Difference between Electrogalvanized Steel and Hot Dip Galvanized Steel

A. Hi, Keith.

Whiskers are believed to be a stress reduction mechanism which I wouldn't expect to be sped up or slowed down by painting. And I would expect the painting itself to dislodge any whiskers which have already formed. But I suppose that paint offers a "binder" effect, so a primer designed for use on galvanized steel should be fine if applied before the materials go into the data center.

I need to add the disclaimer that the subject of "zinc whiskers" is an evolving science without a long and accepted history, so there's a good sized serving of hearsay and guesses in such a reply, but that it's generally believed that hot dip galvanizing is not as subject to zinc whiskers as zinc electroplating (electrogalvanizing).

Regards,

A. Hi, Shane.

Zinc electroplating is done at a temperature that would rarely exceed 100 °F, although the solutions in the pre-cleaning process are hotter and may approach 100 °C. The component will never reach 100 ° C, let alone 150 °C. There are things that can go wrong, I suppose, but overheating of the threaded bolt seems extremely unlikely. There is a potential defect in electroplating called "burning" which is related to excessive current density and which resembles scorching, but it doesn't actually involve high temperatures, it just looks that way :-)

Regards,

A. Hi, Ramarao

Painting sounds simpler, easier, & less expensive, and it may bring the corrosion resistance to an acceptable level -- but it won't comply with the spec, so you would need to get approval. Hot dip galvanization is probably not a problem in your particular case (the usual acid dip that is part of the process will quickly remove the previously electrodeposited coating).

But any time something like this comes up it is important to advise the designer; although I wouldn't expect pipe supports to be hardened steel that could have been damaged by the inadvertent electroplating, high strength components can suffer hydrogen embrittlement during electroplating, so there are some specs that prohibit electroplating for that reason.

Regards,

A. Absolutely not, Ky.

Sandblasting can be a good pretreatment for electrogalvanizing, but the zinc coating is sacrificial and its life will be proportional to its thickness. The sandblasting of the substrate does not change that fact.

Regards,

A. Hi, Hassen. You are apparently calling zinc electroplating "cold galvanizing"; some people call mechanical plating "cold galvanizing"; and still other people call zinc-rich paints "cold galvanizing". So the phrase is slang that has no accepted meaning and may lead to misunderstandings.

The operative phrase is "zinc is zinc", i.e., the corrosion resistance can be expected to be approximately proportional to the zinc thickness.

Although I suppose that it is theoretically possible for zinc electroplating and hot dip galvanizing to be the same thickness, the actual situation is that zinc electroplating is used for thin coatings and hot dipping is used for much thicker coatings, so your question doesn't really come up. I would be surprised to find that electroplated J-bolts are really adequate for general use as anchor bolts.

Regards,

A. Hi cousin Arvind. Those acronyms are not specifications -- they are mere slang. Hopefully, "GI", which stands for "galvanized iron", will actually mean hot dip galvanized steel; and "EG" may mean electrogalvanized steel, and "GP" may mean galvanized plain steel. The problem is we can't toss slang around and then ask for quantitative, finely nuanced definitions of its meaning.

As we have said several times in the above discussion, hot dip galvanized coatings are usually significantly thicker than electrogalvanized coatings, and therefore significantly more corrosion resistant. But that's about as far as slang is able take us. Give us an actual specification, and then we can talk about its corrosion resistance. Thanks!

Regards,

A. Hi, Abdul. Sorry, I don't quite understand your question. But hot dip galvanizing is only used on steel to my knowledge, whereas zinc is electroplated onto steel, copper, aluminum, zinc diecastings, and probably other materials as well. There are entire books just about zinc electroplating, and many books about galvanizing, so it is very difficult to summarize those subjects in a "simple and easy way" that will be useful to you when we don't know your situation or understand your question. Sorry.

Regards,

A. Hi Mike. The galvanizing pretreatment will strip the previous zinc plating, and you will be hot dip galvanizing the whole item.

Regards,

A. Hi Brandon. Electrical current is what drives electroplating; 1 gram equivalent weight of metal is deposited for each 96,485 Amp-seconds of electricity. But electricity essentially follows the path of least resistance, and essentially no current flows to the I.D. and no metal will deposit.

It is not impossible to plate I.D.s for critical applications; using internal anodes and high solution flow rates, it can be done (for example on gun barrels)-- but it is prohibitively expensive for commodity items like conduit. Good luck.

Regards,

A. Hi Nasrul. I am not familiar with such a test, although there may be one... but my suggestion is to ask yourself: "Why does it matter?"

Then, when you answer "It matters because ...", this should guide you towards a much more direct and meaningful test :-)

Regards,

A. Hi Zen. If you have a trained eye, you can usually tell just by looking at it. Electroplating is smooth and bright and uniform in appearance and usually has a chromate conversion coating on it. Galvanizing is mottled, or gritty, or drippy, and usually doesn't have a noticeable conversion coating. Once again we can answer your question best if you tell us why it matters to you? And that way, the readers will learn something from you.

If the coating is 0.0002" thick and you succeed in proving that it is hot dipped despite the low thickness, why does it matter? If the threads of a fastener are jamming and you prove that it was electroplated, despite the heavy coating, why does it matter? If the coating is dull and drippy instead of bright, yet you prove that it was electroplated, so what? If we prove that it was electroplated, but the coating is actually nickel plating or cadmium plating rather than zinc, does that help? If we prove the coating was hot dipped, but it is tin or aluminum rather than zinc, what difference does it make that it was hot dipped? If you have zinc whiskers, why does it matter whether the coating was hot dipped or electroplated?

Regards,

A. Hi Sanjay. Please take the time to re-read the page slowly as I have already described the usual differences in appearance three times! If the parts have obvious heavy drips or an obvious mottled crystalline pattern like frost on a window, they are hot dipped. If they are yellow they were zinc plated and chromated. Otherwise it might be hard to know.

But you have to ask yourself how important it is that the parts be correctly identified? It may be better to sell them off at a discount than to have to declare on your certifications that your declaration is based on untrained amateurs guessing based on something they read on the internet:-)

Regards,

Q. Hello,

Say you have a flat rolled coil which is galvanized. How can you tell the difference if this is electro or hot dip? Zinc coating is supposed to be at 7 microns each side with zero spangle.

A. Hi Tim. Yes, pipe is often galvanized differently. See letter 43365 or search the site for more on that topic. But I don't think that's the heart of the matter.

Self etching primers can be helpful for field application, consumer projects, touchup, etc. -- but they are not the right pretreatment for galvanized parts . . . zinc phosphatization is. My point is that when a self-etching primer fails to deliver good adhesion, the question is not what is wrong with the galvanizing, but what pretreatment you should be doing instead of simply using self-etching primer :-)
If your other parts are okay, carry on. But if the primer isn't adhering to the pipes, send them out for zinc phosphating. Good luck.

Regards,

A. Hi Senthil. This sacrificial coating has been applied across a wide range of thicknesses and in many different service conditions, ranging from essentially non-corrosive air-conditioned offices and computer rooms to severe saline and industrial atmospheres :-)

If you tell us in detail about your application people may be able to offer some predictions about service life. Good luck!

Regards,