Powder Coating vs. Galvanizing

some of these hits about rebar and salt water may have information you can use.

http://www.google.com/search?hl=en&q=%22salt+water%22+%2Brebar&btnG=Google+Search

ditto galvanizing

http://www.google.com/search?hl=en&q=%22salt+water%22+%2Bgalvanizing&btnG=Search

Powder coating works by making an insulator between anode and cathode; The zinc acts as a sacrificial element in the corrosion cell. Powder coating is easily removed by mechanical abrasion and seems to be easily penetrated by water in my experience.

Production finishing magazine has an excellent tutorial on powder coating and corrosion here:http://www.pfonline.com/articles/010002.html

Personally, I've never seen a powder coating standup to much weather let alone concrete chemical activity, but I live with the North American Midwest environment and climate.

However, hot dip galvanize weathers much longer where I live and is standard on many automotive, trailer, and outdoor exposed applications ( civil engineering guard rails for instance.)

My experience, not scientific analysis, would vote for the galvanize every time.

Brief personal aside/rant:

To me, Powder coating is a Marketing term used to sell inferior paints that pass the 'eco freako ' earth friendly test, compared to the real paints that were made up using a n oil based liquid vehicle and real pigment systems.

But I'm a crusty old geezer sometimes. (I painted a pick up truck with oil based ship paint with bis tributyl tin and some other good stuff- using a 4" brush. I never had a spot of rust through that paint.- Powder coat can't last one winter on my back deck on ornamental trellises or furniture let alone exposed to salty roads or corrosive affects of contact with wet concrete.)

Progress my @$$!

milo

well, there are some very good powder coatings that are sprayed onto a hot charged metal and they are attracyed, melt and cure to a very good protective coating. They can also be quite thick.

That said, galvanizing is the gold standard for protecting steel, however it too can be abraded and if the scratches are deep enough the protective action is lost.

You can also use zinc rich silicone paint. This is about 90% zinc dust by weight and as it cures the zince dust touches and gives you the effect of galvanizing to a lesser degree. the advantage is it can be applied to sand blasted surfaces with zero rust /water trapped beneath.

I see that places with steel bridges near salt sand blast them over and over again every year and recoat them, so there will not be a permanent zero maintenence solution. You will have to insopect in detail very often.

For pilings, they often use a hydraulic grade of concrete that is rated foe salt water and they also use rebar that is galvanized and over painted.

http://www.google.com/search?hl=en&q=galvanized+%2Brebar&btnG=Google+Search

Hi,

That begs the question, can zinc rich silicone be powder coated? The main point of the question is that we have ready access to powder coating but galvanising would be more complex logistically.

if the structure can be maintained easily, then a zinc rich coating is fine. If an annual inspection shows a failure point, blast it to bare metal and recoat. Most steel can be purchased pre-galvanized and you only need to worry about cuts/welds/rivets, which can be protected with a zinc rich coating.

I just looked up this.

http://www.google.com/search?hl=en&q=%22powder+coating%22+%2B%22zinc+rich%22&btnG=Google+Search

a number of leads to follow there

I tend to agree with you Milo, just through my experiences hot galv dip is superior to powder coat. I do have one concern, "close proximity to sea water"......what other materials may be connected to it (galvanic action), where the zinc could become the sacrificial anode. I don't see any problems in "desert" areas.......unless frequent flooding occurs.

Powder coating does not use zinc that I am aware of. Powder coating uses plastics and dry paints.

When zinc is applied to steel (by any means it is galvanized) either by hot-dip or elctro-chemical means (sacrificial anode).

There is also electrolysis technologies and sputter as well.

I believe the current in-situ preference for the most aggressive corrosive resistance is a combination of sacrificial anode and sputter technologies. (?) In this method (from memory) The base metal is charged to cathode and a corrosive resisant material such as zinc is 'sputtered' molten over surface and charged as anode. Or something to that effect. Again memory.

My point is, and I am often wrong, that powder coating is not a suitable means or term for the application of zinc to a base metal.

I have been out of industry a while so I'll have to dig around a bit, but that's how I remember it today.

cr3

There is certainly a time and place where powder coating can be quite effective and weather resistant. For instance, all of those dark blue US Postal Service mail boxes on street corners are powder coated, including those in the snowy and road-salty mid-west. And I have specified powder coats that were quite thick and rugged on many products. But one of the major appeals of powder coating is the unique textures that can be achieved, and how nice it can look.

In the end tough, for sheer protection ... ignoring aesthetic issues... yes I will also agree that galvanizing is the way to go.

Thanks for sharing your experience OOBE.

milo

Hot dip zinc is the only practical 'lifetime' anti-corrosion protection for framing member in contact with concrete and subject to salt-air environment. Epoxy coated re-bar in the concrete is strongly recommended as well. 'Cemented in-situ' steel piers IS NOT recommended. Cast in-place anchor pads with bolted connections are the standard. This is so bending loads will not break the concrete surrounding the 'cast-in-place' pier but will be distributed to the anchor pad.

Keith, the photo on the right shows what can happen in reinforced concrete due to corrosion. We can prevent this to a large degree by wiring or having the reo sitting on zinc blocks. Now the question that was asked related to salt water not salt air ....... there is a big difference as to the corrosion potential and it has been my experience that hot galv dip is far from "lifetime anti-corrosion protection"

One of the things that I used to use in any low pressure sea water system where zinc enriched corrosion pieces (same diameter as pipe system flanged and about 12-18'' in length). The flanges were bolted together with brass nuts and bolts with the exception of 1 nut and bolt each end, these being either mild or galv steel. When it was time to renew the corrosion piece one would just chisel the galv nut off and all the brass nuts and bolts were easily undone. On an average they used to last about 12-18 months.

Milo mentions the most important aspects of corrosion prevention comparisons, anodic vs. cathodic protection. This important protection mechanism difference shouldn't be minimized when very-humid, wet or salt-water conditions can exist. Plus, galvanizing isn't typically undermined or compromised by mechanical impact or abrasion like powders can be.

For an outdoor structural application like this one, galvanizing is truly superior to powders. But if galvanizing is too difficult for you to obtain, is it essential for this application?

Powder coatings were initially developed as more environmentally-friendly and more economical-to-apply organic coatings for high-production applications than "wet" coatings. Powder coatings have delivered on those promises, but they tend to favor "decorative" rather than "functional" applications. That still seems to be the case although there are some higher-end polyesters, polypropylenes and polyethylenes with decent performance.

At some point cost becomes an issue for powders when you're looking for the kind of performance that your application requires. I certainly cannot comment on the cost of galvanizing in your part of the world, but in the U.S. it is still relatively economical to obtain.

I would not advise you to pursue powders much further. Others here mentioned zinc-rich materials which would be good possibilities. However, I don't think they can be powder-applied just yet.

IMHO, galvanizing is still the King here.

If you cannot galvanize due to size limitations, try metallizing (Thermal Spray Metal Coating). Pure Zinc can easily be appplied in situ, is a thicker coating than galvanizing (galv ~3 mils v 10 mils for TSMC). When properly applied, a Metalized coating has much higher adhesion than galv, is flexible with minimal porosity. Many bridges are zinc coated, and they show resistance to road salting during winter. Concrete structures (including sea water service) with spalling damage have been repaired /protected with zinc sprayed over concrete. It is expensive, but provides higher corrosion and abrasion protection than either of the powder or galvanizing methods.

Thnaks to all respondents here, plenty to ponder and discuss at our factory meeting today!