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topic 27848

Compare galvanized to powder coat please



A discussion started in 2004 & continuing through 2017

(2004)

Q. On outdoor furnishings, e.g., handrails/guardrails, bike racks, park benches & playground equipment ... how is powder coat finished unit compared to galvanized in terms of rust and corrosion resistance?

Athena Dejarme
architect - San Diego, California, USA


(2004)

A. In brief, galvanizing (hot dipping the item in zinc) offers sacrificial protection. The zinc slowly corrodes, preferentially to the steel, thus protecting it. A good galvanized finish can last many decades of tough exposure. Because of the sacrificial protection, scratches in the coating are not catastrophic. Electrical towers in every corner of the world are almost always galvanized for this reason.

Powder coating is an organic topping that provides barrier-layer protection. As long as there is no mechanical abrasion of the finish, the powder coating can probably last as long as the galvanizing; but once the coating is damaged and breached and the steel exposed, that exposed steel will rapidly rust. Sticking with the same example, electrical towers are not regularly inspected and maintained, and a damaged area might rust to nothing if there was no sacrificial protection.

Again, this is a slight simplification because the powder coating can have a protective pretreatment that can slow the corrosion, but the general answer is that if no damage to the finish can occur, the corrosion resistance may be equal; whereas if mechanical damage can occur, the galvanizing will survive it far better. Similar questions have been asked and answered many times here, Athena, so if you search the site and read those earlier entries you can get some deeper answers. Good luck!

Ted Mooney, finishing.com Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey


March 14, 2011

Q. Trying to help a dairy farm customer (barns high in methane gas, and in a moist environment where water is sprayed on cattle for cooling effect. Which is the type of coating, powdercoat or galvanized is better for longer life in this type environment.

Henry Yonce
- Winter Haven, Florida

March 15, 2011

A. Hi, Henry.

Hopefully someone with actual working experience in these cow barns will chime in, because book knowledge sometimes doesn't anticipate the actual conditions well :-)

While the zinc galvanizing protects the steel, the zinc itself can rapidly corrode if the conditions aren't right. Galvanizing must "breathe" for proper corrosion resistance because what protects the zinc from rapidly corroding itself is tight, adherent, zinc carbonate reaction products rather than zinc oxide or hydroxide. And these carbonate reaction products don't form without good exposure to air (you could look into "wet storage stain" for further detail on this). My inexperienced guess is that powder coating would be better -- but a little knowledge is a dangerous thing. If you can afford it, the ultimate is a "duplex coating" (galvanized followed by powder coating).

Regards,

Ted Mooney, finishing.com
Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey


March 17, 2015

Q. I have purchased a galvanized steel hand railing to be installed on the steps up to the back door of my house. It will be subjected to the four seasons of Iowa weather for it's lifetime. My question is; in the interest of long term rust protection, will I gain anything from having this railing powder coated before I install it? Or will it stand up just fine as a galvanized steel railing? If not, are all powder coatings equal or do I need to find a specific type or place to have my railing coated? Thank you in advance for your reply!

Patrick Simon
- Waterloo, Iowa, USA


March 2015

A. Hi Patrick. If it's actually hot dip galvanized (do you see any spangle or drips to indicate it was dipped in molten zinc?), I think it will be fine for decades. Getting adhesion onto galvanized surfaces can be problematical so I wouldn't powder coat it if the appearance is acceptable to you.

Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey
Striving to live "Aloha"



Best finish for wheels -- galvanizing or powder coating?

October 11, 2016

Q. Hi. I could not find this on your site, but looking for expertise. I am trying to figure out if galvanized wheels or powder coated are better for trailer tires. It is not a boat trailer so they will not be submerged. It will be used year round in Ohio so road salt is a factor. It will be stored outside so all seasonal elements are factors.
I read the first question/answer in this thread discussing whether damage will occur to the finish, which could occur on wheels, so it sounds like galvanized is the best answer but want to know what you think.
Thanks!

Rick Johnson
- Dayton, Ohio, USA


October 2016

A. Hi Rick. It is very difficult for a consumer to determine from basic principles whether one finish will be better or worse than another. For example, there are few finishes that are as corrosion resistant as super high quality chrome plating, but few finishes as rust-prone as low quality chrome plating (which looks very similar).

With that qualification, I think you'll probably be better off with galvanizing. It's a sacrificial coating so, even if not done super well, you should still get good protection from it.

Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey
Striving to live "Aloha"


December 17, 2016

Q. I have a genuine Toyota hitch receiver for my Previa minivan. After 21 years, it has heavily rusted to the point where a local welding shop won't touch it for restoration. This hitch is no longer available from Toyota; however, I've found a used one on Craigslist with little/no corrosion for a very reasonable price.

My hitch was finished with some type of plastic barrier coating. Rust got underneath this, and then the finish made the rust worse by holding moisture in. I can easily flake big chunks of corroded steel with plastic coating still intact. These flakes are perhaps 1/8" thick, and they are heavy, indicating lots of metal in them. I don't know whether the finish was powder coat or something similar; or whether the finish was defective or not.

Assuming the Craigslist hitch was finished the same way as mine, I would like to strip and refinish it, because I don't trust the factory finish. I am considering hot dip galvanizing, powder coating, or both. The receiver installs by square steel tubes that telescope inside the van frame rails, and then bolt in place. So there may be some scratching during installation, but only once, and then the bolts fix the receiver and prevent further movement. Then there would be some scraping inside the receiver tube whenever the drawbar is installed or removed. But the receiver tube, of course, is open to fresh air and would not likely retain moisture. Except for the receiver tube itself, the entire receiver is hidden behind the rear bumper, so aesthetics isn't a consideration. If I finish it with HDG only, I might spray paint the receiver tube black, or I might just leave it.

What finish do you recommend for this?

Dan Joynton
n/a - Colorado Springs, Colorado, USA


December 2016

A. Hi Dan. Your minivan is unlikely to last another 21 years, so corrosion resistance of the receiver may not be a major issue this time. You'll probably find it much easier to locate a shop to powder coat it than to hot-dip galvanize it, and I think powder coating will be fine.

Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey
Striving to live "Aloha"



March 7, 2017

Q. Hi! Just wanted to ask a quick question, although I have to admit that your responses in the previous threads have more or less answered it.

I am a landscape architect and want to use steel angle edging to retain some paving. The side of the edging will be visible so I wanted to finish it with a colour. The edging I am thinking of using is galvanised steel and I was looking at powder coating it black.

Reading through the answers, you mentioned that getting adhesion onto galvanised surfaces might be problematical. Is there a process to make it less problematic or do you have another solution (another painting method perhaps?)

Thank you for your time.

Kind regards

Elizabeth Noltsi
- London, UK


Latex Self-Etching Primer

March 2017

A. Hi Elizabeth. A galvanizing shop can, immediately after galvanizing, apply a zinc phosphate treatment which will insure good adhesion. Unfortunately, it is generally not practical for an end user to buy galvanized material and then have it zinc phosphatized, so your options are probably limited to making sure it's clean (with TSP for example), wiping it with vinegar to acid activate it, rinsing it, and then applying a self-etching primer. Then you can paint or powder coat it any color you want with a good chance of proper adhesion.

Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey
Striving to live "Aloha"



October 24, 2017

Q. I have just had metal railings and gates fitted to my property , they have arrived prepainted black. I asked if they had been galvanized, unfortunately they have not; question is, can I galvanize over the paint?

Jim Duffy
- Scotland Glasgow


October 2017

A. Hi Jim. Galvanizing implies dipping raw steel into molten zinc for an alloying reaction. So it is not applicable to your railings and gates at this point.

I'm not completely sure what you are saying about them having arrived "prepainted black", i.e., are they supposedly completely finished or is this supposed to be just a prime coat?

My personal experience has been that a properly phosphated, then e-coated, then powder coated expanded steel patio set stayed completely rust free on my deck on a salt water lagoon for 15 years, then was submerged in Superstorm Sandy flooding, and although now slightly rusted is still is service 20 years later. So lack of galvanizing does not necessarily sentence it to an early death.

Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey
Striving to live "Aloha"



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