Powder Coating plus Galvanizing Problems and Their Solutions

A. Hi, Mark. We linked your mention of BS 6497 to a source for that one. Regarding AS/NZS 4506, Standards New Zealand warns that "This document has been withdrawn without replacement" ... so hopefully your customer will accept the current spec and forget the withdrawn one :-)

Regards,

I am not sure if you are referring to a powder primer or a liquid paint. Before attempting a two-coat process, you might switch to a TGIC Polyester coating, and see if that eliminates the problem. TGIC coatings allow the gasses to pass through the liquid coating much better than Urethanes (during the cure process).

What chemistry are you using?

A. Sorry for the delay in getting back to you - I was on Holiday for a few weeks.

There are three types of powder coatings that do well in exterior use: TGIC Polyester, Polyester Urethane and Acrylic.

Aluminum wheel manufacturers apply a base coat of TGIC polyester powder, machine the wheels, and then apply an acrylic top coat to provide gloss and to protect the exposed aluminum. Acrylic powders contaminate the polyester products, and cannot be sprayed in the same booth. It has been my experience that the ventilation must be such that the acrylic cannot drift over to the TGIC booth in the air.

The TGIC Polyester base coat may allow the outgassing from the galvanize to occur through the powder as it liquefies, and prevent a problem with subsequent coatings.

You should perhaps discuss this application with a powder coating manufacturer to discover what kinds of problems might occur, and get some kind of commitment from them in terms of performance.

Automotive manufacturers are using powder coatings over frames, engine blocks and other non-exposed parts, but not so much on the body of the vehicle.

My experience has been that for a decorative finish you would be much better off using a liquid primer followed by a catalyzed liquid acrylic or urethane top coat.

Is your reason for using powder coating environmental?

A. Hi Scott,

You are getting good advice from your galvanizer. If your products are going to be offered nationally, then they will see many environments that are more corrosive than the climate in Phoenix.

Zinc phosphating is the pretreatment of choice for galvanized surfaces, but you might be successful with an iron phosphate system that utilizes a high-performing final rinse. If you are using a high-quality powder coating, all the better.

A. Hi Scott. George is right. You really should have a phosphatizing pretreatment whether or not the metal is galvanized. It's important for a lot of reasons . . . it's just that its importance is most quickly perceived on galvanized work because it also improves the paint adhesion.

As for the environmental impact of proper finishing, please consider the billions of dollars of annual corrosion, the countless acres wasted to junk yards and ugly scrap piles, and the continuous and unsustainable waste in re-mining, re-smelting, re-manufacturing, re-packaging, re-transporting, re-selling, and then landfilling millions of tons of metal items to replace improperly finished junk that only lasts a couple of years when it would have easily lasted decades if finished properly.

Phosphates are environmental round-off error! Don't ever let anyone talk you into the penny-wise & pound-ludicrous proposition that easily avoidable early corrosion and unsustainable constant replacement is environmentally preferable to the tiny environmental footprint of proper metal finishing. It is a disgrace when environmental groups focus on the ounce of phosphate while ignoring the tons of steel it saves.

Regards,

A. Hi Phillipe. I can see in the background of that photo something else: galvanized guardrails. Certainly your galvanized plus powder coated fencing should be expected to be in in at least as good shape as guardrails they were only galvanized. It sure looks like that fencing is utterly unsatisfactory. Additionally, either the fencing is hot dip galvanized or it's not (I suspect not), and the manufacturer certainly ought to know what they are selling.

Regards,

A. I believe the devil is in the detail.
"Hot Spray Galvanising"?
I strongly suspect that the structure has not been Hot Dip Galvanised at all.
I can imagine that it has been given a thermal spray.
This is totally inappropriate for a tubular structure.
Obviously, the thermally sprayed Zn has no chance of entering the hollows.
That accounts for the red rust forming from the apertures into the hollows.
Less easily explained is the red rust forming on the vertical external members.
Perhaps these were undersprayed with the Zn coating?
Whoever did this job can only be described as a rank amateur.
In OZ we have a Fair Trading Department of Govt.
Any Building product supplied is deemed to have a 7 year minimum implicit warranty.
This is regardless of any other condition of contract.
I suggest that the goods you have are not of merchantable quality.
I hope your supplier, or his replacement, learns from this very obvious mistake.
Hope this helps,
Regards,
Bill

July 2014 Wow, I wish our corporate-owned government would pass laws like that here, Bill :-) Regards, Ted Mooney, P.E. Striving to live Aloha finishing.com - Pine Beach, New Jersey A. Yes, We, in the antipodes, appear to have more respect for the consumer than in other environs. Regards, Bill William Doherty Trainer - Newcastle, NSW Australia July 28, 2014

Many thanks for all your opinions; these have been taken on board.
Once again many thanks.

A. This steelwork does not give any indication of having been galvanized.
With powder coated galvanized steel you don't get red rust. You might (in a poor powder coating job) get white rust, but not red.

It's not easy to tell if there's galvanizing under powder coating to an inexperienced eye, so would be easy to sell the job as if it had been galvanized.

A. These pieces seem to be NOT hot dip galvanized, I can't see any drainage and ventilation holes on the vertical pipes.

Gentlemen, thank you for your constructive answers -- they are much appreciated. With all your doubts after some considerable time, the manufacturers sent a representative who agreed that the work was not of "satisfactory quality"; they have promised a new gate within 5 weeks. So once again thank you very much.