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

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Q. I am a site supervision Engineer, I am working for an electrical systems CONSULTANT, Riyadh , KSA. I am supervising the Electrical construction work of one of the largest in the Gulf Region. Actually I need some information about one of products which is : The galvanized Switch and Socket Boxes.

My Question is : Some manufacturers use to write " Pre Galvanized switch and socket boxes", Others used to write "Hot Dip Galvanized switch and socket boxes". What is the difference between the two type of steel? The answer is very important for me as a consultant.

So, please your prompt Answer to this question along with any other information about steel types are highly appreciated.

Abdul Rahman baalbaki
electrical systems - Riyadh, KSA


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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.

David Jaye
Houston, Texas

January 4, 2010

Q. I am pretty new to this area, but would like to get to know some more details about 'pre-galvanized' and 'Hot Dip Galvanized' steel.
Is there any way we can figure out the two types by looking at the surface? Or by touching the surface? Is there a difference in color or the way the surface is formed?

I would really appreciate if you can reply ASAP. Thanks in advance.

Vindya Wipulaguna
- Kelaniya, Colombo, Sri Lanka


January , 2010

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,

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

January 21, 2010

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.

Richard Lyons
- Ireland


January 21, 2010

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

Regards,

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


February 7, 2010

thumbsup2Thanks a lot to both of you,Ted and Richard. This was very helpful.

Regards,

Vindya Wipulaguna
- Kelaniya, Colombo, Sri Lanka


April 21, 2010

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

Akif Qadeer
- Mumbai, MH , India


December 7, 2010

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 fabrications, plating thickness by this method will reach minimum up to 45 µn;/m.

thanks

Mahmoud Abu Al Ata
- Istanbul, Turkey

February 28, 2012

thumbsup2Thanks all of you guys, it help a lot to my review/design works in Cable Trays/Ladders.

wilfredo_meneses
Wilfredo H. Meneses
- Doha, Qatar


May 24, 2013

Q. Hello... I would like to know if anyone can help me with a question about pre galvanised steel and coating zinc over the welds and cut points.
We have a project which is basically fabricating and supplying pre-galvanised 40 x 40 x 1.6 SHS panels plus miscellaneous plates to the Gas Fields, and we need to paint all welds to client protective coatings technical specification "Surface preparation SSPC SP11, AS1627 ST3 using Epoxy Zinc Primer 75-100". There is no requirement for 2nd & 3rd coat in the spec; therefore I would appreciate your advice to source a suitable product for use in the elements. Is the product you have identified the most suitable for this application?

I was told by a paint supplier that if we use epoxy zinc rich paint it will react with the pre-galvanised steel and in time it will fail, they said it fight against itself ?
They said to use just epoxy zinc?
Can anyone help me understand it?

Thanks

Jeremy Ingledew
- Toowoomba, Queensland, Australia
  ^- Privately contact this inquirer -^


June 4, 2013

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. I recommend Zinga from Belgium. It's in market as a spray as well as in paint; it is gray color. After repairing with Zinga, match the color with aluminium spray paint.
Please try.

Umesh Dalela
- Sana'a, Yemen



June 2, 2013

Q. I'm supervising the production of some steel products. The case I have a question about is a product which consists of some round bars (Diameter 120-160 mm) and wires (Diameter 3 mm) which are welded together.

The product should be galvanized. According to the standard ASTM A123, minimum thickness of zinc coating is 100 microns for product diameter of 12-16 mm and is 50 microns for product diameter of 3 mm.

If the product is galvanized after the welding process, its not satisfying the coating thickness in all parts; and if each part is galvanized before welding individually, and then welded together, coating evaporation and peeling occurs (because weld area is relatively high).

Ali Rafiei
- Arak, Iran
  ^- Privately contact this inquirer -^

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