Difference between Electrogalvanized and Hot Dip Galvanized Steel

A. Hi Tyler. I don't think I or anyone here said quite that ...

I have claimed that, for zinc coatings, service life is roughly proportional to the zinc thickness: "zinc is zinc" -- although galvanizers claim the zinc-iron alloying extends the life, and platers claim the purity of the zinc plating deposit extends the life. When coating is done post-fab, heavy hot-dip galvanizing might be 10X as thick as thin zinc electroplating. But if the galvanizing is done pre-fab, while processing sheet, some sort of wiping action is common and hot-dip may not be be much heavier than zinc electroplating. My guess until you say otherwise is that 11 gauge legs are fabricated from pre-coated sheet.

If you can say that your coatings will be substantially thicker than the alternatives, the service life should be substantially longer than the alternatives. If your zinc coatings are the same thickness, the service life will be about the same. If the parts are painted or powder coated after the zinc coating, it can be pretty hard to make predictions :-)

Regards,

A. Hi Wojciech. After many pages of postings on this subject we are into endless repetition, but ...

For practical purposes, "zinc is zinc" so the corrosion resistance of equal thicknesses will be about equal. Electrogalvanizing will usually prove more practical and cheaper for thin coatings, and hot dipping will prove more practical and economical for thick coatings. Wire gets a bit more complicated regarding practical thicknesses and costs because it is often drawn down to a thinner gauge after coating. You might have to shop around regarding relative cost for in-between applications. Good luck.

Regards,

A. Hi Madanmohan. The atmospheric corrosion resistance of zinc coated steel is pretty much proportional to the thickness of the zinc. In the case of post-plated fabrications, the thickness of zinc on hot-dipped articles almost always greatly exceeds the thickness of zinc on electroplated (electrogalvanized) articles, and therefore they are significantly more corrosion resistant.

But in the case of pre-plated sheet, my understanding is that the hot dip galvanize lines usually have "wipers" to reduce the thickness of the zinc coating and hold it to a particular thickness/weight. So in this case, I think you have to go by what the coating thickness/weight specification is, rather than ideological factors. In other words, the specification should tell you the zinc thickness, and the thicker coating will be the more corrosion resistant.

Regards,

A. Hi Pratik. Neither!

These zinc coatings are useful towards preventing corrosion under atmospheric conditions; and thick zinc coatings like hot dip galvanizing can be useful for saltwater exposure. But depending on the "electrolyte", it may well attack a zinc coating much faster than it attacks steel. For example, zinc dissolves almost instantly in hydrochloric acid. A quick rule of thumb is probably that zinc is not chemically resistant.

Regards,

A. Hydrogen Embrittlement is probably a smaller concern with galvanizing, but if they are supposed to be galvanized, that means hot dip galvanized to me regardless of advantages or disadvantages.

When someone offers to sell me 'gold', I don't want to have to use the the longer form 'real genuine gold, atomic number 79 on the periodic table, not fake gold-colored cheaper imitation stuff'. And when 'galvanizing' is specified, it should mean galvanizing, i.e., hot dipping, without the need to say 'real genuine hot dipped in molten zinc galvanizing, not fake electrogalvanized cheaper zinc electroplated stuff' :-)

Regards,

A. Hi Amol. I am no expert and have no galvanizing experience at all, but I understand that they are dipped, then 'spun' in a centrifugal machine to minimize drips blocking the threads of the screws. I also understand that to minimize thread interference, the nut 'blanks' are usually galvanized first, then drilled and tapped; although this means the thread is bare steel, the galvanizing of the screw thread, plus everything else being galvanized, protects them.

A. Dear Mr. Amol,

You can Galvanize threaded nuts and bolts with the help of Centrifugal machine.

A. Ted is exactly correct in his description of hot-dip galvanized fasteners. An alternative is mechanical galvanizing, which gives equivalent zinc thickness (roughly two mils).

Excellent point Tom. Hot dip galvanizing and zinc electroplating are not the only ways to apply a zinc coating, and sometimes neither is the best way!

When very thin coatings are required on very small fasteners, zinc electroplating is the usual choice. When very thick coatings are required on very large fasteners, hot dipping is the usual choice. In the broad middle range, mechanical galvanizing or other approaches may be better than either.

Regards,

? Hi Kumar. You probably can't. Why does it matter to you?

Regards,

A. Hi Fabrice. It's not clear whether you are talking about a coating for the flanges or the nut & bolt. If you're talking about the nut & bolt, it's not clear what the coating on the flanges is. In any case, HDG gives greater thicknesses and therefore probably offers superior corrosion resistance.

Regards,

A. Hi Aushi. Certainly there is ... there are many different coating and corrosion-resisting technologies. Tom Rochester just mentioned mechanical galvanizing, and there's sherardizing, flame spraying, zinc-rich paints, etc. What is your situation?

Regards,

A. Hi Leif. It's hard to compare adhesion strengths between different technologies because they are all subject to being done poorly, with resulting low bond strengths. If done right, both galvanizing and zinc plating will "stick" far more than is necessary for suitable pipeline service, although you may be right that electroplating sticks better in the ideal case.

But it is much easier to hot dip galvanize the interior of a pipe than to electroplate it with zinc, so I am not familiar with any pipe that has an interior coating of zinc plating anyway. If there is such a product line, please advise. Thanks.

Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

A. Hi Charles -- thanks for your patience! But proceeding from the abstract like this begins with a slight misquote of me; so then I have to note, for the benefit of other readers rather than you, that that isn't exactly what I said, etc., etc :-)
Instead, please tell us what you are doing and describe the details of your actual situation, and how it relates to hot-dipping vs. electrogalvanization (if it does).

The basic idea is that electroplating, pickling, cathodic electrocleaning, and some other metal finishing operations generate hydrogen. If the steel is soft mild steel, it's no problem; but in hard high strength steels this can cause hydrogen embrittlement, which means the steel can fracture under unexpectedly low loads. Hydrogen embrittlement is usually addressed by immediate baking, although some steels are so hard that they should not be pickled or plated at all. You seem to be describing a formable steel that doesn't sound real hard, but I think the steel vendor can give you a quick answer as to whether this grade of their steel can suffer hydrogen embrittlement. Then we can proceed with your questions from there. Good luck.

Regards,

A. Hi Sami. If it's a question of zinc plating vs. hot dip galvanizing, the galvanizing will be much thicker and much more corrosion resistant, and therefore probably more appropriate for offshore use -- but things can be electroplated with many other metals which may be appropriate for different applications. As a trivial and obvious example, a hydraulic piston must be hard chrome plated; you can't galvanize it. Further, if components can be painted, or zinc plated and painted, or galvanized and painted, this will probably be better than bare galvanizing in most cases.

You can get answers to highly specific questions here, but I'm sure you understand that nobody becomes a finishing expert, able to independently and knowledgeably specify finishes, from a few paragraphs of Q&A's :-)
Finishing has been my career for 50 years and in many cases I would not attempt to specify a finish without some specialized help :-)

Regards,

A. Hot dip galvanizing would be less unsuitable than electroplating. There is a vast amount of literature available on what coatings are recommended for offshore use and merely coating them with zinc by any process is simply inadequate. Typical offshore use would have a base of hot dip galvanizing followed by two or more coats of paint.