Pre-Galvanized vs Hot Dipped Galvanized vs Dura-Green

[Stringpark]

thanks peakbagger. Yeah, the IronRidge tool factors snow load and wind uplift based on location and the tilt of the array. As you mentioned, I'm sure its just a rough estimate, but their guess is better than mine.

[peakbagger]

All these design tools are going to have big factors of safety for the worst case conditions that they are designing to. The vast majority of the time failures are related to installation issues where short cuts were taken by the installer.

On industrial roofs I would have to meet with out underwriters rep and prove that the underlying decking had adequate pull out strength for fasteners along the edges of the roof where wind uplift was most significant. We had to use the same type of fastener as approved by the manufacturer. We would shoot in a fastener and then the rig would pull on it with the specified pull out load. If the fastener pulled early we had to come up with a different fastener or layout. In one case the decking looked perfect from down below but we couldn't get anywhere near the pull out load we needed. The solution was we would have to replace the deck but the facilities manager decided to skip the fix and wait a year and pay a couple of folks overtime to torch apply a new membrane over the failed roofing. It lifted up a couple of years later in a windstorm but the manager was long gone.

[emartin00]

What level of galvanizing are they using for the pre-galv material? It can range in thickness from G-30 to G-140 (ASTM A653). UL2703 requires a minimum of G-90 for any solar racking components, so I would make sure they meet that coating thickness.

[bcroe]

What level of galvanizing are they using for the pre-galv material? It can range in thickness from G-30 to G-140 (ASTM A653). UL2703 requires a minimum of G-90 for any solar racking components, so I would make sure they meet that coating thickness.

And is there any way to actually check it? Long ago printed circuits we ordered had choices of
plating levels. I designed in some patterns to allow actually measuring it in product, and
SURPRISE we were not getting what was specified. Bruce Roe

[J.P.M.]

And is there any way to actually check it? Long ago printed circuits we ordered had choices of
plating levels. I designed in some patterns to allow actually measuring it in product, and
SURPRISE we were not getting what was specified. Bruce Roe

Well yea - same thing metal fabricators do - buy it to spec and get a material cert. with the material. It'll cost you though. Q.C. costs $$.

[ICPete]

With pre-galvanized material (like schedule 40 pipe) the process is called "inline" and they use an air knife to remove the excess molten zinc and control the precise coating thickness. The value of G-60 is 0.60 oz/SF (that's total weight of zinc on both sides of the wall) and equals roughly 0.0005" thickness on each side. So G-90 comes to about 0.00075" thickness. I spoke to a local hot-dip galvanizing plant and learned that the usual ASTM A123 standard calls for finished zinc thickness that is a function of plate thickness, but for 1/4" stock it should come out around 0.003" thick, or about four times the thickness of G-90. I also learned that the cost of getting raw pipe galvanized to this standard would actually be roughly half the cost differential between buying plain iron pipe and the pre-galvanized pipe. The galvanizing process used by the local plant (and traditional around the world for hot-dip) is first an alkaline cleaning bath, then a pickling bath to remove light rust and mill scale, and finally the dip into the huge kettle of molten very pure zinc. Depending on the steel alloy involved, the zinc forms various thicknesses of intermetallic iron-zinc compounds, with the outermost layer usually very pure zinc. The website for the American Galvanizers Association has lots and lots of interesting information on the galvanizing process. It sure convinced me that this is how I want to protect my PV-mounting structure. For example, we're talking about >80 years lifetime in a rural/suburban environment, compared to maybe 20-30 years for the G-90.

I would also love to see the actual dipping process, but I suspect the plant won't allow visitors/customers anywhere near it, for safety reasons.

[bcroe]

I hate to get into painting and rust, and galvanizing is not compatible with drilling/cutting/welding in the field.
My method is 6061 aluminum with 18-8 SS bolts. I do not know the lifetime, but after 6 outdoor years here it
looks just like the day it was erected. Bruce Roe

[Mike90250]

..... I also learned that the cost of getting raw pipe galvanized to this standard would actually be roughly half the cost differential between buying plain iron pipe and the pre-galvanized pipe. The galvanizing process used by the local plant (and traditional around the world for hot-dip) is first .........

What country do you live in that is allowed to do hot dip for less than factory galvi ?

[ICPete]

Hi Mike,

USA. Not sure what you mean by "allowed". The pricing I've managed to find is simply quoted pricing that appears to be available to anyone.

My example was from Online Metals and Metal Depot. However a better comparison is DiscountSteel, which sells 3-inch sch 40 in both plain steel and galvanized steel. The former is roughly $200 per 21-ft length; the latter is roughly $300. The cost I was quoted for hot-dip galvanizing, for pieces over 50 lbs each, was $0.53 per pound. A 21-ft length of this pipe weighs about 160 lbs; so that would cost about $85 to galvanize. My original comparison, perhaps not as much "apples to apples", was that Metal Depot had the galvanized pipe for $400 per length, vs Online Metals had plain for $162. So it was that comparison that led me to be surprised and impressed that local hot-dip galvanizing could cost less than the difference for factory galvanizing.

I agree this seems odd, since the local hot dipping process is applying 4X the thickness of zinc.

I still need to find a local supplier of pipe to get a true comparison. The freight estimate for LTL from DiscountSteel to my location comes to roughly $60 per length (based on an order of 20 lengths).

I would also suggest that if I weren't also galvanizing a bunch of related parts, I probably wouldn't go to the extra trouble and transport of getting the raw pipes and sending them out, vs simply buying pre-galvanized in the first place.

Pete

[Bala]

The gal steel you buy is not a comparison for dipping, even more so now as quality is not what it once was. I have many exaples of it rusting around my house.

Best option is to buy plain steel, pefabricate It, then have it dipped.

if you want to paint it you will then need to have it wip blasted, this is a very light blast to allow paint to stick.

That will give you a very long life product but the cost may then be closer to alloy or stainless.

[ICPete]

OK I have another data point, info from a nearby wholesale supplier.
Imported black steel (3" Sch 40) at 5.15/ft vs imported galvanized at 6.90/ft; both based on 16 lengths of 21 feet each.
However he had NO data or info as to what the galvanized spec on his pipe actually is. I've seen information putting pre-galvanized pipe at anywhere from .00076" coating thickness to three times that, depending on the source. So it seems caveat emptor applies.
The salesman also pointed out that both his domestic and imported black steel pipe comes with a protective lacquer coating, so that would need to be removed before galvanizing.
So this cost differential of 1.75/ft makes more sense, and as Mike suspected, is less than half the cost of getting hot-dipping done after fabrication.
Pete

[Mike90250]

I didn't think the EPA/OHSA allowed any molten metal work in the states anymore. ohhh..... that's dangerous if you fall in.