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Galvanizing 101: galvanized vs. hot-dipped galvanized

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What is the difference between "galvanized" and "hot-dipped galvanized", if any. Is there a standard.

Bill G deleted
hobbyist - Jacksonville, Florida


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Words are rarely exact, Bill, they keep morphing, but these two terms mean the same thing to me and most finishing professionals. There are a number of standards for galvanizing but I don't have a good list offhand. If you search our site with the words galvanize and standard you'll find some good references; and maybe try G60 and G90 too.

Please try to cast your question with the situation that motivates it, and we can probably offer further help; because in some contexts (like when trying to sell an item by making it sound better than it is), people may use the word electrogalvanized when the article is actually only electroplated with zinc. From there, they may then think it's legitimate to shorten it to galvanized -- and before you know it they're selling zinc electroplated finishes (which are much thinner and less corrosion resistant than galvanized) and calling them galvanized :-(

pic of Ted Mooney Teds signature
Ted Mooney, P.E. RET
finishing.com
Brick, New Jersey

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There IS a difference between galvanized and hot dipped galvanized objects, such as nails. I can come to this conclusion because the new standard for pressure treated lumber, ACQ states that "The chemicals used in ACQ will corrode ordinary galvanized fasteners, therefore special consideration must be taken when working with ACQ lumber. Hot dipped or stainless steel fasteners MUST be used with ACQ." I've heard of the phrase "electro-galvanized", I wonder if that has to do with the difference between the two.

Victor Dauragon
- Wapato, Washington


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Thanks, Victor. Words like "galvanized" can be imprecise except in a specific context -- which is why I asked Bill G to clarify his situation. But at least nine out of ten professionals would take 'galvanized' to mean 'hot dip galvanized' rather than zinc electroplated ('electro-galvanized') -- that is a term which most professionals simply never use. So ACQ really should do a re-write after consulting with metal finishing professionals :-)

There is no need for the term "electro-galvanized" because it means precisely the same thing as "zinc electroplated" -- a term which has been universally used, understood, and accepted for over a hundred years. So we have to suspect that the term "electro-galvanized" was coined by a salesman so that a company which was providing the far less expensive and less durable zinc electroplating could ride the coat tails of the premium finish through obfuscation :-)

pic of Ted Mooney Teds signature
Ted Mooney, P.E. RET
finishing.com
Brick, New Jersey


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I work for a large general contractor and we're building a hospital which has a spec that all outlet boxes in the wall are to be hot dipped galvanized or sherardized. The electrical contractor is having a fit over this since all of the threads will be destroyed in the box. The boxes are manufactured zinc plated galvanized. Is there a difference? Should we go ahead and have the boxes hot dipped or convince the architect that they are good the way they are.

Simon Peters
- Panorama City, California USA


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Hello, Peter. Because it's a hospital, where critical electronic life-support equipment will be used, my suspicion is that the architect is concerned about zinc whiskers -- a stress phenomenon, to which zinc electroplated surfaces are definitely subject and galvanized surfaces are believed to be much less so.

Whether it's over-reaction on his part is a matter of opinion, but I believe the specification may have been intentional not irrational. Yes, threads are cut differently to allow for the thickness of galvanizing. But it may be okay to galvanize the boxes, then chase the threads, even if this removes all of the zinc on the threads. Good luck.

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Ted Mooney, P.E. RET
finishing.com
Brick, New Jersey


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I went through the hot dipped vs. zinc electroplating debate for pullboxes at LAX. What I found was that if you were to take a sheet metal 4S, 5S or pullbox and have it hot dipped, it not only would warp and loose it's thinner features (such as threads) it also would no longer be a UL approved device.

Hot dipped specifications seem to appear on electrical materials mainly because of outdated specifications. Clean air regulations have forced manufacturers to adopt new processes, and construction specifications have not kept up with these requirements.

Scott Klosterman
electrical contractor - LA, California


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Thanks, Scott. You may be correct that hot dipping thinwall devices can warp them -- every finish has advantages and disadvantages. Acres of computer room flooring has been removed and thrown away because it was zinc electroplated and therefore susceptible to zinc whiskers, which short out computers. It has been replaced with hot dip galvanized flooring at enormous rework cost because most people believe that hot dip galvanized finishes are not susceptible to zinc whiskers. People have died in hospitals when apnea monitors have shorted out due to zinc whiskers from zinc plated enclosures.

So, while I agree that some specs which call for hot dipping may be archaic, some are deliberate and based on much more current thinking that you may realize. It is crucial that if we feel a spec is wrong we must alert the designer and get the spec changed, not unilaterally make a substitution -- because we simply don't know what issues the designer was planning for.

Hot dip galvanizing definitely changes thread dimensions, but it is common to cut threads to allow for that dimensional change. Countless hot dipped nuts and bolts are in place on electrical transmission towers, bridges, and other installations; it is not an outmoded finish, and it offers tremendous corrosion resistance. Good luck in working it out, and thanks again.

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Ted Mooney, P.E. RET
finishing.com
Brick, New Jersey


August 13, 2008appended

Hi everyone,

I believe that the thickness of the zinc coating for electro-galvanizing is far more controllable than the hot-dipped. Can anyone please give me an advise with regards to which is better in resistance to corrosion.

I am hoping to get comments and reply from you guys.

Thanks and Best Regards.

Darryl Ablaza
project engineer - Singapore


August 14, 2008

Hi, Darryl. As you can see, instead of starting a duplicate thread, we pasted your inquiry onto a thread that already partially answers it. Yes, electro-galvanizing is more controllable, but the thickness is far less and the corrosion resistance is therefore far less.

Regards,

pic of Ted Mooney Teds signature
Ted Mooney, P.E. RET
finishing.com
Brick, New Jersey


August 16, 2008appended

What's the difference between "galvanized" and "hot dip zinc" coated?
There is a difference or is it that the procedure of making a hot dip zinc with the process gives the result "galvanized coated"?

steve dada
global parts group - lebanon


May 8, 2010

Hi. What is the difference between galvanized steel and hot dip galvanized; which one is best for kitchen project (ladder).

ramesh kumar
- Qatar


May 2010

Hi, Steve. I do not believe there is any difference between the phrases "galvanized" and "hot dip zinc coated".

Hi, Ramesh. A kitchen is not a highly corrosive environment, so I don't think you need a super corrosion-resistant finish; any zinc coating procedure may work fine. To me, "galvanized" means "hot dip galvanized" with no distinction between them. But I cannot stop other people from using words in the fashion they use them, even if I consider it a sloppy fashion. Some other people have apparently started using the word galvanized to also refer to the much cheaper and much less corrosion resistant zinc electroplating. When people use words that way, you have no choice but to ask them what THEY mean when they use the term. Good luck.

Regards,

pic of Ted Mooney Teds signature
Ted Mooney, P.E. RET
finishing.com
Brick, New Jersey


August 11, 2011

thumbsup2Thanks for this thread. It became clear to me the difference between Hot-dipped and Electro-galvanized. The other term for hot-dipped w/c I confused is galvanized.

Edwin N. Cabilan
- Cavite, Philippines


December 5, 2011

Hello and this is an interesting thread. I have recently completed some salt bath testing on pregalvanised product vs Hot dipped galvanised with results that seem to go against your thoughts. The hot dipped galv product failed both the scribed and unscribed tests at the 672 hour mark whereas the pregalved product showed no corrosion on the unscribed test at 1008 hours and less than 1 mm crawl on the scribed test at 1008. We had 3 hot dipped pieces and 2 pregalvanised pieces. I hear all the time that hot dipped is better and I am confused to know why? It seems that once any weakness is found in the hot dipped product the crawl between the hot dipped layer and the product it is supposed to protect becomes the weakness which is not found on Pre Galvanised product?

Michael Backhurst
- Hamilton, New Zealand


December 6, 2011

Thanks, Michael. Actually I thought it was a rather dull thread that only became interesting when you posted findings that do not match conventional thought on the subject :-)

But actually, there are several things that have to be said about your results.

First, salt spray testing is a QA measure to be employed to make sure that an established process has not gone south. It is NOT an appropriate way to gauge the corrosion resistance of one finish vs. another. Let me explain that rather than just claiming it --

Although galvanizing protects steel cathodically, what protects the zinc? It protects itself with a coating of its own corrosion products. If it did not protect itself, it would just quickly react with the environment and dissolve away like uncoated steel does, or like zinc does when put into hydrochloric acid. Indeed, if galvanized sheets are left tightly stacked in a moist environment, they do corrode that way, they get "wet storage stain", i.e., the zinc corrodes into voluminous powdery or crusty white patches. But that doesn't happen in actual service because in service the zinc slowly reacts with carbon dioxide in the air to produce tightly adhered, glassy feeling, zinc carbonate reaction products which are very effective in sealing the virgin zinc away from the air and slowing corrosion to such a crawl that galvanized structures can last 75 years and more.

But what happens in a salt-fog chamber? You are emulating the conditions for wet storage stain, not real life. The carbonate reaction products never form and the zinc just rapidly corrodes away into loose, non-protective, powdery corrosion products.

The second problem here is that proper pretreatment is a necessary part of a coating system. The pre-galv apparently received an adequate chromate pre-treatment. A chromate dip is not an appropriate pre-paint treatment for hot dip galvanized surfaces though; they require phosphatization.

Practically speaking, your results seem to indicate that the pre-galv + chromate + paint will probably be a robust and affordable system. But they don't actually indicate that this system is superior to hot-dip galvanizing alone, let alone a proper duplex system of galvanizing + pretreatment + painting. Thanks for the interesting posting.

Regards,

pic of Ted Mooney Teds signature
Ted Mooney, P.E. RET
finishing.com
Brick, New Jersey


December 6, 2011

thumbsup2Hello Ted and thank you for taking the time to review my question and posting an informative answer for me.

Being new to the steel industry and more specifically Steel Tubular fencing, I am questioning "why" a lot of the time to understand our own product and how it compares to other similar products for both strength and weaknesses.

This particular question came up when I lost a tender based on a supposed fact that hot dipped Galv will always outperform pre Galv'd product. In fact the unflattering comment that our product was not fit for purpose in a marine environment was made. The marine environment was 500 mtrs from a beach. I expected at the least to be comparable to Hot dipped at that stage.
I wanted a clarification on where this "fact" came from and secondly why this would have been the case?

Upon contacting our coatings partner, I was offered an opportunity to submit some product for testing and to get some "facts" which I have sent to you. I was also offered some additional coating methods that would further enhance the life time of our product by giving a more durable coat than we normally use.

After reading your reply it appears I also made a mistake in thinking the process used for the Hot Dipped Galvanised product in the testing was an industry standard and we were comparing apples with apples. I will also now undertake the testing on the products without powder coating to see what difference this makes.

Your answer while more than satisfactory to me now leaves myself other questions to ponder and uncover. Again thank you in taking the time to reply.

Regards

Michael Backhurst
- Hamilton, New Zealand


December 7, 2011

Hi, Michael.

Finishing.com recently completed a project for the IZA wherein we tested a number of different finishes for zinc diecastings; we used the CAMRI test rather than a salt spray test to evaluate them. You may wish to google that term as it may prove to both be a better indicator of corrosion resistance, as well as being very easy and economical to perform in-house.

Regards,

pic of Ted Mooney Teds signature
Ted Mooney, P.E. RET
finishing.com
Brick, New Jersey


December 8, 2011

As Ted has explained, salt spray testing is not a way to predict the life of galvanized products, hot dip or pregalv.
But actual real life testing in outdoor conditions has been done in the UK and this shows the life expectancy to be directly proportional to the thickness of the zinc. The main difference between pregalv (such as you would get that had ultimately come from the likes of Bluescope in Waiuku, NZ), and post fabrication hot dip galvanized is thickness of coating.
Typically pregalv at 20-25 microns and hot dip at 3-4 times that. So life expectancy is 3-4 times in hot dip than in pregalv.

Geoff Crowley
galvanizing &
   powder coating shop
Glasgow, Scotland


December 12, 2011

Taken from wikipedia on Salt Spray Test :

Hot-dip galvanized surfaces are not generally tested in a salt spray test (see ISO 1461 or ISO 10684). Hot-dip galvanizing produces zinc carbonates when exposed to a natural environment, thus protecting the coating metal and reducing the corrosion rate. The zinc carbonates are not produced when a hot-dip galvanized specimen is exposed to a salt spray fog, therefore this testing method does not give an accurate measurement of corrosion protection. ISO 9223 gives the guidelines for proper measurement of corrosion resistance for hot-dip galvanized specimens.

CS Leong
- Selangor, Malaysia


December 14, 2011

Hi there,
Currently, my company is doing this steel gratings project and client wants us to galvanize the gratings.

The gratings are expected to have high strength property as lorries will always go above them and have high corrosion resistance since the client's place is very near to beach.

Should I go for hot-dip process or electro-galvanized is enough?

thanks

Atrash Ghani
- Kuala Terengganu, Malaysia


December 16, 2011

Sir:

Use hot dip galvanizing.

Regards,

Dr. Thomas H. Cook
- Hot Springs, South Dakota


December 20, 2011

Dear Sir,

Electro-galvanization will give a very thin layer which will be corroded away in no time. You will want Hot-Dipped Galvanized (HDG). Most outdoor items are HDG. You can also refer to this thread :

http://www.finishing.com/221/64.shtml

Regards,

CS Leong
- Selangor, Malaysia


August 7, 2013

Q. I am trying to decide on the best material for a wall mounted mailbox for prevention of rusting/corrosion.

Is there a difference between electro-galvanized 20 gauge steel with powder coating and galvanized steel 16 gauge with powder coating?

Would "Hot Rolled Pickled and Oiled Steel (HRP&O)" be considered a good product for a mailbox?

D Bischof
- Plantation, Florida, USA


August 9, 2013

A. Hi D. "Specifications" in sales literature are designed to induce you to buy, not to technically inform you, and therefore they are usually not very meaningful. One mailbox claims electrogalvanizing while the other just says "galvanized" so -- while we'd like to hope that it means hot dip galvanized, which is thicker and more corrosion resistant -- it's probably not realistic to count on it. However, 16 gauge steel is much heavier than 20 gauge, so it's probably a better choice.

HRP&O has no zinc coating on it at all, so it is probably not as corrosion resistant as either of the others.

Regards,

pic of Ted Mooney Teds signature
Ted Mooney, P.E. RET
finishing.com
Brick, New Jersey

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