Galvanizing 101: galvanized vs. hot-dipped galvanized

A. 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 :-(

A. 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.

A. Thanks, Victor. Words like "galvanized" are imprecise except in a specific context -- which is why I asked Bill G to clarify his situation. But nine out of ten galvanizing 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 have consulted with metal finishing professionals before issuing that text :-)

There never was a need for the phrase "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 century. So we have to suspect that the term "electro-galvanized" was coined by a salesman so that his company, which was providing the far less expensive and less durable zinc electroplating, could ride the coat tails of the premium finish through obfuscation :-)

A. 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.

A. 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.

A. Thanks, Scott. You may be correct that hot dipping thinwall devices will warp them -- every finish has advantages and disadvantages.

But on the other hand, acres & 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 and cleanup cost because most people believe that hot dip galvanized finishes are not susceptible to zinc whiskers, or at least far less so. Also people have died in hospitals when apnea monitors have shorted out due to zinc whiskers from their zinc electroplated enclosures.

So, while I agree that some specs which call for hot dipping may be archaic, some are deliberate and based on far more current thinking that you may realize. It is crucial that if we feel a spec is wrong we 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, or to drill and tap nuts after galvanizing. 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 corrosion resistance far superior to any other finish. Good luck in working it out, and thanks again.

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 usually far less and the corrosion resistance is therefore far less.

Regards,

A. Hi, Steve. To me there is no 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 we cannot stop other people from using words in the fashion they use them, even if we 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, and to specify things by specification numbers, rather than slang. Good luck.

Regards,

Thanks 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.

 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,

A. 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,

A. 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.

A. 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.

A. Sir:

Use hot dip galvanizing.

Regards,

A. 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 :

www.finishing.com/221/64.shtml

Regards,

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,

  Hi Louisi. Share and share alike. Plus if you can tell us what you know about the rates for hot-dipped galvanized finishing, that will be very useful towards helping other readers understand your question in sufficient depth to be able to answer you. Thanks!

Regards,

A. Hi Tony. So we don't confuse other readers, we should note that zinc may be applied to steel by electroplating (electrogalvanizing) or by hot-dipping, or by a few other methods. The coating may be applied to the stock before fabrication (pre-plating, pre-galvanizing) or after fabrication (post-plating, post-galvanizing) ... so hot dipped and pre-galv are not really opposites.

If you are going to bend and form the rod, though, I think you want a thin zinc coating, usually associated with pre-galvanizing or zinc electroplating rather than the heavy coating usually associated with post galvanizing.

Regards,

A. Hi Richard. Zinc electroplating tends to be very smooth and shiny, but without the spangled crystalization patterns of a frosted window. Hot dip galvanizing does tend to be flat gray and is occasionally a bit lumpy (from runs). I don't know much about hot dip galvanizing myself, although the site has several very knowledgable regular readers; until such time as they respond, you can search the site for "spangle", and I think you'll find it can be minimized and maximized through small additions of alloying metals. In any case, if it's spangled or it's heavy and flat gray, it's hot dip galvanizing, not electroplating.

Regards,

  Thank you for your reply and comments, I will get a sample of both hot dip galvanizing as well as zinc electroplating to more fully illustrate the difference(s).
Sincerely,

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Ed. note: For additional perspective, please see thread 22164 where Richard has also posted.