatment. Although there are proprietary pretreatments that could be tried, conventional zinc phosphate pretreatment, as described above by Geoff Crowley, is the standard. Please review this thread, search the site for zinc phosphatizing, and try to get a good book on the subject. Best of luck.

Regards,

Hi Sellappan. Thanks for the kind words.

opinion! I'd be interested in alternatives to phosphating like nanocoatings if the quality of finished metal goods like patio furniture, lawn equipment and bathroom accessories was not in a nosedive/freefall. Although we've known how to finish things right for decades, we decided to not do so anymore to save an ounce of phosphate, and the critical first step is to start doing it right again. Such metal items should last 25 years like they used to ... but today we re-mine, re-smelt, re-fabricate, re-finish, re-package, re-ship, re-merchandize and re-landfill a ton of steel every 3 or 4 years instead of every 25 in our pennywise & pound-foolish quest to that ounce of phosphate. This assault on sustainability -- brought about by misguided regulatory authorities, environmentalists, & cost cutters -- is so obvious and critical that we need to concentrate on the importance of a return to strong corrosion resistance standards before we go offer them yet more straws to clutch at like alternative "nanotech" treatments.

I'm not fully understanding your "brush off blasting" proposition. Is this a prelude to nano-coating and in lieu of acid etching, or is it the whole proposed pretreatment? I'm not familiar with it in practice, but I don't see why steel OR stainless steel shot would cause a galvanic problem with a steel or a galvanized surface, but blasting alone is not adequate pretreatment.

Regards,

Dear Mr. Ted,

I fully agree with you about age old Zinc phosphating. The advantages are many of which robustness of zinc phosphate is topmost. You can see on your own eyes whether zinc phosphate coating happened on the substrate. But the nano chemicals are getting coated in the range of milligrams per sqmts. You can't visualize whether coating happened or not (on Galvanized surface). However when salt spray tests are checked Nano chemicals offers a very good resistance as comparable to zinc phosphating.

Q. Clarification of my question: Our substrate is HDG. We prefer a sweep blasting or brushoff blasting (comparable to Sa1) just to remove the oxides in the first stage. Then we want to follow a standard pre-treatment system with Degreasing + conversion coating and no acid etching. We can't do acid etching on spray PT line. Would like to confirm whether this (sweep blasting + Pre-treatment without acid etching) will do justice.

Thanks

Hi again. Frankly, I don't know whether blasting plus degreasing and conversion coating will suffice. Hopefully a galvanizer who has done it will chim in :-)

Regards,

A. Hi Tony.

Short of multimillion dollar 'clinical trial' experiments where nobody knows which process steps were used (and which will probably never happen), we can never get to a definitive 'proven best' sequence, but certainly there are workable sequences and we appended your inquiry to a thread where Geoff Crowley tells you what he does, and where we've added a link to an article by finishing consultant Rodger Talbert for another take.

The best answer of course, if possible, is to have both processes done by the same shop so they can accept responsibility.

A. Hi Karan. You probably won't get full pinhole-free powder coverage on a welded mesh panel, which is good justification for galvanizing or electrocoating it before powder coating.

Although on better surfaces 1000 hours might be possible with the powder coating alone, I'm not sure that you'll get it on a welded mesh panel even with galvanizing. But this might depend, of course, on how you will be defining failure.

Regards,

A. Salt spray testing is inappropriate for testing anything with galvanizing. It is testing the wrong thing, and will give misleading results.
Galvanizing protects by forming a layer of zinc carbonate and zinc oxide complexes on the surface. This is replenished by the underlying zinc which is eventually consumed.
In salt spray testing the salt spray removes this protective layer, so dramatically shortening the apparent life.
This sort of accelerated weathering test is valuable in some coatings, but not galvanizing. If you wish to estimate the life expectancy of a galv coating this is not the way to go (and isn't the purpose of salt spray -- to estimate life in a shortened time)?

A. Hi Doug. Personally, I would probably settle for refinishing a 15 year old fence in place.

Apparently you are willing to take the fencing out and send it to a powder coating shop for refinishing, and that's better of course. I suspect that the shop will remove the old powder coating with some kind of solvent which might partially remove the zinc coating; if they use a caustic stripper it might completely remove it. They will then, hopefully, zinc phosphatize the fencing; in that event the white rust is not an issue and how much if any zinc is left on vs. removed becomes a matter of not much importance.

I think you should tell them you want them to strip the powder, zinc phosphatize, and re-powdercoat and see if they agree. Ideally it would be electrocoated between the phosphatizing and powder coating but that's probably unrealistic.

Regards,

 Thanks Ted. I will talk to the powder-coating shop with your suggestions. I've been warned that the labour costs may be prohibitive, but we'll see.
Doug

A. Hi Mangesh. I may be misunderstanding your photographs or your situation, but burrs and dents like this have nothing to do with your chemical treatment. You either remove them mechanically, depending on the size of your pieces, with buffing equipment or tumbling/vibratory finishing ... or you reject them at incoming inspection.

Regards,