Difference between pre-galvanized and hot-dip galvanized steel?

A. In answer to your question, they may both be hot dip galvanized. The difference is probably whether the material was galvanized before or after fabrication. Most boxes are made from sheet that has been "mill galvanized", which means the coil manufacturer galvanized the sheet on a continuous line. This coating is generally very thin (.5 - 1.5 mils) and during the fabrication by the box manufacturer, any cut edges will be left exposed. Also, any welds will damage the initial coating.

A box galvanized after fabrication will usually have at least twice the coating thickness, and any evidence of fabrication should have the benefit of being coated. And yes, is is usually much cheaper to build a box out of pre-galvanized steel.

A. Hi, Vindya. The distinction relates to whether holes were drilled or edges sheared or parts welded together after galvanizing. If the component has no holes or sheared edges or welds you can't tell the difference because there is no difference (although David Jaye is probably right that the thickness of galvanizing on fabricated components is probably heavier than on plain sheet). But if edges are sheared or holes are drilled after galvanizing, there will be exposed steel.

Regards,

A. It is very easy to tell the difference between fabrications made from pre galvanized steel and those which are hot dipped after fabrication. The edge finish and weld appearance are some of the obvious ways as already mentioned. In my experience post fabrication hot dipping produces a less consistent finish, tends to contain inclusions leaving the surface rough in places and will leave drip marks and build up in areas where the molten zinc cannot fully drain.

Post fabrication galvanizing will provide superior corrosion protection as the minimum thickness of zinc will most likely exceed that which you will find on pre galvanized material. The downside is that if a high quality aesthetic finish is required (powder coated or wet painted) a significant amount of surface finishing may be required prior to painting. It's worth noting that in the case of pre galvanized sheet, the exposed edges left after cutting/punching still remain protected from corrosion. I can't explain the science behind this but I have experience of this working over many years of outdoor exposure.

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Thanks, Richard, that was helpful. The exposed edges are protected from corrosion in the same way that a zinc anode helps protect the steel hull of a ship.

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
January 21, 2010

Thanks a lot to both of you,Ted and Richard. This was very helpful.

Regards,

A. The difference is very obvious by naked eyes.
Just see once keeping side by side.

A. Pregalvanized method: once the sheet metal plated in the continuous galvanizing (melted zinc) bath, before fabrication, the plating thickness will be minimum 12 µm, and the materials plated by this way will be protected against corrosion on cut surfaces due to the cathodic effect between zinc and iron (steel); it's valid and effective up to 2 mm thickness.

Hot dip galvanization method: this method is the process of plating the sheet steel materials by dipping into zinc melt after fabrication, plating thickness by this method will reach minimum up to 45 µm.

thanks

Thanks all of you guys, it help a lot to my review/design works in Cable Trays/Ladders.

A. Dear Friend
In Fabrication line: pre-galvanized structural steel is: "already hot dip galvanized".
For necessary repairing on pre-galvanized steel or galvanized steel, normally we use zinc-rich paints. After repairing with that, match the color with aluminium spray paint.
Please try.

A. Hi Raul. We attached your inquiry to a thread where Richard L tells us that years of experience have shown him that the cut edges are often not a problem; it is often possible to 'roll' the edges so there is no exposed steel if necessary though.

500 hours of salt spray resistance seems improbable to me. Please remember than salt spray testing is a very very silly way to try to judge galvanizing though. The reason that hot dip galvanizing can be the most durable of all metal finishes (often able to last 75-100 years or even more without maintenance) is that the zinc slowly reacts with carbon dioxide in the air over the months & years, forming a hard, glassy, tightly sealed zinc carbonate surface; it is this corrosion product not the original zinc coating which offers the tremendous corrosion resistance. When you do a salt spray test, deliberately preventing the formation of this corrosion product, the salt spray chamber is testing something that has almost nothing to do with the actual corrosion resistance :-(

Please don't be a party to such foolishness. Good luck.

Regards,

A. Hi Gandeel. Probably not. The problem is that 'hot-dipped galvanizing' has a clear meaning, but after people started calling zinc electroplating 'electrogalvanizing', they also started shortening it to 'galvanizing'. Depending on who uses the word 'galvanizing' and the circumstances, they might mean hot dip galvanizing or they might mean zinc electroplating (usually very thin and not usually good enough for outdoor exposure). If the accessories don't say 'hot dipped' and don't look hot dipped, they probably aren't, and they probably won't hold up.

Regards,

A. Hi Kalla. Please remember that pre-galvanized and hot dip galvanized are not 'opposites' or 'alternatives' in the way you are employing those words. It's pre-galvanized vs. post-galvanized, and hot dip galvanized vs. electrogalvanized (zinc electroplated).

But in any case, zinc covered surfaces receive varying chromate conversion chemistry, oxidize to varying levels depending on environmental conditions, and are not reliable contact surfaces. It is one thing to ground zinc plated conduit and junction boxes to minimize the likelihood of electrical shocks, but I think it is quite another thing to depend on them to reliably carry electricity.

Please tell us why this question is of concern.

Regards,