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Adhesion problems on hot dip galvanized articles
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I run a powder coating business in which we coat various types of substrates. Our prep systems consists of an alkaline cleaner, rinse, sulphuric acid pickling, rinse, iron phosphate, rinse and a final acidified rinse (chromated). On the odd occasion we need to powder coat hot-dip galvanized articles, but for some reason the adhesion on these articles are very poor. The articles are checked for water breaks after the alkaline cleaning stage, rinsed for a minute or two and the left in the phosphate solution for 7 to 10 minutes. Thereafter rinsed in water and passivated before powder coated.
- Am I using the correct prep system (iron or zinc phosphating?) or is this a common problem?
- What is the chemistry behind an iron phosphate on a zinc surface?
- Do I need to phosphate the articles at all or can I just coat them?
Thank you for your response.
Laurence van Niekerk|
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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 http://www.galvanizeit.org/publications/publications.htm, and there is also a book on the subject titled Duplex Systems: Hot-dip Galvanizing plus Painting [link is to info about book on Amazon]. Good luck.
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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.
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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.
Ling Hao- Grand Rapids, Michigan
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Craig / Ted/ et al Thanks a million for your responses. Firstly I reduced the bath pH to 3.5 and observed a dull gray colour on the galvanized surface. (Would assume this must be the zinc phosphate or an etched surface). Thereafter I increased the pH to above 4.5 to evaluate adhesion properties. Unfortunately I could detect no improvement with either procedure. Ted, I have applied for a copy of the AGA booklet, but as yet have not received anything. Unfortunately I can not close shop until I receive this booklet, so I must persist in my questions. I contacted the supplier of the hot dip galvanized items and he confirmed that the items were chromated. It is however difficult to control this aspect of incoming work as we have various customers who require powder coating of galvanized items.
More questions:
Thank you |
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!
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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.
R.Sivakumar--
I have problems in degreasing the galvanised sections which I'm having even after degreasing them the " water break " test fails can you give me a good reason why this happens when the degreasing pointage is set correctly there is no oil on the bath the dip timing was also extended
Thanks
ANIL TIWARI- PUNE ( Maharashtra ) INDIA
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Can you tell me the proper procedure for painting a galvanize garage door. I painted it last summer and its all peeling. It was primed with Tremclad galvanize primer. Hope you can help me. Thank You. Jacques PerrierHi, Jacques. A galvanize primer is essential, as is scrubbing well with trisodium phosphate [linked by editor to product info at Amazon] 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. Regards,
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Our company ran into a problem using galvanized, chem-treated sheet steel to make billboard panels. The standard posting paste is water-based and apparently the chem-treatment prevented adhesion. Is there anyway to remove this effect in the stock we have remaining?
Jesse Robertson.
To be able to paint hot dip galvanized steel and to do it properly you need at minimum 5 steps.
- Cleaning (Non Hydroxide Cleaner)
- Grain Refiner/Rinse
- Zinc Phosphate
- Rinse
- 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.
Tom Roberts- Tucson, Arizona
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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 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 passified 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 trisodiumphosphate, which is a half-decent multi-metal cleaner itself!
Hope this helps.
Steve Cox- Toronto, ON, Canada
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August 24, 2009
My name is Debbie. I have two issues at hand. The first issue is that I need to know how to tell if my front entry door to my home is made of regular steel or galvanized steel. My second issue is if it is galvanized steel, how do I paint it so that the paint sticks. homeowner - Bowling Green, Ohio August 25, 2009 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, then rinse and prime it as soon as it dries. Regards,
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