Issues Powder Coating Galvanized Steel

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A. Hi Rudresh. Although there are several possible causes for your problem and we have no data from you yet, everywhere I go I see paint and powder coating peeling off of galvanized surfaces because the galvanized surface has not been properly pretreated.

Galvanized surfaces require a full zinc phosphatization pretreatment, and unfortunately I don't know what you mean by "GG chemical" ... but I suspect that your pretreatment is not correct for galvanized surfaces. Please describe the steps in your pretreatment process.

If you are confident that your pretreatment system is robust, there are other thoughts on this page, regarding whether the surface was specified as not chem-treated (chromated), properly de-gassed, etc. Thanks.

Luck & Regards,

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


(Free!)

A. You can powder coat galvanized steel just fine, just make sure you don't get passivated galvanized. There is a little more involved with treating galvanized, but with the right process it can be successfully done.

Just remember, Iron Phosphate will not form in the areas where there is no iron. What you get will be a clean part, a little etch (micro) and possibly some small amount of phosphates of the coating metals.

An adhesion promoting final rinse can also be a huge benefit if you have durability standards to meet.

A. Hi,

I am uncertain as to the effect of bismuth on the problem, but here are a few thoughts:

1. De-gas before coating 30 mins + at 250 °C

2. Use a anti-gassing powder which remains liquid longer and allows the gas bubbles to pass thru the wet film.

3. Try sweep blasting or acid etching.

A. Hi,

Normal builds of powder are just fine, i.e., 60-90 microns. More to the point is pretreatment, as if not correct the powder will delaminate away from weak areas (edges, holes, damage) on extended exterior exposure. If you are looking for longterm performance consult BS 6497, powder coating of architectural galvanised steel.

A. Jagathees,

You already tried to eliminate the first two possible cause that come into mind when having these kind of problems. Obviously they didn't work out.

Some more suggestions:

- is the powder suited for application on hot dip zinc? Your powder supplier should be able to tell you.

- what about the thickness of the zinc-layer? I've experienced problems with zinc films that where too thick as result of using steel that was not suitable for HDG.

Good luck

A. Dear Sirs,

Your problem is quite a common one - small blister on the surface of the cured powder. This is due to entrapped air in the substrate that expands on heating and creates the defects as it tries to escape during the baking process.

Answer: Ask your powder supplier to give you a sample of an "anti-gassing powder". This is a standard powder to which an additive is post added. This additive slows down the cure thus allowing the air to escape prior to the fuse phase of the bake.

Hope this helps -- if not: heat the items and before the items cools apply a thin coat of a solvent-based epoxy clear coat -- as the item cools the substrate will draw in the epoxy resin and seal up the capillaries. Now you should be able to powder coat without any defects.

A. Dear Jagathees Chandran

First why are you curing the tank 50 min at 200 °C -- did your supplier data sheet say that ? I think the time is more so the galvanized coat evaporated some gas dust to the zinc brightener which may make this problem so you must checked your powder supplier . and you can make a steel sample and if the problem disappears, check your galvanized steel if the problem persists still you must tell you powder supplier to give you a special powder for coating galvanized steel (anti-gassing) thanks.

A. Hi Mark from a fellow Brummy,

Although you do not state if the problem is confined to the weld mark area or a general overall problem but I will try to answer your 3 Q's. Yes to all three. Temperature has a great deal to do with adhesion. Failure to bake for the recommended time and temperature is one cause. Porosity of the galv will cause defects such as pin-holing.
A waterborne primer or etch primer (wash primer) will overcome all your problems, although I can not see how you would confine it's application to one particular area i.e. the weld. Coating the complete tube would be most beneficial.

A. Sorry, Amrish, the answer is "c". :-)

c). MS Sheet - Pre Treatment - Galvanizing - Pre Treatment - Powder Coating

Proper cleaning, acid activation, and fluxing must precede galvanizing, but phosphatizing or equivalent pretreatment must precede powder coating as well. Good luck.

? What is "electro-galvanized"? Either its galvanized or electroplated.

A. Both zinc coatings require pretreatment, the same treatment.
Adhesion to zinc surfaces is the issue, not how the zinc got there.

Pretreatment - possible 4-8 stages including either chromate or zinc phosphate.

It's not a phrase that you or I like, Geoff, but it's out there. "Electrogalvanized" means zinc electroplated.

A. Zinc electroplating (electrogalvanizing) virtually always includes chromate conversion coating, Amrish. So if you powder coat it while it's still fresh, nothing should be required but mild cleaning.

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


(Free!)

A. Hi Ashoka. Geoff's advice is surely good because he does it. But if you want further info, the galvanizing associations call this "duplex coating" and have white papers of suggestions.

Regards,

A. Hi Mangesh. You require a proper zinc phosphate process on the sheet metal. This is often called a 7-step process, but it's debatable exactly what those seven steps are, so it's probably possible that satisfactory pretreatment can be done in 5 steps, to wit: cleaning, rinsing, zinc phosphatizing, rinsing, passivation dip.

Regards,

A. It's water in the porosity of the galvanizing.
Do better drying.

A. Hi Rethish. I think zinc phosphatizing is the most reasonable pretreatment. Please search the site for "zinc phosphating", for a good introduction to the steps and to ask further questions about those steps.

Regards,

A. Hi Mohammed. Were the tubes zinc phosphatized before powder coating. Did you determine that the tubes were not chem treated?

If you are not involved in specifying the tubes and have no control over it, and you are just trying to paint on site rather than market the tubes, the best you can do is probably to lightly sand them and pretreat via a self-etching primer before powder coating. If you are marketing the tubes they must not be chem treated, but zinc phosphatized before powder coating. Good luck.

Regards,

A. It is very common to peel off powder coating from the galvanized sheet or surface. We call it impact failure or impact test failure. This is because the galvanized surface are non-porous and due to that powder coating powder does not go deeper inside the galvanic surface which causes weak intersection polymerization; due to that powder coating on galvanic surface become prone to peel off with small value to impact.