Adhesion problems on hot dip galvanized articles

Duplex Systems: Painting Over Hot-Dip Galvanized Steel
by American Galvanizers Association


(Free!)

A. Galvanizing means applying a hot-dipped zinc coating. It is usually possible to paint a galvanized surface, but it is different than painting a steel surface, and it is easier to paint a galvanized surface that was intended to be painted than one which received instead a final chromate dip.

This response has come up so many times that we added it to our FAQs, but there is a booklet entitled "Painting Galvanized Steel Structures" that has some of the information you seek. Single copies are available free from the AGA at https://www.galvanizeit.org/publications/publications.htm, and there is also a book on the subject titled Duplex Systems: Hot-dip Galvanizing plus Painting ⇦[this on Amazon affil links]. Good luck.

A. Laurence:

The chemistry of an iron phosphate on a zinc surface is simple. It can deposit a nice zinc phosphate coating under the right conditions or it can do nothing at all to the surface. Everything depends on the pH of the phosphate solution.

If the pH is on the high end say 4.5 to 6.0 then it cannot pickle the surface so it will not deposit anything.

If you lower the pH to 3.0 to 4.0, then you begin to pickle off some of the zinc. The zinc stays local to the surface and redeposits on the surface as zinc phosphate.

You do not need to change the bath to a zinc phosphate bath, because an iron phosphate bath done right, pulls the zinc out of the parts and puts it right back on as a zinc phos coating.

For improved adhesion on the zinc try adding a little fluoride 400-600 ppm to the phosphate bath. This gives a little bit of extra bite into the metal.

Hope this was helpful.

A. I suggest to identify the adhesion problem between phosphate coating and powder coating or between steel substrate and the phosphate coating.

If the former is a problem, I guess the phosphated sheets were contaminated with oil or dust in waiting room or the powder coating was improperly cured. It has to be noted that phosphate coating consists of inorganic salts, the thermal properties of which are definitely different from those of organic powder coatings.

If the latter is a problem, there is something wrong with the phosphating process, such as surface cleaning, pH value, temperature, agitation, and concentrations. Adding some wetting agents is helpful to improve the adhesion of phosphate coating on steel substrate.

A. Laurence:

I suspect that your experiment with the pH was thwarted by the Chromate. Here are my answers to your further questions. Yes you can remove the chromate in Nitric acid, but you'll have a bath full of zinc and chrome. If adhesion is acceptable to your customers a clean surface is all that you should need to paint over. Zinc phosphate is a misnomer. the crystal contains both iron and zinc. The ferrous phosphate basically becomes a sludge. Unknown on the chromate conversion coating.

I would review what adhesion requirements that the customers have and run sample lots to determine if the current system meets requirements and compare that to just an alkaline clean and rinse before painting.

Good Luck!

A. Hi Laurence,

It depends on how desperate you are. "Time saving" (abrading) and grit blasting have been used under circumstances such as yours.

Just trying to help.

A. Hi anil Tiwari
Please use synthetic detergents at 65° C to solve your problem; If it still persists, Use Chemical Oil removers that operate under ambient conditions for removal of Oil.

A. Hi, Jacques. A galvanize primer is essential, as is scrubbing well with trisodium phosphate ⇦ this on eBay or Amazon [affil links] before priming. Light sanding may help too. Alas, if it was galvanizing that was not intended to be painted, as discussed off-and-on above, really good adhesion may not be possible without phosphating in a shop.

Regards,

A. To be able to paint hot dip galvanized steel and to do it properly you need at minimum 5 steps.

1. Cleaning (Non Hydroxide Cleaner)
2. Grain Refiner/Rinse
3. Zinc Phosphate
4. Rinse
5. Sealer

Do not attempt to use Iron Phosphate unless you just want to use the fluoride (Assuming your Iron Phos Product contains Fluoride) for an etch. If you attempt to develop a crystalline structure using Iron Phosphate you will reapply zinc as a powder (akin to talcum powder). Adhesion will be worse than a simple etch. If you are going to do it, do it right.

A. I was able to troubleshoot this exact problem on a 5 Stage Iron Phosphate line.

The HDG parts had an inconsistent coating evident by different reactions to copper sulphate ⇦ this on eBay or Amazon [affil links] along the surface. They were also over 18 feet long, so a manual pre-clean or blast wasn't feasible. Discussions with the company doing the galvanizing didn't help.

The cleaner and phosphate stages had to remain compatible for all regular substrates, so the basic parameters such as pH, concentration, pressure, time etc. could not be permanently changed.

The easiest solution was a set of knockdown risers at the entrance using a 5 %(v/v) Sodium Hydroxide-based cleaner formulation. I believe that this is removing any oxide and/or passivated areas on the surface. The parts then run through the regular multi-metal cleaner (which has to be tightly monitored for free caustic drag-in). Adhesion is uniform and excellent.

We are now experimenting with ramping the regular Potassium Hydroxide-based cleaner with NaOH just before a run, and then buffering it with Phosphoric Acid after. The Phosphoric will produce trisodium phosphate ⇦ this on eBay or Amazon [affil links] , which is a half-decent multi-metal cleaner itself!

Hope this helps.

A. Hi, Debbie. If the paint stuck once, it should stick again. Use a primer made for galvanized surfaces on both of your doors. It will work fine on the front door, galvanized or not, and will work as well as possible on the garage door.

Try washing the garage door with vinegar ⇦in bulk on eBay or Amazon [affil links] , then rinse and prime it as soon as it dries.

Regards,

A. Hi Sarang. We revised your letter so as to not promote one brand over the other (which often leads to difficulties; this site is for technical discussions rather than testimonials anyway).

If you properly pretreat the fans, the adhesion will probably be fine with either. I suspect that the one with poor adhesion has a chromate treatment after the galvanizing, but I'm sure they'd appreciate learning that their fan is hard to powder coat while their competitor's is easy so they can fix the issue. Good luck. Interesting!

Regards,

"Phosphating of Metals"
by Werner Rausch

on AbeBooks

or eBay or

Amazon

(affil links)

A. Hi Harish. Yes, phosphating should be done before powder coating galvanized sheets -- just as it should be done before powder coating plain steel sheets. Please study "zinc phosphating" or zinc phosphatization".

Blisters in salt spray testing (or at other times) usually indicate poor adhesion because of the surface not being properly pretreated, i.e., not being clean, or lacking proper phosphatization. But we would need to know the details of your pretreatment process to know in more detail where the defect lies. Good luck.

Regards,

! Painting of galvanized steel is topic that comes up from time to time on this great site.

Typically the issue (and it would be the same for any other zinc-based coating such as zinc electroplate, zinc-rich coatings, zinc alloy coatings, mechanical zinc, etc. etc.) is that the zinc oxide on the surface prevents adhesion although sometimes surface contamination is the problem.

The SSPC and the American Galvanizers Association are having a webinar on November 13th at 1:30 pm EDT that those facing such problems may find useful.

https://register.gotowebinar.com/register/2607915527077126657

A. One thing that still puzzles me after 23 years of posting tens of thousands of Q&A's here is that nearly every zinc plater says the zinc must be chromate conversion coated or the paint won't stick ... and galvanizers says don't chromate quench the zinc or the paint won't stick :-)

Regards,

A. We chromate passivate all galvanizing and then powdercoat about a third of that galvanized. We use (currently) Cr6 (seeking a reliable Cr3). The PC pretreatment process (dip based) includes manganese phosphate. Adhesion on that doesn't seem a problem, at least testing shows it to not be.

A. My employer's facility does both electro galvanizing and coil coating. If the coil has a chrome conversion coating on it, whether it was applied in house or not, it cannot be painted on the coil coating line. The paint will not adhere.

When coils are to be painted on the coil coating line a treatment system must first be applied. On zinc based substrates we use zinc phosphate, or complex oxide depending on the end use. Both these systems use a chrome rinse prior to painting.

It's virtually the same chemical in both instances.